All About CD Ma

8/6/2019 All About CD Ma

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Summary

Code division multiple access (CDMA) is a wireless communications technology that uses the principle of

spread spectrum communication. The intent of CDMA technology is to provide increased bandwidth in a

limited frequency system, but has also other advantages including extended range and more secure

communications. In a CDMA system, a narrow band message signal is multiplied by a spreading signal,

which is a pseudo-noise code sequence that has a rate much greater than the data rate of the message. CDMA

uses these code sequences as a means of distinguishing between individual conversations. All users in the

CDMA system use the same carrier frequency and may transmit simultaneously.

CDMA is a driving technology behind the rapidly advancing personal communications industry. Because of

its greater bandwidth, efficiency, and multiple access capabilities, CDMA is becoming a leading technology

for relieving the spectrum congestion caused by the explosion in popularity of cellular mobile phones, fixed

wireless telephones, and wireless data terminals. Since becoming an officially recognized digital cellular

protocol, CDMA is being rapidly implemented in the wireless communications networks of many large

communications corporations.

Introduction

CDMA stands for "Code Division Multiple Access." It is a form of spread-spectrum, an advanced digital

wireless transmission technique. Instead of using frequencies or time slots, as do traditional technologies, it

uses mathematical codes to transmit and distinguish between multiple wireless conversations. Its bandwidth is

much wider than that required for simple point-to-point communications at the same data rate because it uses

noise-like carrier waves to spread the information contained in a signal of interest over a much greater

bandwidth. However, because the conversations taking place are distinguished by digital codes, many users

can share the same bandwidth simultaneously. The advanced methods used in commercial CDMA technology

improve capacity, coverage and voice quality, leading to a new generation of wireless networks.

Old-fashioned radio receivers separate stations and channels by filtering in the frequency domain. CDMA

receivers, conversely, separate communication channels by a pseudo-random modulation that is applied andremoved in the digital domain. Multiple users can therefore occupy the same frequency band. This universal

frequency reuse is crucial to CDMA's distinguishing high spectral efficiency. CDMA has gained international

acceptance by cellular radio system operators as an upgrade because of its universal frequency reuse and

noise-like characteristics. CDMA systems provide operators and subscribers with significant advantages over

analog and conventional TDMA-based systems. The main advantages of CDMA are as follows:

1. Increased capacity

2. Improved voice quality, eliminating the audible effects of multipath fading

3. Enhanced privacy and security

4. Improved coverage characteristics, which reduce the number of cell sites

5. Simplified system planning reduces deployment and operating costs

6. Reduced average transmitted power, thus increasing talk time for portable devices7. Reduced interference to other electronic devices

8. Reduction in the number of calls dropped due to handoff failures

9. Development of a reliable transport mechanism for wireless data communications

10. Coexistence with previous technologies, due to cdma and analog operating in two spectras with no

interference


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Background

To understand why there is a demand for CDMA, it is necessary to understand the technology that existed

prior to its introduction and to know the background behind previous spread-spectrum systems. Spread

spectrum communications have been used for encrypting military communication for many years. Its

strengths in the military arena lie in its ability to resist enemy jamming and to provide secure communications.

It is difficult to interfere with or intercept a CDMA signal because of its use of a spread signal. The great

attraction of CDMA technology from the beginning was its inherent ability to boost communications capacity

and reuse frequencies to a degree unheard of in narrowband multiple access wireless technology. Its civilian

mobile radio application was proposed theoretically in the late 1940's, but its practical application in the

market did not take place until 40 years later due to the many technical obstacles that still needed to be

overcome. The viability of CDMA technology was dismissed by TDMA (Time Division Multiple Access)

supporters as a technology that worked fine in theory but would never work in practice. The rapid

development of high density digital ICs, however, combined with the realization that regulating all transmitter

powers to the lowest level required for a link would achieve optimal multiple access communication, allowed

CDMA to materialize as a working technology. In 1991, the promising results of the first field trials

demonstrated that CDMA could work as well in practice as it did in theory. Commercial CDMA was

introduced, tested, standardized, and initially deployed in less then seven years, a relatively rapid maturation

cycle compared to other technologies such as TDMA. The first commercial CDMA service was launched inHong Kong in 1995, followed by a launch in Korea and Pennsylvania. It has rapidly become the primary

choice of carriers in the U.S. Now 11 of the top 14 cellular carriers, 10 of the top 17 PCS carriers, the 2 largest

PCS C block bidders and 60% of POPs have selected CDMA for their new digital network.

FDMA, TDMA and CDMA

FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access) and CDMA (Code

Division Multiple Access) are the three basic multiple access schemes.

