Custom Search

AMPLIFIERS

Signal amplification performs two important functions:
• Increases the resolution of the inputted signal
• Increases it's signal-to-noise ratio
Operational amplifiers over the years have proven to be the best choice
for signal amplifications because of the following characteristics offered
by Operational amplifiers over other kinds of amplifiers
• High open-loop gain
• High Bandwidth
• High input impedance
• Low/Non offset voltage
• Fast/High slew rate
• Low output impedance
• Low noise

Introduction To Filter

What is a filter?
A filter is a device that passes electric signals at certain frequencies or frequency ranges while
preventing the passage of others.
Applications:
• Filter circuits are used in a wide variety of applications.
• In the field of telecommunication, band-pass filters are used in the audio frequency
range (0 kHz to 20 kHz) for modems and speech processing.
• High-frequency band-pass filters (several hundred MHz) are used for channel
selection in telephone central offices.
• Data acquisition systems usually require anti-aliasing low-pass filters as well as lowpass
noise filters in their preceding signal conditioning stages.
• System power supplies often use band-rejection filters to suppress the 60-Hz line
frequency and high frequency transients.
• In addition, there are filters that do not filter any frequencies of a complex input
signal, but just add a linear phase shift to each frequency component, thus
contributing to a constant time delay. These are called all-pass filters.
• At high frequencies (> 1 MHz), all of these filters usually consist of passive
components such as inductors (L), resistors (R), and capacitors (C). They are then
called LRC filters.
• In the lower frequency range (1 Hz to 1 MHz), however, the inductor value becomes
very large and the inductor itself gets quite bulky, making economical production
difficult.
• In these cases, active filters become important. Active filters are circuits that use an
operational amplifier (op amp) as the active device in combination with some
resistors and capacitors to provide an LRC-like filter performance at low frequencies.


Application as Interface Circuit:
That connects the real world analog signal to the digital signal processor and provides band
limiting before the signal can be sampled for further processing with the digital technique and
reconstruction back to the analog world.
Why We Use Analog Filter?
• The practical active filter makes use of operated amplifier together with capacitor and
resistor is known as Analog filters.
• Many filtering applications are now handle with digital signal processing techniques
and digital filters.
• Analog continues time filter provides a more economical solution.
• For analog filter we must consider such factors as:
Factor considered while filter designing:
1. Technology derived for the system implementation.
2. Availability of DC supplies for the active devices and power
consumption.
3. Availability of voltage source for Op-Amp.
4. Cost.
5. Range of frequency of operation.
6. Sensitivity to parameter changes and stability.
7. Weight and size of the implemented circuit.
8. Noise and dynamic range of the realize filter.
These factors are to choose an analog or digital filter for a particular application.
Introduction to Filter Design:-
• The filter design is a special case of electric wave’s filter which makes the properties of
the Op-Amp.
• Such filter are sometimes called Active filter, currently called Analog Filter.
• In modern integrated circuits technology both analog and digital filter may be
implemented on the same chip.
Circuit Diagram:-
• If we know the circuit and the form of input then we may determine the output V2, this is
known as Circuit Analysis.
• If we know the input and the output or the ration that is |T| then we found the response in
the circuit is known as Circuit Designing.
• We have to design a circuit therefore we concern with designing in the filter Analysis.
• The circuit that we have design for the variation of the magnitude, phase or related
quantities they are known as Filter.
• In electrical engineering we filter the signal in terms of voltages at a specified frequency.
If a signal is made up of 2 tones, one at high frequencies called Piccolo and other is low
frequency called Tuba, imagine a filtering action by which one tone or the other is
suppressed.

Filter Terminologies

Brief definitions of terms related to filter design:
Convolution
Neighborhood operation in which each output pixel is a weighted sum of neighboring input
pixels. Image processing operations implemented with convolution include smoothing,
sharpening, and edge enhancement.
Correlation
Neighborhood operation in which each output pixel is a weighted sum of neighboring input
pixels. Correlation is closely related mathematically to convolution.
Bandwidth - few terms in signal processing have more definitions than this one. We'll define
bandwidth as the frequency width of the passband of a filter. For a low-pass filter, the bandwidth
is equal to the cutoff frequency. For a bandpass filter, the bandwidth is typically defined as the
frequency difference between the upper and lower -3 dB points.
Attenuation - an amplitude loss, usually measured in dB, incurred by a signal after passing
through a filter.
Cascaded filters - the implementation of a filtering system where multiple individual filters are
connected in series. That is, the output of one filter drives the input of the following filter.
Center Frequency (fo) - the frequency lying at the midpoint of a bandpass filter.
Cutoff Frequency - the upper passband frequency for low-pass filters, and the lower passband
frequency for highpass filters. A cutoff frequency is determined by the -3 dB point of a filter
magnitude response relative to a peak passband value.
Decibels (dB) - a logarithmic unit of attenuation, or gain, used to express the relative voltage or
power between two signals. For filters we use decibels to indicate cutoff frequencies (-3 dB) and
stopband signal levels (-20 dB)
Digital filter- computational process, or algorithm, transforming a discrete sequence of numbers
(the input) into another discrete sequence of numbers (the output) having a modified frequency
domain spectrum. Digital filtering can be in the form of a software routine operating on data
stored in computer memory or can be implemented with dedicated digital hardware.
Filter Coefficients - the set of constants, also called tap weights, used to multiply against
delayed signal sample values within a digital filter structure. Digital filter design is an exercise in
determining the filter coefficients that will yield the desired filter frequency response. For an FIR
filter, the filter coefficients are, by definition, the impulse response of the filter.
Filter Order - a number describing the highest exponent in the numerator or denominator of the
z-domain transfer function of a digital filter. For FIR filters, there is no denominator in the
transfer function and the filter order is merely the number of taps used in the filter structure. For
IIR filters, the filter order is equal to the number of delay elements in the filter structure.
Generally, the larger the filter order, the better the frequency magnitude response performance of
the filter.
Frequency Magnitude Response - a frequency domain description of how a filter interacts with
input signals.
Impulse Response - a digital filter's time domain output sequence when the input is a single
unity-valued sample (an impulse) preceded and followed by zero-valued samples. Using perhaps
the most powerful principle in signal processing, we can say that a linear digital filter's frequency
domain response can be calculated by taking the discrete Fourier transform of the filter's time
domain impulse response.
Linear Phase Filter- a filter that exhibits a constant change in output phase angle as a function
of frequency. The resultant filter phase plot vs frequency is a straight line. As such, a linear
phase filter's group delay is a constant. In order to preserve the integrity of their informationcarrying
signals, linear phase is an important criteria for filters used in communication systems.
Phase Response - the difference in phase, at a particular frequency, between an input sinewave
and the filter's output sinewave at that frequency. The phase response, sometimes called phase
delay, is usually depicted by a curve showing the filter's phase shift vs frequency.
Ripple - Ripple refers to fluctuations (measured in dB) in the passband, or stopband, of a filter's
frequency magnitude response curve.
Rolloff - a term used to describe the steepness, or slope, of the filter response in the transition
region from the passband to the stopband.
Transfer Function - a mathematical expression of the ratio of the output of a filter over the
input of the filter. Given the transfer function we can determine the filter's frequency magnitude
and phase responses.
Transition Region - the frequency range between the passband and the stopband of a filter.
Passband - that frequency range over which a filter passes signal energy.
Stopband - that band of frequencies attenuated by a digital filter

