GSM security

GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using a pre-shared key and challenge-response. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional USIM, that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user - whereas GSM only authenticates the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation. GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in other countries. Serious weaknesses have been found in both algorithms: it is possible to break A5/2 in real-time with a ciphertext-only attack, and in February 2008, Pico Computing, Inc revealed its ability and plans to commercialize FPGAs that allow A5/1 to be broken with a rainbow table attack.^[15] The system supports multiple algorithms so operators may replace that cipher with a stronger one.

Subscriber Identity Module (SIM)

ne of the key features of GSM is the Subscriber Identity Module, commonly known as a SIM card. The SIM is a detachable smart card containing the user's subscription information and phone book. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries.

In Australia, North America and Europe many operators lock the mobiles they sell. This is done because the price of the mobile phone is typically subsidised with revenue from subscriptions, and operators want to try to avoid subsidising competitor's mobiles. A subscriber can usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the handset themselves. While most web sites offer the unlocking for a fee, some do it for free. The locking applies to the handset, identified by its International Mobile Equipment Identity (IMEI) number, not to the account (which is identified by the SIM card).

In some countries such as Bangladesh, Belgium, Costa Rica, Indonesia, Malaysia, Hong Kong and Pakistan, all phones are sold unlocked. However, in Belgium, it is unlawful for operators there to offer any form of subsidy on the phone's price. This was also the case in Finland until April 1, 2006, when selling subsidized combinations of handsets and accounts became legal, though operators have to unlock phones free of charge after a certain period (at most 24 months).

Network structure

The structure of a GSM network

The network behind the GSM seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.

• the Base Station Subsystem (the base stations and their controllers).
• the Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network.
• the GPRS Core Network (the optional part which allows packet based Internet connections).
• all of the elements in the system combine to produce many GSM services such as voice calls and SMS.

Voice codecs

GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 5.6 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate (5.6 kbit/s) and Full Rate (13 kbit/s). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.

GSM was further enhanced in 1997^[14] with the Enhanced Full Rate (EFR) codec, a 12.2 kbit/s codec that uses a full rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.

GSM frequencies

GSM networks operate in a number of different frequency ranges (separated into GSM frequency ranges for 2G and UMTS frequency bands for 3G). Most 2G GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated. Most 3G GSM networks in Europe operate in the 2100 MHz frequency band.

The rarer 400 and 450 MHz frequency bands are assigned in some countries where these frequencies were previously used for first-generation systems.

GSM-900 uses 890–915 MHz to send information from the mobile station to the base station (uplink) and 935–960 MHz for the other direction (downlink), providing 125 RF channels (channel numbers 0 to 124) spaced at 200 kHz. Duplex spacing of 45 MHz is used.

In some countries the GSM-900 band has been extended to cover a larger frequency range. This 'extended GSM', E-GSM, uses 880–915 MHz (uplink) and 925–960 MHz (downlink), adding 50 channels (channel numbers 975 to 1023 and 0) to the original GSM-900 band. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate for all 8 channels is 270.833 kbit/s, and the frame duration is 4.615 ms.

The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.

Cellular radio network

GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity.

There are five different cell sizes in a GSM network—macro, micro, pico, femto and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen metres; they are mainly used indoors. Femtocells are cells designed for use in residential or small business environments and connect to the service provider’s network via a broadband internet connection. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.

Cell horizontal radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometres. The longest distance the GSM specification supports in practical use is 35 kilometres (22 mi). There are also several implementations of the concept of an extended cell^[12], where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.

Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when a lot of call capacity is needed indoors; for example, in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from any nearby cell.

The modulation used in GSM is Gaussian minimum-shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent channel interference).

[edit] Interference with audio devices

Some audio devices are susceptible to radio frequency interference (RFI), which could be mitigated or eliminated by use of additional shielding and/or bypass capacitors in these audio devices. However, the increased cost of doing so is difficult for a designer to justify.^[13]

It is a common occurrence for a nearby GSM handset to induce a "dit, dit di-dit, dit di-dit, dit di-dit" audio output on PAs, wireless microphones, home stereo systems, televisions, computers, cordless phones, and personal music devices. When these audio devices are in the near field of the GSM handset, the radio signal is strong enough that the solid state amplifiers in the audio chain act as a detector. The clicking noise itself represents the power bursts that carry the TDMA signal. These signals have been known to interfere with other electronic devices, such as car stereos and portable audio players. This also depends on the handset's design, and its conformance to strict rules and regulations allocated by the US body, the FCC, in part 15 of its rules and regulations pertaining to interference with electronic devices

GSM

GSM world coverage as of 2008

GSM (Global System for Mobile communications: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. Its promoter, the GSM Association, estimates that 80% of the global mobile market uses the standard.^[1] GSM is used by over 3 billion people across more than 212 countries and territories.^[2][3] Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signaling and speech channels are digital, and thus is considered a second generation (2G) mobile phone system. This has also meant that data communication was easy to build into the system.

The ubiquity of the GSM standard has been an advantage to both consumers (who benefit from the ability to roam and switch carriers without switching phones) and also to network operators (who can choose equipment from any of the many vendors implementing GSM^[4]). GSM also pioneered a low-cost (to the network carrier) alternative to voice calls, the short message service (SMS, also called "text messaging"), which is now supported on other mobile standards as well. Another advantage is that the standard includes one worldwide emergency telephone number, 112.^[5] This makes it easier for international travellers to connect to emergency services without knowing the local emergency number.

Newer versions of the standard were backward-compatible with the original GSM phones. For example, Release '97 of the standard added packet data capabilities, by means of General Packet Radio Service (GPRS). Release '99 introduced higher speed data transmission using Enhanced Data Rates for GSM Evolution (EDGE).

Spread Spectrum Characteristics of CDMA

Most modulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread spectrum techniques use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems. These systems were designed using spread spectrum because of its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudorandom code; this code makes the spread spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudorandom sequence used to encode the data. CDMA is also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal.^[4]

CDMA can also effectively reject narrowband interference. Since narrowband interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information. Convolution encoding and interleaving can be used to assist in recovering this lost data. CDMA signals are also resistant to multipath fading. Since the spread spectrum signal occupies a large bandwidth only a small portion of this will undergo fading due to multipath at any given time. Like the narrowband interference this will result in only a small loss of data and can be overcome.

Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudorandom codes will have poor correlation with the original pseudorandom code, and will thus appear as another user, which is ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay, the multipath signals will arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudorandom codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it is thus ignored.

Some CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a single correlator tuned to the path delay of the strongest signal. ^[5]

Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems frequency planning is an important consideration. The frequencies used in different cells need to be planned carefully in order to ensure that the signals from different cells do not interfere with each other. In a CDMA system the same frequency can be used in every cell because channelization is done using the pseudorandom codes. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; however, planning of the different pseudorandom sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell.^[6]

Since adjacent cells use the same frequencies, CDMA systems have the ability to perform soft handoffs. Soft handoffs allow the mobile telephone to communicate simultaneously with two or more cells. The best signal quality is selected until the handoff is complete. This is different from hard handoffs utilized in other cellular systems. In a hard handoff situation, as the mobile telephone approaches a handoff, signal strength may vary abruptly. In contrast, CDMA systems use the soft handoff, which is undetectable and provides a more reliable and higher quality signal.^[6]

Advantages of Asynchronous CDMA over other techniques

1. Efficient Practical utilization of Fixed Frequency Spectrum

Asynchronous CDMA's main advantage over CDM (Synchronous CDMA), TDMA and FDMA is that it can use the spectrum more efficiently in mobile telephony applications. (In theory, CDMA, TDMA and FDMA have exactly the same spectral efficiency but practically, each has its own challenges - power control in the case of CDMA, timing in the case of TDMA, and frequency generation/filtering in the case of FDMA.) TDMA systems must carefully synchronize the transmission times of all the users to ensure that they are received in the correct timeslot and do not cause interference. Since this cannot be perfectly controlled in a mobile environment, each timeslot must have a guard-time, which reduces the probability that users will interfere, but decreases the spectral efficiency. Similarly, FDMA systems must use a guard-band between adjacent channels, due to the unpredictable doppler shift of the signal spectrum which occurs due to the user's mobility. The guard-bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum.

2. Flexible Allocation of Resources

Asynchronous CDMA offers a key advantage in the flexible allocation of resources i.e. allocation of a PN codes to active users. In the case of CDM, TDMA and FDMA the number of simultaneous orthogonal codes, time slots and frequency slots respectively is fixed hence the capacity in terms of number of simultaneous users is limited. There are a fixed number of orthogonal codes, timeslots or frequency bands that can be allocated for CDM, TDMA and FDMA systems, which remain underutilized due to the bursty nature of telephony and packetized data transmissions. There is no strict limit to the number of users that can be supported in an Asynchronous CDMA system, only a practical limit governed by the desired bit error probability, since the SIR (Signal to Interference Ratio) varies inversely with the number of users. In a bursty traffic environment like mobile telephony, the advantage afforded by Asynchronous CDMA is that the performance (bit error rate) is allowed to fluctuate randomly, with an average value determined by the number of users times the percentage of utilization. Suppose there are 2N users that only talk half of the time, then 2N users can be accommodated with the same average bit error probability as N users that talk all of the time. The key difference here is that the bit error probability for N users talking all of the time is constant, whereas it is a random quantity (with the same mean) for 2N users talking half of the time.

In other words, Asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals. CDM (Synchronous CDMA), TDMA and FDMA systems cannot recover the underutilized resources inherent to bursty traffic due to the fixed number of orthogonal codes, time slots or frequency channels that can be assigned to individual transmitters. For instance, if there are N time slots in a TDMA system and 2N users that talk half of the time, then half of the time there will be more than N users needing to use more than N timeslots. Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal code, time-slot or frequency channel resources. By comparison, Asynchronous CDMA transmitters simply send when they have something to say, and go off the air when they don't, keeping the same PN signature sequence as long as they are connected to the system.

3. Privacy protection in Spread Spectrum CDMA due to anti-jamming capabilities of PN sequences

Asynchronous CDMA

The previous example of orthogonal Walsh sequences describes how 2 users can be multiplexed together in a synchronous system, a technique that is commonly referred to as Code Division Multiplexing (CDM). The set of 4 Walsh sequences shown in the figure will afford up to 4 users, and in general, an NxN Walsh matrix can be used to multiplex N users. Multiplexing requires all of the users to be coordinated so that each transmits their assigned sequence v (or the complement, -v) starting at exactly the same time. Thus, this technique finds use in base-to-mobile links, where all of the transmissions originate from the same transmitter and can be perfectly coordinated.

On the other hand, the mobile-to-base links cannot be precisely coordinated, particularly due to the mobility of the handsets, and require a somewhat different approach. Since it is not mathematically possible to create signature sequences that are orthogonal for arbitrarily random starting points, unique "pseudo-random" or "pseudo-noise" (PN) sequences are used in Asynchronous CDMA systems. A PN code is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receivers. These PN codes are used to encode and decode a users signal in Asynchronous CDMA in the same manner as the orthogonal codes in synchrous CDMA (shown in the example above). These PN sequences are statistically uncorrelated, and the sum of a large number of PN sequences results in Multiple Access Interference (MAI) that is approximated by a Gaussian noise process (following the "central limit theorem" in statistics). If all of the users are received with the same power level, then the variance (e.g., the noise power) of the MAI increases in direct proportion to the number of users. In other words, unlike synchronous CDMA, the signals of other users will appear as noise to the signal of interest and interfere slightly with the desired signal in proportion to number of users.

All forms of CDMA use spread spectrum process gain to allow receivers to partially discriminate against unwanted signals. Signals encoded with the specified PN sequence (code) are received, while signals with different codes (or the same code but a different timing offset) appear as wideband noise reduced by the process gain.

Since each user generates MAI, controlling the signal strength is an important issue with CDMA transmitters. A CDM (Synchronous CDMA), TDMA or FDMA receiver can in theory completely reject arbitrarily strong signals using different codes, time slots or frequency channels due to the orthogonality of these systems. This is not true for Asynchronous CDMA; rejection of unwanted signals is only partial. If any or all of the unwanted signals are much stronger than the desired signal, they will overwhelm it. This leads to a general requirement in any Asynchronous CDMA system to approximately match the various signal power levels as seen at the receiver. In CDMA cellular, the base station uses a fast closed-loop power control scheme to tightly control each mobile's transmit power. See Near-far problem for further information on this problem.