FDMA divides radio channels into a range of radio frequencies and is used in the traditional analog cellular

system. With FDMA, only one subscriber is assigned to a channel at a time. Other conversations can access

this channel only after the subscriber's call has terminated or after the original call is handed off to a differentchannel by the system. FDMA cellular standards include AMPS (Advanced Mobile Phone Service) and TACS

(Total Access Communications System). TDMA is a common multiple access technique employed in digital

cellular systems. It divides conventional radio channels into time slots to obtain higher capacity. Its standards

include North American Digital Cellular, Global System for GSM (Mobile Communications), and PDC

(Personal Digital Cellular). As with FDMA, no other conversations can access an occupied TDMA channel

until the channel is vacated. CDMA uses a radically deferent approach. It assigns each subscriber a unique

"code" to put multiple users on the same wideband channel at the same time. The codes, called "pseudo-

random code sequences", are used by both the mobile station and the base station to distinguish between

conversations. The IS-95 CDMA standard was adopted by the TIA (Telecommunications Industry

Association) and became a digital cellular standard in 1992. The J-STD-008 standard for personal

communications services was also accepted by ANSI. CDMA is the first digital technology, which meets the

exacting standards of the CTIA (Cellular Telecommunications Industry Association). Depending on the levelof mobility of the system, it provides 10 to 20 times the capacity of AMPS, and 4 to 7 times the capacity

ofTDMA.. CDMA is the only one of the three technologies that can efficiently utilize spectrum allocation

and offer service to many subscribers without requiring extensive frequency planning. All CDMA users can

share the same frequency channel because their conversations are distinguished only by digital code, while

TDMA operators have to coordinate the allocation of channels in each cell in order to avoid interfering with

adjacent channels. The average transmitted power required by CDMA is much lower than what is required by

analog, FDMA and TDMA technologies.


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Current Cellular Standards

The cellular solution, originally designed by Bell Telephone laboratories in the 1970's makes use of multiple

fixed stations. Each station, located in what is termed a "cell" services subscriber stations within a limited

geographical area. Cellular companies are

each granted 25 Mhz of the spectral division in the 800-900 MHz region, each split between the two directions

of communications. Typical analog systems such as AMPS employ FDMA schemes that divide the spectral

allocations into uniform frequency channels in the range of 25-30 kHz wide. Applying simple algebra shows

the approximate number of channels to be around 416. This number, although appearing somewhat large, is

rather small with respect to data communications.

Different types of cellular systems employ various methods of "multiple access" methods, meaning that

multiple, simultaneous users can be supported. These users share a common pool of radio channels and can

gain access to any channel. Just as each

telephone call is granted a specific line for discourse, each subscriber is assigned a unique channel to

propagate data transmission. Only one subscriber at a time is assigned to each channel; no other conversations

can access it until the call is completed. These channels are a limited resource of cellular companies, as are the

number of phone lines are for Ma Bell. Solutions to achieve greater capacity are central to cellular principles.

Spectral allocations are limited for each cell, due in part to regulatory agencies limiting the bandwidth in order

for communication companies to create highly efficient solutions. This spectral efficiency is measured in

Erlangs per unit service area, per MHz. Quite simply, this dimensionless unit of telephone traffic intensity,

known as the Erlang blocking probability (typically 0.05), is equal to calling rate multiplied by the average

call length. This shows the capacity for a channel to be completely occupied for some given time frame, with

higher values representing higher channel usage. Due to the explosive growth of the cellular industry

exceeding initial predictions of analysts, subscribers in many urban cities often experience "blocking" with the

trend increasing as the number of wireless LAN's and personal cellular radios continue to grow. Anyone

who has tried to make a call and has been prevented or "blocked" will understand this concept. One in six Los

Angeles subscribers experience blocking during peak hours. Many subscribers also experience "dropped calls"

when leaving one cell and moving into another when the new cell can't allocate a carrier channel to the

mobile. Consequently this leads to poor customer relations which forces the cellular providers to arrive atsolutions that achieve high spectral efficiency to increase cell capacity.

Central to the cellular concept is frequency reuse, which is critically dependent upon the fact that the carrier

wave power decays with increasing distance. With this information, and some physics (which we won't get

into), a cellular division of frequency channels can be implemented. It's the same rationale when traveling

long distances; your favorite radio show on a familiar frequency is not the same in each city. The channel is

allocated to another radio station far enough apart where signals won't interfere with each other. By reusing

channels in multiple cells, the system can grow without geographical limits.

Here each cell represents an allocation of channels where no adjacent cells share common frequencies, with atypical maximum subscriber load at about 350 users. This idealized depiction is a hypothetical representation


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of true cellular systems that is good for modeling, but unfortunately not substantial enough for real world

implementations of cellular technology.

