CDMA CODE DIVISION MULTIPLE ACCSES AND RADIO COM

8/9/2019 CDMA CODE DIVISION MULTIPLE ACCSES AND RADIO COM

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CDMACode Division Multiple Access

Muhannad alfegawi aljazaerli


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Multiple Access

For radio systems there are two resources, frequencyand time.

Division by frequency, so that each pair ofcommunicators is allocated part of the spectrum for all of

the time, results in Frequency Division Multiple Access(FDMA).

" Division by time, so that each pair of communicators isallocated all (or at least a large part) of the spectrum forpart of the time results in Time Division Multiple Access

(TDMA).

" In Code Division Multiple Access (CDMA), everycommunicator will be allocated the entire spectrum all ofthe time. CDMA uses codes to identify connections.


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ntroduction to Spread Spectrum

Communications

CDMA is a form ofDirect Sequence SpreadSpectrum communications. In general, SpreadSpectrum communications is distinguished bythree key elements:

1. The signal occupies a bandwidth much greaterthanthat which is necessary to send the information. Thisresults in many benefits, such as immunity tointerference and jamming and multi-useraccess,which well discuss later on.

2. The bandwidth is spread by means of a code which isindependent of the data. The independence of thecode distinguishes this from standard modulationschemes in which the data modulation will alwaysspread the spectrum somewhat.


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3. The receiversynchronizes to the code to recoverthe data. The use of an independent code andsynchronous reception allows multiple users toaccess the same frequency bandat the same

time.

In order to protect the signal, the code used ispseudo-random. It appears random, but isactually deterministic, so that the receiver can

reconstruct the code for synchronous detection.This pseudo-random code is also calledpseudo-noise (PN).


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CODING CDMA uses unique spreading codes to spread the

baseband data before transmission.

The signal is transmitted in a channel, which is belownoise level.

The receiver then uses a correlatorto despread the

wanted signal, which is passed through a narrowbandpass filter.

Unwanted signals will not be despread and will not passthrough the filter.

Codes take the form of a carefully designed one/zerosequence produced at a much higher rate than that ofthe baseband data.

The rate ofa spreading code is referred to as chip raterather than bit rate.


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Three Types of Spread Spectrum

CommunicationsThere are three ways to spread the bandwidth of the

signal:1. Frequency hopping. The signal is rapidly switched

between different frequencies within the hopping

bandwidth pseudo-randomly, and the receiverknowsbefore hand where to find the signal at any given time.

2. Time hopping. The signal is transmitted in shortbursts pseudo-randomly, and the receiverknowsbefore hand when to expect the burst.

3. Direct sequence. The digital data is directly coded ata much higher frequency. The code is generatedpseudo-randomly, the receiverknows how togenerate the same code, and correlates the receivedsignal with that code to extract the data.


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General Model of Spread

Spectrum System


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Direct Sequence Spread Spectrum

CDMA is a Direct Sequence Spread Spectrum system.

The CDMA system works directly on 64 kbit/sec digitalsignals. These signals can be digitized voice, ISDNchannels, modem data, etc.

Figure 1 shows a simplified Direct Sequence SpreadSpectrum system. For clarity, the figure shows onechannel operating in one direction only.

Signal transmission consists of the following steps:

1. A pseudo-random code is generated, different for each channeland each successive connection.

2. The Information data modulates the pseudo-random code (theInformation data is spread).

3. The resulting signal modulates a carrier.


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4. The modulated carrier is amplified and broadcast.

Signal reception consists of the followingsteps:1. The carrier is received and amplified.

2. The received signal is mixed with a local carrier torecover the spread digital signal.

3. A pseudo-random code is generated, matching theanticipated signal.

4. The receiver acquires the received code and phaselocks its own code to it.

5. The received signal is correlated with the generatedcode, extracting the Information data


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Direct Sequence Direct Sequence is the most famous Spread Spectrum

Technique. The data signal is multiplied by a PseudoRandom Noise Code (PN-code).

A PN-code is a sequence ofchips valued -1 and 1 (polar)or0 and 1 (non-polar). The number of chips within onecode is called the period of this code. A PN-code is a

noise-like code with certain properties Several classes ofbinary (2-phase) PN-codes exist: M-

sequences (base), Gold-codes and Kasami-codes.There exists also 4-phase codes, these aren't taken intoaccount yet. A PN-code can be created by means of one or

more shiftregisters. When the length of such ashiftregister is , in general the following can be said aboutthe period : N=2^n-1

In the most simple case a complete PN-code is multipliedwith a single databit. The bandwidth of the data signal is

now multiplied by a factor , this factoris said to be theprocessing gain


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CDMA in a DSSS Environment


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Direct Sequence Spread

Spectrum Example


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Spectrum Transmitter


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Spectrum Receiver


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SpectrumU

sing BPSK Example


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Seven Channel CDMA

Encoding and Decoding


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CODES

CDMA codes are not required to provide callsecurity, but create a uniqueness to enable callidentification.

Codes should not correlate to other codes or timeshifted version of itself.