Steps in CDMA Modulation

CDMA is a spread spectrum multiple access^[1] technique. A spread spectrum technique is one which spreads the bandwidth of the data uniformly for the same transmitted power. Spreading code is a pseudo-random code which has a narrow Ambiguity function unlike other narrow pulse codes. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is simply logically XOR (exclusive OR) added with the faster code. The figure shows how spread spectrum signal is generated. The data signal with pulse duration of T_b is XOR added with the code signal with pulse duration of T_c. (Note: bandwidth is proportional to 1 / T where T = bit time) Therefore, the bandwidth of the data signal is 1 / T_b and the bandwidth of the spread spectrum signal is 1 / T_c. Since T_c is much smaller than T_b, the bandwidth of the spread spectrum signal is much larger than the bandwidth of the original signal. The ratio T_b / T_c is called spreading factor or processing gain and determines to certain extent the upper limit of total number of users supported simultaneously by a base station^[2]

Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. The best performance will occur when there is good separation between the signal of a desired user and the signals of other users. The separation of the signals is made by correlating the received signal with the locally generated code of the desired user. If the signal matches the desired user's code then the correlation function will be high and the system can extract that signal. If the desired user's code has nothing in common with the signal the correlation should be as close to zero as possible (thus eliminating the signal); this is referred to as cross correlation. If the code is correlated with the signal at any time offset other than zero, the correlation should be as close to zero as possible. This is referred to as auto-correlation and is used to reject multi-path interference.^[3]

In general, CDMA belongs to two basic categories: synchronous (orthogonal codes) and asynchronous (pseudorandom codes).

[edit] Code Division Multiplexing (Synchronous CDMA)

Synchronous CDMA exploits mathematical properties of orthogonality between vectors representing the data strings. For example, binary string "1011" is represented by the vector (1, 0, 1, 1). Vectors can be multiplied by taking their dot product, by summing the products of their respective components. If the dot product is zero, the two vectors are said to be orthogonal to each other (note: if u=(a,b) and v=(c,d), the dot product u.v = a*c + b*d). Some properties of the dot product aid understanding of how W-CDMA works. If vectors a and b are orthogonal, then

Each user in synchronous CDMA uses a code orthogonal to the others' codes to modulate their signal. An example of four mutually orthogonal digital signals is shown in the figure. Orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other. In the case of IS-95 64 bit Walsh codes are used to encode the signal to separate different users. Since each of the 64 Walsh codes are orthogonal to one another, the signals are channelized into 64 orthogonal signals. The following example demonstrates how each users signal can be encoded and decoded.

CDMA(Code division multiple access)

Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne and CDMA2000 (which are often referred to as simply "CDMA"), which use CDMA as an underlying channel access method.

One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a bandwidth of frequencies. This concept is called multiplexing. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of "spread-spectrum" signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.

An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.

• PC World
• »Phones

In Pictures: A History of Cell Phones

From Motorola's first phone, which weighed in at 2 pounds, to Apple's upcoming iPhone, here's a look at how cell phones have evolved over the years.

Liane Cassavoy

May 7, 2007 1:00 pm

Hefty: Motorola DynaTAC 8000X (1982)

In 1973, Motorola showed off a prototype of the world's first portable cellular telephone. That phone, which measured more than a foot long, weighed almost 2 pounds, and cost $3995, ultimately became commercial available in 1983. Known as the Motorola DynaTAC 8000X, its battery could provide 1 hour of talk time, and its memory could store 30 phone numbers. It may not have been pretty, but it did let you talk while on the go--if you could lift it, that is.

Heftier: Nokia Mobira Senator (1982)

It may look more like a boombox than a portable phone, but this boxy, bulky device was actually Nokia's first mobile (if you can call it that) phone. Introduced in 1982, the Nokia Mobira Senator was designed for use in cars. After all, you wouldn't want to use this phone while walking: It weighed about 21 pounds.

Pre-iPhone: BellSouth/IBM Simon Personal Communicator (1993)

A cell phone with added PDA functions isn't news today. But in 1993, it was a novel idea. The Simon Personal Communicator, jointly marketed by IBM and BellSouth, was the first mobile phone to add PDA features. It was a phone, pager, calculator, address book, fax machine, and e-mail device in one package, albeit a 20-ounce package that cost $900.

Ahead of Its Time: Motorola StarTAC (1996)

Before the Motorola StarTAC was introduced in 1996, cell phones were more about function than fashion. But this tiny, lightweight phone ushered in the concept that style was just as important, ultimately paving the way for today's sleek-looking phones like the Motorola Razr. This 3.1-ounce clamshell-style phone, which could easily be clipped to a belt, was the smallest and lightest of its time. In fact, it was smaller and lighter than many of to

DotComs Ran on These: Nokia 6160 (1998) or Nokia 8260 (2000)

In the late 1990s, Nokia's candybar-style cell phones were all the rage. Sporting a monochrome display, an external antenna, and a boxy, 5.2-inch tall frame, the Nokia 6160 was the company's best-selling handset of the 1990s. The somewhat sleeker Nokia 8260, introduced in 2000, added a colorful case and lost some of the 6160's bulk: it stood only about 4 inches tall and weighed 3.4 ounces, compared with almost 6 ounces for the 6160

Early Smart Phone: Kyocera QCP6035 (2000)

If you're one of the many fans of the Palm OS-based Treo phone, you might want to thank Kyocera. The company's QCP6035 smart phone, which hit the retail market in early 2001 and cost between $400 and $500 (depending on the carrier), was the first Palm-based phone to be widely available to users. It included a measly 8MB of memory, and sported a bland monochrome display, but it paved the way for future products

PDA to Phone: Handspring Treo 180 (2001)

Back when Palm and Handspring were still rivals, Handspring made waves with the Treo 180. More PDA than phone, the Treo 180 came in two versions: one with a QWERTY keyboard for typing (pictured), and another (the Treo 180g) that used Graffiti text input instead. Like the Kyocera QCP6035, it featured a monochrome screen, but boasted 16MB of memory.