Spreading the Spectrum

Spread spectrum multiple access transmits the entire signal over a bandwidth that is much greater than that

required for standard narrow band transmissions in order to gain signal-to-noise (S/N) performance. In

channels with narrowband noise, increasing the transmitted signal bandwidth results in an increased

probability that the received information will be correct. Because each signal is a compilation of many smaller

signals at the fundamental frequency and its harmonics, increasing the frequency results in a more accurate

reconstruction of the original signal. The effective drawback of narrowband data communications is the

limitation of bandwidth; thus signals must be transmitted with enough power so the corruption by gaussian

noise isn't as effective and the probability that the data received is correct will remain low. This means that the

effective SNR must be high enough so that the receiver can recover the transmitted code without error.

From a system viewpoint, the performance increase for very wideband systems is referred to as "process

gain". This term is used to describe the received signal fidelity gained at the cost of bandwidth. Errors

introduced by a noisy channel can bereduced to any desired level without sacrificing the rate of information transfer using Claude Shannon's

equation describing channel capacity:

C =Wlog2(1+S/N)

where C = Channel capacity in bits per second, W = Bandwidth, S/N = Energy per bit/Noise power.

The benefits of increasing bandwidth become more clear. The S/N ratio may be decreased without decreasing

the bit error rate. This means that the signal may be spread over a large bandwidth with smaller spectral power

levels and still achieve the required data rate. If the total signal power is interpreted as the area under the

spectral density curve, then signals with equivalent total power may have either a large signal power

concentrated in a small bandwidth or a small signal power spread over a large bandwidth.

A CDMA spread spectrum signal is created by modulating the radio frequency signal with a spreading

sequence (a code consisting of a series of binary pulses) known as a pseudo-noise (PN) digital signal because

they make the signal appear wide band and "noise like". The PN code runs at a higher rate than the RF signal

and determines the actual transmission bandwidth. Messages can also be cryptographically encoded to anylevel of secrecy desired with direct sequencing as the entire transmitted/received message is purely digital.


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An SS receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only

the desired coded information from all possible signals. A SS correlator can be thought of as a specially

matched filter -- it responds only to signals that are encoded with a pseudo noise code that matches its own

code. Thus an SS correlator (SS signal demodulator) can be "tuned" to different codes simply by changing its

local code. This correlator does not respond to man made, natural or artificial noise or interference. It

responds only to SS signals with identical matched signal characteristics and encoded with the identical

pseudo noise code.

Many spread spectrum radios can share the same frequency band, provided that each system uses a unique

spreading code to reduce interference between the different radios. Because only the receiver with the

identical code can despread the signal to recover the signal, SS radios can tolerate a high level of interference

unlike conventional radios, providing much greater capacity increase in frequency reuse. SSMA is not very

bandwidth efficient when used by a single user. However, since many users can share the same spread

spectrum bandwidth without interfering with one another, SS systems become bandwidth efficient in multiple

user environments. This reason makes SS communication an ideal choice for metropolitan areas with large

blocking rates. Frequency reuse is universal, that is, multiple users utilize each CDMA carrier frequency. The

reuse pattern is now...

The spread of energy over a wide band, or lower spectral power density, makes SS signals less likely to

interfere with narrow band communications, because the spreaded signal power is near that of gaussian noise

levels. Narrow band communications, conversely, cause little to no interference to SS systems because the

correlation receiver effectively integrates over a very wide bandwidth to recover an SS signal. The correlator

then "spreads" out a narrow band interferer over the receiver's total detection bandwidth.

Spread Spectrum: down to the bits

CDMA technology focuses primarily on the "direct sequence" method of spread spectrum. Direct sequence is

spread spectrum technique in which the bandwidth of a signal is increased by artificially increasing the bit

data rate. This is done by breaking each bit into a number of sub-bits called "chips". Assuming this number is

10, each bit of the original signal would be divided up into 10 separate bits, or "chips." This results in an

increase in the data rate by 10. By increasing the data rate by 10, we also increase the bandwidth by 10.

The signal is divided up into smaller bits by multiplying it by a Pseudo-Noise code, PN-code. A PN-code is a

sequence of high data rate bits ("chips") ranging from -1 to 1 (polar) or 0 to 1 (non-polar). When referring to

the number of "chips" used, we mean

the number of small data bits in the PN-code per single bit of the original signal. Simply by multiplying theoriginal modulated signal by this high data rate PN-code will result in dividing the signal into smaller bits, and

hence, increase its bandwidth. This process is


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shown in the figure below.

The greater number of "chips" used results in a wider bandwidth proportional to the number of "chips".

The basic operation of the transmitter and receiver for spread spectrum will now be described briefly. Let'sassume there are two transmitters (refer to the figure below) with two different messages to be transmitted.