Significance of 3dB Frequency

• The 3dB frequency is the frequency at which the power is reduced by one half.
• Another way of saying it is the frequency at which the voltage is reduced to 70.7% of its full
value.
• An audio amplifier will have a lower and an upper 3dB frequency, so will a bandpass filter.
• A high‐pass filter will have only one 3dB frequency, that of the lowest frequency for which it is
much use; and conversely for a low‐pass filter.
• A power ratio of 2 is equal to 3 dB. ( take the log of 2 check above equation) Conversely, a
power ratio of 1/2 is equal to ‐3 dB. (NOTE: The ‐3 dB point is often referred to as a "half‐power
point," as when describing, say, a frequency response curve)
• A voltage or current ratio of 2 is equal to 6 dB. Conversely, a voltage or current ratio of 1/2 is
equal to ‐6 dB. Since power is E^2/ R.
• The 3dB point, or 3dB frequency, is the point at which the signal has been attenuated by 3dB (in
a bandpass filter). This is generally considered the point for determining the filter's bandwidth.
The bandwidth is defined as the difference between the upper and lower 3dB points.
• For example, if you have a filter which has a lower 3dB point of 720 MHz, and an upper 3dB
point of 730 MHz, you have a bandwidth of 10 MHz.

Wide area network

A wide area network (WAN) is a computer network that covers a broad area (i.e., any network whose communications links cross metropolitan, regional, or national boundaries ^[1]). This is in contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively.

WANs are used to connect LANs and other types of networks together, so that users and computers in one location can communicate with users and computers in other locations. Many WANs are built for one particular organization and are private. Others, built by Internet service providers, provide connections from an organization's LAN to the Internet. WANs are often built using leased lines. At each end of the leased line, a router connects to the LAN on one side and a hub within the WAN on the other. Leased lines can be very expensive. Instead of using leased lines, WANs can also be built using less costly circuit switching or packet switching methods. Network protocols including TCP/IP deliver transport and addressing functions. Protocols including Packet over SONET/SDH, MPLS, ATM and Frame relay are often used by service providers to deliver the links that are used in WANs. X.25 was an important early WAN protocol, and is often considered to be the "grandfather" of Frame Relay as many of the underlying protocols and functions of X.25 are still in use today (with upgrades) by Frame Relay.

Academic research into wide area networks can be broken down into three areas: Mathematical models, network emulation and network simulation.

Performance improvements are sometimes delivered via WAFS or WAN optimization.

Several options are available for WAN connectivity:

Local area network

A local area network (LAN) is a computer network covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport. The defining characteristics of LANs, in contrast to wide-area networks (WANs), include their usually higher data-transfer rates, smaller geographic area, and lack of a need for leased telecommunication lines.

ARCNET, Token Ring and many other technologies have been used in the past, and G.hn may be used in the future, but Ethernet over twisted pair cabling, and Wi-Fi are the two most common technologies currently in use.

As larger universities and research labs obtained more computers during the late 1960s, there was increasing pressure to provide high-speed interconnections. A report in 1970 from the Lawrence Radiation Laboratory detailing the growth of their "Octopus" network^[1][2] gives a good indication of the situation.

Cambridge Ring was developed at Cambridge University in 1974^[3] but was never developed into a successful commercial product.

Ethernet was developed at Xerox PARC in 1973–1975,^[4] and filed as U.S. Patent 4,063,220. In 1976, after the system was deployed at PARC, Metcalfe and Boggs published their seminal paper, "Ethernet: Distributed Packet-Switching For Local Computer Networks."^[5]

ARCNET was developed by Datapoint Corporation in 1976 and announced in 1977.^[6] It had the first commercial installation in December 1977 at Chase Manhattan Bank in New York.^[7]

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.