Swivel It: Danger Hiptop (2002)

Before the T-Mobile Sidekick became Hollywood's "it" phone, it was known as the Danger Hiptop. PC World liked it so much that we named it our product of the year in 2003. While its voice capabilities were only mediocre, this was one of the first devices to offer truly functional mobile Web browsing, e-mail access, and instant messaging. Plus, it pioneered that nifty swiveling design.

CrackBerry Phone: BlackBerry 5810 (2002)

Before the BlackBerry 5810 came along in early 2002, Research In Motion's devices were best known for their data capabilities: Push e-mail technology, Organizer features, and thumb keyboards. The 5810--the first BlackBerry to offer voice capabilities--changed that perception. This device added a GSM cell phone to the package, albeit one that required the use of a headset (it lacked both a speaker and a microphone

Photo Opp: Sanyo SCP-5300 (2002)

Today, most cell phones come with a built-in camera. But, just a few years ago, a camera phone was hard to come by. In 2002, Sanyo and Sprint debuted the Sanyo SCP-5300 PCS phone, which they claimed was the first mobile phone available in America with a built-in camera. (A camera phone from Sharp had been available in Japan for a few years.) At its highest resolution, it captured VGA (640 by 480) images--a far cry from today's 5-megapixel camera phones like the Nokia N95.

Bad Buzz: Nokia N-Gage (2003)

Nokia's N-Gage also created plenty of buzz when it was launched in 2003, but, unfortunately, most of the buzz was bad. This combination cell phone/gaming device was supposed to lure gamers away from their portable devices. Instead, it earned scorn for its odd curved design, and the fact that you had to hold the phone on its side to place a call. Later versions (like the N-Gage QD, launched in 2004) fixed many of the problems with the original device. But for many, the damage was done.

Sleek: Motorola Razr v3 (2004)

Cell phones continued to get thinner and more stylish over the years, but it was the debut of the Motorola Razr v3 in 2004 that took design to another level. With its super-slim lines and sleek metallic look, the Razr quickly became the must-have accessory. Three years later, it remains one of the most popular handsets on the market (according to market data from The NPD Group, various versions of the Razr were 3 of the 4 best-selling handsets in 2006), and is one of the few phones offered by almost every

Out of Tune: Motorola Rokr (2005)

It promised to bring together the best of two worlds: Apple's excellent iTunes music player and Motorola's cell phone design expertise. The Motorola Rokr, released in September 2005, was the first music phone to incorporate Apple's music software. It allowed users to transfer songs purchased from iTunes to the phone for listening on the go. Unfortunately, users found song transfers to be painfully slow, and many were stymied by the 100-song limit imposed on their music collections. Still, this handset paved the way for today's music phones, including those (like the Motorola Slvr and Razr V3i) that support iTunes

Good Looks: BlackBerry Pearl (2006)

Research In Motion continued its efforts to shed its strictly-business image with the consumer-friendly Pearl. This phone, with its slim design and SureType keyboard, looked the part. It went further than the 7100t, however: the Pearl was the first BlackBerry to include a camera and an audio/video player. Combine these multimedia features with BlackBerry's excellent e-mail service, and you have one impressive device.

Coming Soon: Apple iPhone (2007)

After months of speculation and rumors, Apple confirmed the news in January: The company does indeed plan to launch a cell phone. The device, which is expected to be available from AT&T/Cingular in June, will feature an innovative design: it lacks a numeric keypad. Instead, it will feature a touch-sensitive screen. The iPhone will also reportedly include a 2-megapixel camera, the ability to sync your iTunes collection to the phone, and it will run Mac OS X. Whew. We can't wait to get a look at

A Brief History of the Mobile Phone (1973-2007)

1973

• Motorola touts a prototype of the world's first mobile cellular phone, the Motorola DynaTAC 8000X. It's more than a foot long, weighs nearly 2 pounds and sells for $4,000. However, it wasn't commercially available until a decade later.

1982

• Finnish handset maker Nokia introduces its first mobile phone, the Nokia Mobira Senator. The device looks very much like a portable radio and it weighs a whopping 21 pounds. Yikes.

1993

• BellSouth/IBM unveil the world's first mobile phone with PDA features, including phone and pager functionality, calculator and calendar applications, as well as fax and e-mail capability. The BellSouth/IBM Simon Personal Communicator weighs 21 ounces and sells for $900.

1996

• Motorola debuts its StarTAC mobile phone, merging fashion and functionality into the cell phone. It weighs 3.1 ounces--light by even today's standards--and it is a clam shell device.

2000

• Kyocera introduces its QCP6035 mobile phone, the very first widely available Palm OS-based phone. It costs between $400 and $500 but only included 8MB of memory.

2001

• Before Palm acquired Handspring, the company released its Handspring Treo 180 cellular phone, which came in two versions. The Treo 180 was available with a QWERTY keyboard as well as in a separate version with text input method called Graffiti.

2002

• The Danger Hiptop, which later became known as the T-Mobile Sidekick, hits the mobile space. It is one of the first mobile devices to include a quality Web browser, reliable e-mail access and instant messaging, as well a unique swiveling form factor. (PCWorld.com later went on to name the device its 2003 product of the year.)
• The BlackBerry 5810 hits the market in 2002, and though it's not the first BlackBerry, it's the first such device from Research In Motion (RIM) to include voice functionality--though a headset is required because it doesn't have an external microphone or speaker.
• Sanyo and Sprint make the Sprint SCP-5300 PCS available, and both companies claim it's the first mobile phone in the United States to include a digital camera. Image quality is, however, less than impressive.

2004

• Motorola announces its RAZR v3 cell phone and starts a trend toward ultra-thin, stylish phones that's still influencing mobile device manufacturers today. The RAZR v3 is a "cool" device that everyone, from high schoolers to businessmen, wants. It's still one of the most popular mobile phones, and its one of the few handsets offered by the majority of major cellular carriers.

2006

• RIM, known for its high-end business phones and reliable "push" e-mail technology, makes its first foray into the consumer space with the BlackBerry Pearl 8100. The device is the first from RIM to include a digital camera and media player and it's also the smallest, thinnest BlackBerry--currently, the company's 8800 series of devices are the thinnest it offers. (Read CIO.com's review for more on the BlackBerry Pearl.)