We should keep in mind that each transmitter can be thought of as separate cell phones. The messages M1(t)

and M2(t) first go through a modulator to modulate the message at a higher carrier frequency. For spread

spectrum, all messages are modulated on the same carrier frequency. The output for each of the modulators is

S1(t) and S2(t). After the modulator, each signal is multiplied by its own unique Pseudo-Noise code, C1(t) and

C2(t). These are the high data rate bit patterns which spreads the signal's bandwidth. For this example, we will

assume the range values for the PN-code is -1 and 1. After spreading the bandwidth, each signal is

transmitted. Because many signals can be transmitted from different transmitters at the same time, we

represent these transmissions by simply summing their spectrums.


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At the receiver end, the incoming signal is the spread spectrum signal. In order for a receiver to extract a

single message, it must multiply the incoming signal by the correct PN-code. Because we chose the PN-code

to range from -1 to 1, this technique of multiplying by the PN-code works perfectly. Since the original signal

at the transmitter end was multiplied by the PN-code, and again multiplied by the same PN-code at the


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receiver end, we effectively canceled out the PN-code for that particular message. The figure below helps

illustrate how the PN-code is eliminated.

By eliminating the PN-code, we eliminate the spread spectrum effects for that particular message signal. The

receiver circuit that does this is called a correlator, and it collapses the spread signal back down to just the

original narrow bandwidth centered at the

modulated carrier frequency. The resulting signal is then passed through a bandpass filter (BPF) centered at

the carrier frequency. This operation selects only the desired signal while rejecting all surrounding frequencies

due to other messages in the spread spectrum. This rejection is known as the processing gain of the

despreading correlation process. Lastly, the desired signal is demodulated to eliminate the carrier frequency.

Processing gain is a direct consequence of the direct sequence radio signal spreading and despreading process.

It refers to the increase in signal-to-noise ratio that results from this process, and is required for successful

data communications. Processing gain increases as the number of chips per data bit increases, and this can be

manipulated by the system designer to get the desired effect.

The Near-Far Problem

CDMA has not been previously implemented due to its "Near-Far Problem." Let's assume there are two users,

one near the base and one far from the base.

The propagation path loss difference between these extreme users may be many tens of dB. In general, the

strongest received mobile signal will capture the demodulator at the base station. In CDMA, stronger receivedsignal levels raise the noise floor at the

base station demodulators for the weaker signals, thereby decreasing the probability that weaker signals will

be received.


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To help eliminate the "Near-Far Problem", CDMA uses power control. The base station rapidly samples the

radio signal strength indicator levels of each mobile and then sends a power change command over the

forward radio link. This sampling is done 800 times per second and can be adjusted in 84 steps of 1 dB. The

purpose of this is so that the received powers from all users are roughly equal. This solves the problem of a

nearby subscriber overpowering the base station receiver and drowning out the signals of far away

subscribers. An extra benefit is extended battery life. That is, when a mobile unit is close to a base station, its

power output is lower. In other words, the mobile unit transmits only at the power necessary to maintain

connection.

Soft Handoff

Mentioned previously were methods of current cellular technology that uses the "hard handoff" method when

mobiles are changing cells. Because each mobile is on a limited channel within a specific cell, the transmitting

base station must try to allocate a new

channel to a new mobile. The problem arises when the mobile is active and also changing cells. At the very

least, the person will here some static or a glitch of some sort because the transmission had to be placed on a

new carrier wave. This is relatively acceptable, except in cases when there are no more channels available toany mobile. In this case the call is just dropped. Since the bandwidth in CDMA schemes is common to all

users, channel allocation is not required. As shown in the animation, the current cell of the mobile is

responsible for all transmissions. As the mobile nears the boundary of a neighboring cell, it receives

transmissions from both cells. The mobile will receive some message from one cell, and some from the other

until it has moved into one or the other cells. This is known as a "soft handoff" because the user never

experiences any glitch and certainly never a dropped call.

Commercial Applications


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CDMA's primary commercial application will definitely be with cellular wireless phones and pagers, but there

are also some other uses of the technology being applied commercially. Wireless local loop technology is a

way for a local cluster of telephones to be installed in areas that would normally require expensive or

disrupting physical wire installation. The basic idea is that a central CDMA hub is installed where the phone

lines come into an area, be it an office building, apartment complex, or even a subdivision The hub interfaces

with the standard phone lines coming into such an area, and then CDMA-compatible phones are installed in

each office or apartment - wherever a wired phone would normally have to be installed. The phones are not

designed to be battery-powered portables; they get their power from standard 110V sources, but because they

are not tied to a phone jack, they can be moved to different areas easily. This is especially important for

preexisting structures where adding more phone lines would be a problem.

Wireless local loop can also be applied in the other direction. If a location does not have wired telephone

service but has access to CDMA cellular coverage, a building's standard wired phones can be connected to a

wireless local loop hub that allows many phones to share a predetermined number of CDMA cellular

transceivers. The service is almost transparent in that the users' phone act as if they were connected to a

physical phone line except when all the transceivers in the wireless local loop hub are in use.

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