Spreading codes are noise like pseudo-randomcodes, channel codes are designed formaximumseparation from each other and cell identificationcodes are balanced not to correlate to other codesof itself.


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WCDMA uses Direct Sequence spreading, wherespreading process is done by directly combiningthe baseband information to high chip ratebinary code.

The Spreading Factoris the ratio of the chips(UMTS = 3.84Mchips/s) to baseband informationrate. Spreading factors vary from 4 to 512in FDDUMTS.

Spreading process gain can in expressed indBs (Spreading factor128 = 21dB gain).


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WCDMA Spreading

TDD WCDMA uses spreading factors 4 - 512 tospread the base band data over~5MHz band.Spreading factor in dBs indicates the process gain.Spreading factor128 = 21 dB process gain).Interference margin is calculated from that:

Interference Margin = Process Gain - (Required

SNR + System Losses)

Required Signal to Noise Ration is typically about 5

dB System losses are defined as losses in receiver

path. System losses are typically 4 - 6 dBs


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Cont.

When using Frequency Hopping, the carrier

frequency is 'hopping' according to a known

sequence (of length). In this way the bandwidth is

also increased. There are two kinds of Frequency Hopping

Techniques.

Slow Frequency Hopping (SFH)

In this case one or more data bits are transmitted withinone Frequency Hop. An advantage is that coherent data

detection is possible. A disadvantage is that if one

frequency hop channel is jammed, one or more data bits

are lost. So we are forced to use error correcting codes.


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Cont.

Fast Frequency Hopping (FFH)

In this technique one data bit is divided over

more Frequency Hops. Now error correcting

codes are not needed. An otheradvantage isthat diversity can be applied. Every frequency

hop a decision is made whether a -1 or a 1 is

transmitted, at the end of each data bit a

majority decision is made. The appliedmodulation technique should be FSK or

MFSK.


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Frequency Hopping Spread

Spectrum System (Transmitter)


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Frequency Hopping Spread

Spectrum System (Receiver)


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Slow and Fast FHSS

Frequency shifted every Tc seconds

Duration of signal element is Ts seconds

Slow FHSS has Tc u Ts Fast FHSS has Tc < Ts

Generally fast FHSS gives improved

performance in noise (or jamming)


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Slow Frequency Hop Spread

Spectrum Using MFSK (M=4, k=2)


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Fast Frequency Hop Spread

Spectrum Using MFSK (M=4, k=2)


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Frequency Hopping Example


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Hybrid System: DS/(F)FH

The DS/FFH Spread Spectrum technique is acombination ofdirect-sequence and frequency-hopping. One data bit is divided over

frequency-hop channels (carrier frequencies).In each frequency-hop channel one completePN-code of length is added to the data signal(see figure, where is taken to be 5). Using theFFH scheme in stead of the SFH scheme

causes the bandwidth to increase, this increasehowever is neglectable with regard to theenormous bandwidth already in use.


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POWER CONTROL

CDMA is interference limited multiple access system.Because all users transmit on the same frequency,internal interference generated by the system is the mostsignificant factor in determining system capacity and

call quality.

The transmit powerfor each user must be reduced tolimit interference, however, the power should beenough to maintain the required Eb/No (signal to noiseratio) for a satisfactory call quality.

Maximum capacity is achieved when Eb/No of everyuser is at the minimum level needed for the acceptablechannel performance.


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Cont.

As the MS moves around, the RF environmentcontinuously changes (a) due to fast and slowfading, (b)external interference, (c) shadowing ,

and other factors. The aim of the dynamic power control is tolimit transmitted power on both the links whilemaintaining link quality under all conditions.

Additional advantages are longer mobilebattery life and longerlife span of BTS poweramplifiers


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HANDOVER

Handoveroccurs when a call has to be passed fromone cell to another as the user moves between cells.

In a traditional "hard" handover, the connection to thecurrent cell is broken, and then the connection to thenew cell is made. This is known as a "break-before-

make" handover.

Since all cells in CDMA use the same frequency, it ispossible to make the connection to the new cell beforeleaving the current cell. This is known as a "make-b

efore-b

reak"

or"soft

"handover.

Soft handovers require less power, which reducesinterference and increases capacity.

Mobile can be connected to more than two BTS the

handover.


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Cont.

One of the main advantages ofCDMA systemsis the capability of using signals that arrive in thereceivers with different time delays. Thisphenomenon is called multipath.

FDMA and TDMA, which are narrow bandsystems, cannot discriminate between themultipath arrivals, and resort to equalization tomitigate the negative effects of multipath.

Due to its wide bandwidth and rake receivers,CDMA uses the multipath signals and combinesthem to make an even stronger signal at thereceivers.


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CDMA subscriber units use rake receivers.

This is essentially a set ofseveral receivers.

One of the receivers (fingers) constantlysearches for different multipaths and feeds the

information to the otherthree fingers. Each

finger then demodulates the signal

corresponding to a strong multipath. Theresults are then combined together to make the

signal stronger.


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CDMA CODE DIVISION MULTIPLE ACCSES AND RADIO COM

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