2007

• Apple releases the iPhone, a beautifully designed device that includes an innovative--and much hyped up--touch screen navigation interface, which doesn't require the use of a stylus. The device is available exclusively through AT&T in the United States, and it comes in a 4GB version for $499 and an 8GB version for $599.

What's missing? Is there another notable event in the

history of mobile phones that's not on the list? If so, please let me know. (There are a few more events mentioned in the PCWorld piece. To read about them, as well as see images of the above mentioned devices,check out

4G

As the limitation of the 3G, people are try to make new generation of mobile communication, this is the 4^th generation. This 4G system is more reliable,

Nowadays, some companies have started developing the 4G communication system, this technology can have a high uplink rate up to 200Mbps, more data can transfer in the mobile phone. So the 4G mobile can have more function such as work as the television. Some telecommunication companies claimed that they would applied this 4G system to the business and it will bring more convenience to people.

History of Cellular Phones

The basic concept of cellular phones began in 1947 when researchers looked at crude mobile (car) phones and realized that by using small cells (range of service area) with frequency reuse could increase the traffic capacity of mobile phones substantially, however, the technology to do it was nonexistent.

Anything to do with broadcasting and sending a radio or television message out over the airwaves comes under a Federal Communications Commission (FCC) regulation that a cellular phone is actually a type of two-way radio. In 1947, AT&T proposed that the FCC allocate a large number of radio spectrum frequencies so that wide-spread mobile phone service could become feasible and AT&T would have a incentive to research the new technology. We can partially blame the FCC for the gap between the concept of cellular phone service and it's availability to the public. Because of the FCC decision to limit the cellular phone frequencies in 1947, only twenty three cellular phone conversations could occur simultaneously in the same service area - not a market incentive for research.

The FCC reconsidered it's position in 1968, and stated "if the technology to build a better mobile phone service works, we will increase the cellular phone frequencies allocation, freeing the airwaves for more mobile phones." AT&T - Bell Labs proposed a cellular phone system to the FCC of many small, low-powered broadcast towers, each covering a 'cell' a few miles in radius, collectively covering a larger area. Each tower would use only a few of the total frequencies allocated to the cellular phone system, and as cars moved across the area their cellular phone calls would be passed from tower to tower.

Individual Inventors & Mobile Phone Patents

Dr. Martin Cooper for Motorola. US03906166 09/16/1975 Radio telephone system Martin Cooper, Richard W. Dronsuth, ; Albert J. Mikulski, Charles N. Lynk Jr., James J. Mikulski, John F. Mitchell, Roy A. Richardson, John H. Sangster

Related Class Patents Related to cellular phone patents and cordless phones, 64 patents listed.

By 1977, AT&T Bell Labs constructed and operated a prototype cellular phone system. A year later, public trials of the new cellular phone system were started in Chicago, IL with over 2000 trial cellular phone customers. In 1979, the first commercial cellular phone system began operation in Tokyo. In 1981, Motorola and American Radio phone started a second U.S. cellular radio-phone system test in the Washington/Baltimore area. By 1982, the slow moving FCC finally authorized commercial cellular phone service for the USA. A year later, the first American commercial for analog cellular phone service or AMPS (Advanced Mobile Phone Service) was offered in Chicago, IL by Ameritech. Despite the incredible demand, it took cellular phone service 37 years to become commercially available in the United States.

Consumer demand quickly outstripped the cellular phone system's 1982 standards, by 1987, cellular phone subscribers exceeded one million, and the airways were crowded. Three ways of improving services existed:

• one - increase cellular phone frequencies allocation
• two - split existing cellular phone cells
• three - improve the cellular phone technology
  

The FCC did not want to handout any more bandwidth and building/splitting cells would have been expensive and add bulk to the cellular phone network. To stimulate the growth of new cellular phone technology, the FCC declared in 1987 that cellular phone licensees may employ alternative cellular phone technologies in the 800 MHz band. The cellular phone industry began to research new transmission technology as an alternative.

For more information on Cellular Phone Technology, read Cellular Telephony by Brian Oblivion.

How Mobile Telephone Calls Are Handled

Telephone customer (1) dials 'Long Distance' and asks to be connected with the mobile services operator, to whom he gives the telephone number of the vehicle he wants to call. The operator sends out a signal from the radio control terminal (2) which causes a lamp to light and a bell to ring in the mobile unit (3). Occupant answers his telephone, his voice traveling by radio to the nearest receiver (4) and thence by telephone wire.

To place a call from a vehicle, the occupant merely lifts his telephone and presses a 'talk' button. This sends out a radio signal which is picked up by the nearest receiver and transmitted to the operator.[BLR1]

(The above accompanies a Bell Laboratories Record illustration, from the 1946 article first describing the system. It's a 346k download.)

The 20 watt mobile sets did not transmit back to the central tower but to one of five receivers placed across the city.[BLR2] Once a mobile went off hook all five receivers opened. The Mobile Telephone Service or MTS system combined signals from one or more receivers into a unified signal, amplifying it and sending it on to the toll switchboard. This allowed roaming from one city neighborhood to another. Can't visualize how this worked? Imagine someone walking through a house with several telephones off hook. A party on the other end of the line would hear the person moving from one room to another, as each telephone gathered a part of the sound.

One party talked at a time with MTS. You pushed a handset button to talk, then released the button to listen. (This eliminated echo problems which took years to solve before natural, full duplex communications were possible.) Mobile telephone service was not simplex operation as many writers describe, but half duplex operation. Simplex uses only one frequency to both transmit and receive. In MTS the base station frequency and mobile frequency were offset by five kHz. Privacy is one reason to do this; eavesdroppers could hear only one side of a conversation. Like a citizen's band radio, a caller searched manually for an unused frequency before placing a call. But since there were so few channels this wasn't much of a problem. This does point out radio-telephones' greatest problem of the time: too few channels.

This system presaged many cellular developments, indeed, Bell Laboratories' D.H. Ring articulated the cellular concept one year later in an unpublished paper. Young states all the elements were known then: a network of small geographical areas called cells, a low powered transmitter in each, the cell traffic controlled by a central switch, frequencies reused by different cells and so on. Young states that from 1947 Bell teams "had faith that the means for administering and connecting to many small cells would evolve by the time they were needed." [Young] While recognizing the Laboratories' prescience, more mobile telephones were always needed. In every city where mobile telephone service was introduced waiting lists developed, growing every year. By 1976 only 545 customers in New York City had Bell System mobiles, with 3,700 customers on the waiting list. Around the country 44,000 Bell subscribers had AT&T mobiles but 20,000 people sat on five to ten year waiting lists. [Gibson] Despite this incredible demand it took cellular 37 years to go commercial from the mobile phone's introduction. But the FCC's regulatory foot dragging slowed cellular as well. Until the 1980s they never made enough channels available; as late as 1978 the Bell System, the Independents, and the non-wireline carriers divided just 54 channels nationwide. [O'Brien] That compares to the 666 channels the first AMPS systems needed to work.

In mobile telephony a channel is a pair of frequencies. One frequency to transmit on and one to receive. It makes up a circuit or a complete communication path. Sounds simple enough to accommodate. Yet the radio spectrum is extremely crowded. In the late 1940s little space existed at the lower frequencies most equipment used. Inefficient radios contributed to the crowding, using 60 kHz to send an signal that can now be done with 10kHz or less. But what could you do with just six channels, no matter what the technology? Users by the scores vied for an open frequency. You had, in effect, a wireless party line, with perhaps forty subscribers fighting to place calls on each channel. Most mobile telephone systems couldn't accommodate more than 250 people. There were other problems.

Radio waves at lower frequencies travel great distances, sometimes hundreds of miles when they skip across the atmosphere. High powered transmitters gave mobiles a wide operating range but added to the dilemma. Telephone companies couldn't reuse their precious channels in nearby cities, lest they interfere with their own systems. They needed at least seventy five miles between systems before they could use them again. While better frequency reuse techniques might have helped, something doubtful with the technology of the times, the FCC held the key to opening more channels for wireless.

In 1947 AT&T began operating a "highway service", a radio-telephone offering that provided service between New York and Boston. It operated in the 35 to 44MHz band and caused interference from to time with other distant services. Even AT&T thought the system unsuccessful.

In that same year the Bell System asked the FCC for more frequencies. The FCC allocated a few more channels in 1949, but gave half to other companies wanting to sell mobile telephone service.

Berresford says "these radio common carriers or RCCs, were the first FCC-created competition for the Bell System" He elaborates on the radio common carriers, a group of market driven businessmen who pushed mobile telephony in the early years further and faster than the Bell System:

The telephone companies and the RCCs evolved differently in the early mobile telephone business. The telephone companies were primarily interested in providing ordinary, 'basic' telephone service to the masses and, therefore, gave scant attention to mobile services throughout the 1950s and 1960s. The RCCs were generally small entrepreneurs that were involved in several related businesses-- telephone answering services, private radio systems for taxicab and delivery companies, maritime and air-to-ground services, and 'beeper' paging services. As a class, the RCCs were more sales-oriented than the telephone companies and won many more customers; a few became rich in the paging business. The RCCs were also highly independent of each other; aside from sales, their specialty was litigation, often tying telephone companies (and each other) up in regulatory proceedings for years.

As proof of their competitiveness, the RCCs serviced 80,000 mobile units by 1978, twice as many as Bell. This growth built on a strong start, the introduction of automatic dialing in 1948. On March 1, 1948 the first fully automatic radiotelephone service began operating in Richmond, Indiana, eliminating the operator to place most calls. [McDonald] The Richmond Radiotelephone Company bested the Bell System by 16 years. AT&T didn't provide automated dialing for most mobiles until 1964, lagging behind automatic switching for wireless as they had done with landline telephony. (As an aside, the Bell System did not retire their last cord switchboard until 1978.) Most systems, though, RCCs included, still operated manually until the 1960s. Interestingly, some claim the Swedish Telecommunications Administration's S. Lauhrén designed the world's first automatic mobile telephone system, with a Stockholm trial starting in 1951.

I've found no literature to support a claim they were the first, before the 1948 Richmond Telephone Company service. For completeness, I should mention the following.

Anders Lindeberg of the Swedish Museum of Science and Technology does point out the link I provide in the preceding paragraph is "a summary from an article in the yearbook "Daedalus" (1991) for the Swedish Museum of Science and Technology http://www.tekmu.se/

The Swedish original article is much more extensive than the summary." He adds that "The Mobile Phone Book" by John Meurling and Richard Jeans, ISBN 0-9524031-02 published by Communications Week International, London in 1994 does briefly describe the "MTL" from 1951.

Speaking of Sweden, let's go to Europe to read about a typical radio-telephone unit, something similar to American installations:

It was in the mid-1950's that the first phone-equipped cars took to the road. This was in Stockholm - home of Ericsson's corporate headquarters - and the first users were a doctor-on-call and a bank-on-wheels. The apparatus consisted of receiver, transmitter and logic unit mounted in the boot of the car, with the dial and handset fixed to a board hanging over the back of the front seat. It was like driving around with a complete telephone station in the car. With all the functions of an ordinary telephone, the telephone was powered by the car battery. Rumor has it that the equipment devoured so much power that you were only able to make two calls - the second one to ask the garage to send a breakdown truck to tow away you, your car and your flat battery. . . These first car phones were just too heavy and cumbersome - and too expensive to use - for more than a handful of subscribers. It was not until the mid-1960's that new equipment using transistors were brought onto the market. Weighing a lot less and drawing not nearly so much power, mobile phones now left plenty of room in the boot - but you still needed a car to be able to move them around.

In 1956 the Bell System began providing manual radio-telephone service at 450 MHz, a new frequency band assigned to overcrowding. AT&T did not automate this service until 1969. In 1958 the innovative Richmond Radiotelephone Company improved their automatic dialing system. They added new features to it, including direct mobile to mobile communications.

Other independent telephone companies and the Radio Common Carriers made similar advances to mobile-telephony throughout the 1950s and 1960s. If this subject interests you, The Independent Radio Engineer Transactions on Vehicle Communications, later renamed the IEEE Transactions on Vehicle Communications, is the publication to read during those years.

In that same year the Bell System petitioned the FCC to grant 75 MHz worth of spectrum to radio-telephones in the 800 MHz band. The FCC had not yet allowed any channels below 500MHz, where there was not enough continuous spectrum to develop an efficient radio system. Despite the Bell System's forward thinking, the FCC sat on this proposal for ten years and only considered it in 1968 when requests for more frequencies became so backlogged that they could not ignore them.

In 1964 the Bell System introduced Improved Mobile Telephone Service or IMTS, a replacement to the badly aging Mobile Telephone System. It worked in full-duplex so people didn't have to press a button to talk. Talk went back and forth just like a regular telephone. It finally permitted direct dialing, automatic channel selection and reduced bandwidth to 25-30 kHz.

Before leaving conventional radio telephony I should mention fraud. As telephone folks were well acquainted with landline toll fraud, begun in earnest in the late 1960s, so they were aware of wireless fraud. Here's a summary from a 1985 article in Personal Communications Technology Magazine: "The earliest form of mobile telephony, unsquelched manual Mobile Telephone Service (MTS), was vulnerable to interception and eavesdropping. To place a call, the user listened for a free channel. When he found one, he would key his microphone to for service: 'Operator, this is Mobile 1234; may I please have 555-7890.' The operator knew to submit a billing ticket for account number 1234 to pay for the call. So did anybody else listening to the channel--hence the potential for spoofing and fraud.

Squelched channel MTS hid the problem only slightly because users ordinarily didn't overhear channels being used by other parties. Fraud was still easy for those who turned off the squelch long enough to overhear account numbers.

Direct-dial mobile telephone services such as Improved Mobile Telephone Service (IMTS) obscured the problem a bit more because subscriber identification was made automatically rather than by spoken exchange between caller and operator. Each time a user originated a call, the mobile telephone transmitted its identification number to the serving base station using some form of Audio Frequency Shift Keying (AFSK), which was not so easy for eavesdroppers to understand.

Committing fraud under IMTS required modification of the mobile--restrapping of jumpers in the radio unit, or operating magic keyboard combinations in later units--to reprogram the unit to transmit an unauthorized identification number. Some mobile control heads even had convenient thumb wheel switches installed on them to facilitate easy and frequent ANI (Automatic Number Identification) changes."

Mobile Phone History

Digital wireless and cellular roots go back to the1940s when commercial mobile telephony began. Compared to today's furious pace of development, it may seem odd that wireless didn't come along sooner. There are many reasons for that. Technology, disinterest, and to some extent regulation limited early United States radio-telephone development. As the vacuum tube and the transistor made possible the early telephone network, the wireless revolution began only after low cost microprocessors and digital switching became available. And while the Bell System built the finest landline telephone system in the world, they never seemed truly committed to mobile telephony. Their wireless engineers were brilliant and keen but the System itself held them back. Federal regulations also hindered many projects but in Europe, where state run telephone companies controlled their own telecom development, although, admittedly, without competition, wireless came no sooner, and in most cases, later. Starting in 1921 in the United States mobile radios began operating at 2 MHz, just above the present A.M. radio broadcast band. [Young] These were chiefly experimental police department radios, with practical systems not implemented until the 1940s. [FCC] Police and emergency services drove mobile radio pioneering, with little thought given to private telephone use.

In 1934 the United States Congress created the Federal Communications Commission. In addition to regulating landline interstate telephone business, they also began managing the radio spectrum. It decided who would get what frequencies. It gave priority to emergency services, government agencies, utility companies, and services it thought helped the most people. Radio users like a taxi service or a tow truck dispatch company required little spectrum to conduct their business. Radio telephone used large frequency allocations to serve a few people. The FCC designated no radio-telephone channels until after World War II.

On June 17, 1946 in Saint Louis, Missouri, AT&T and Southwestern Bell introduced the first American commercial mobile radio-telephone service. Mobiles used newly issued vehicle radio-telephone licenses granted to Southwestern Bell by the FCC. They operated on six channels in the 150 MHz band with a 60 kHz channel spacing. [Peterson] Bad cross channel interference, something like cross talk in a landline phone, soon forced Bell to use only three channels. In a rare exception to Bell System practice, subscribers could buy their own radio sets and not AT&T's equipment. Installed high above Southwestern Bell's headquarters at 1010 Pine Street, a centrally located antenna transmitting 250 watts paged mobiles and provided radio-telephone traffic on the downlink. Operation was straightforward, as the following describes:

Mobile phone networks

The most common example of a cellular network is a mobile phone (cell phone) network. A mobile phone is a portable telephone which receives or makes calls through a cell site (base station), or transmitting tower. Radio waves are used to transfer signals to and from the cell phone. Large geographic areas (representing the coverage range of a service provider) may be split into smaller cells to avoid line-of-sight signal loss and the large number of active phones in an area. In cities, each cell site has a range of up to approximately ½ mile, while in rural areas, the range is approximately 5 miles. Many times in clear open areas, a user may receive signals from a cellsite 25 miles away. All of the cell sites are connected to cellular telephone exchanges "switches", which connect to a public telephone network or to another switch of the cellular company.

As the phone user moves from one cell area to another cell, the switch automatically commands the handset and a cell site with a stronger signal (reported by each handset) to switch to a new radio channel (frequency). When the handset responds through the new cell site, the exchange switches the connection to the new cell site.

With CDMA, multiple CDMA handsets share a specific radio channel. The signals are separated by using a pseudonoise code (PN code) specific to each phone. As the user moves from one cell to another, the handset sets up radio links with multiple cell sites (or sectors of the same site) simultaneously. This is known as "soft handoff" because, unlike with traditional cellular technology, there is no one defined point where the phone switches to the new cell.

Modern mobile phone networks use cells because radio frequencies are a limited, shared resource. Cell-sites and handsets change frequency under computer control and use low power transmitters so that a limited number of radio frequencies can be simultaneously used by many callers with less interference.

Since almost all mobile phones use cellular technology, including GSM, CDMA, and AMPS (analog), the term "cell phone" is used interchangeably with "mobile phone". However, satellite phones are mobile phones that do not communicate directly with a ground-based cellular tower, but may do so indirectly by way of a satellite.

Old systems predating the cellular principle may still be in use in places. The most notable real hold-out is used by many amateur radio operators who maintain phone patches in their clubs' VHF repeaters.

There are a number of different digital cellular technologies, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), Code Division Multiple Access (CDMA), Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), 3GSM, Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN).

Coverage comparison

Following table shows the dependency of frequency on coverage area of one cell of a CDMA2000 network:^[3]

Frequency (MHz)	Cell radius (km)	Cell area (km2)	Relative Cell Count
450	48.9	7521	1
950	26.9	2269	3.3
1800	14.0	618	12.2
2100	12.0	449	16.2

Frequency choice

The effect of frequency on cell coverage means that different frequencies serve better for different uses. Low frequencies, such as 450 MHz NMT, serve very well for countryside coverage. GSM 900 (900 MHz) is a suitable solution for light urban coverage. GSM 1800 (1.8 GHz) starts to be limited by structural walls. This is a disadvantage when it comes to coverage, but it is a decided advantage when it comes to capacity. Pico cells, covering e.g. one floor of a building, become possible, and the same frequency can be used for cells which are practically neighbours. UMTS, at 2.1 GHz is quite similar in coverage to GSM 1800. At 5 GHz, 802.11a Wireless LANs already have very limited ability to penetrate walls and may be limited to a single room in some buildings. At the same time, 5 GHz can easily penetrate windows and goes through thin walls so corporate WLAN systems often give coverage to areas well beyond that which is intended.

Moving beyond these ranges, network capacity generally increases (more bandwidth is available) but the coverage becomes limited to line of sight. Infra-red links have been considered for cellular network usage, but as of 2004^[update] they remain restricted to limited point-to-point applications.

Cell service area may also vary due to interference from transmitting systems, both within and around that cell. This is true especially in CDMA based systems. The receiver requires a certain signal-to-noise ratio. As the receiver moves away from the transmitter, the power transmitted is reduced. As the interference (noise) rises above the received power from the transmitter, and the power of the transmitter cannot be increased any more, the signal becomes corrupted and eventually unusable. In CDMA-based systems, the effect of interference from other mobile transmitters in the same cell on coverage area is very marked and has a special name, cell breathing.

Old fashioned taxi radio systems generally use low frequencies and high sited transmitters, probably based where the local radio station has its mast. This gives a very wide area coverage in a roughly circular area surrounding each mast. Since only one user can talk at any given time, coverage area doesn't change with number of users. The reduced signal to noise ratio at the edge of the cell is heard by the user as crackling and hissing on the radio.

One can see examples of cell coverage by studying some of the coverage maps provided by real operators on their web sites. In certain cases they may mark the site of the transmitter, in others it can be calculated by working out the point of strongest coverage.

Frequency reuse

The increased capacity in a cellular network, compared with a network with a single transmitter, comes from the fact that the same radio frequency can be reused in a different area for a completely different transmission. If there is a single plain transmitter, only one transmission can be used on any given frequency. Unfortunately, there is inevitably some level of interference from the signal from the other cells which use the same frequency. This means that, in a standard FDMA system, there must be at least a one cell gap between cells which reuse the same frequency.

The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation).

In case of N sector antennas on the same base station site, each with different direction, the base station site can serve N different sectors. N is typically 3. A reuse pattern of N/K denotes a further division in frequency among N sector antennas per site. Some current and historical reuse patterns are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 (GSM).

If the total available bandwidth is B, each cell can only utilize a number of frequency channels corresponding to a bandwidth of B/K, and each sector can use a bandwidth of B/NK.

Code division multiple access-based systems use a wider frequency band to achieve the same rate of transmission as FDMA, but this is compensated for by the ability to use a frequency reuse factor of 1, for example using a reuse pattern of 1/1. In other words, adjacent base station sites use the same frequencies, and the different base stations and users are separated by codes rather than frequencies. While N is shown as 1 in this example, that does not mean the CDMA cell has only one sector, but rather that the entire cell bandwidth is also available to each sector individually.

Depending on the size of the city, a taxi system may not have any frequency-reuse in its own city, but certainly in other nearby cities, the same frequency can be used. In a big city, on the other hand, frequency-reuse could certainly be in use.

Cellular telephone frequency reuse pattern. See U.S. Patent 4,144,411

Although the original 2-way-radio cell towers were at the centers of the cells and were omni-directional, a cellular map can be redrawn with the cellular telephone towers located at the corners of the hexagons where three cells converge.^[1] Each tower has three sets of directional antennas aimed in three different directions and receiving/transmitting into three different cells at different frequencies. This provides a minimum of three channels for each cell. The numbers in the illustration are channel numbers, which repeat every 3 cells. Large cells can be subdivided into smaller cells for high volume areas.^[2]

[edit] Movement from cell to cell and handover

In a primitive taxi system, when the taxi moved away from a first tower and closer to a second tower, the taxi driver manually switched from one frequency to another as needed. If a communication was interrupted due to a loss of a signal, the taxi driver asked the base station operator to repeat the message on a different frequency.

In a cellular system, as the distributed mobile transceivers move from cell to cell during an ongoing continuous communication, switching from one cell frequency to a different cell frequency is done electronically without interruption and without a base station operator or manual switching. This is called the handover or handoff. Typically, a new channel is reserved for the mobile unit on the new base station which will serve it. The mobile unit then automatically switches from the current channel to the new channel and communication continues.

The exact details of the mobile system's move from one base station to the other varies considerably from system to system. For example, in all GSM handovers and W-CDMA inter-frequency handovers the mobile station will test the reserved channel before changing channels. Once the channel is confirmed okay, the network will command the mobile unit to switch to the new channel and at the same time start bi-directional communication on the new channel. In CDMA2000 and W-CDMA same-frequency handovers, both channels will actually be in use at the same time (this is called a soft handover or soft handoff). In IS-95 inter-frequency handovers and older analog systems such as NMT it will typically be impossible to test the target channel directly while communicating. In this case other techniques have to be used such as pilot beacons in IS-95. This means that there is almost always a brief break in the communication while searching for the new channel followed by the risk of an unexpected return to the old channel.

If there is no ongoing communication or the communication can be interrupted, it is possible for the mobile unit to spontaneously move from one cell to another and then notify the base station with the strongest signal.