cdma

Code Division Multiple AccessEng. Mshari Alabdulkarim

Code Division Multiple Access

Outline: Introduction.Spread Spectrum Methods.Technologies based on CDMA.Performance analysis of IEEE 802.11 protocol.Major Development in CDMA.


Introduction:

Code division multiple access (CDMA) is a fixed assignment access technique that uses spread spectrum and a special kind of coding scheme to allow the transmitters to share the media at the same time.

Why it is considered as a form of spread spectrum technique?It is considered as a form of the spread-spectrum techniques, because the modulated coded signal has a higher bandwidth than the data being transmitted.


Introduction (Cont.):

How does it support more than one group in the same channel?It allows multiple groups to share the same channel at the same time by assigning each group to different code.

CDMA Modulation Steps:

1. Generate a local pseudo-random code with a higher rate than the data to be transmitted.

2. XOR the data which need to be transmitted with the generated code.

b T

c T

Data Signal

Pseudo-random Code

Transmitted Signal

Time

Code Division Multiple AccessIntroduction (Cont.):

Channel Encoder Modulator Channel Demodulator Channel

Decoder

Pseudo-noiseGenerator

Pseudo-noiseGenerator

General Model of Spread Spectrum Digital Communication System

Input data

Output data





What can we gain from the spreading of the spectrum?

The immunity from the various types of noise. The immunity from the multipath distortion. It can be used as an encrypting technique. Several users can use the same channel with little interference.



Generating Pseudo-Random Codes:

The base station generates a unique pseudo-random code for every connection and these codes should have the following properties:

It should be deterministic. It must appear random to a listener who does not have a prior

knowledge of the code. The cross-correlation between any two codes must be small. The code should have a long period.



Code Correlation:

Each user uses different code to modulate his signal. The best performance will be when the users’ signals are separated

as much as possible.

The separation can be done by correlating the received signal with the local generated code.

If the correlation was high, then we exclude this code and we search for another code with less correlation.



Correlation Functions:

1. Cross-Correlation: The correlation of two different codes. “Used to separate the transmitters’ signals from each other”.

2. Auto-Correlation: The correlation of a code with a time-delayed version of itself. “Used to reject multipath interference”.


Spread Spectrum Methods:

Direct Sequence Spread Spectrum (DSSS):

Each bit from the data which need to be sent is represented by multiple bits in the transmitted data, by using the spreading code.

The original data is spread out across a wider frequency band by

using the spreading code.

The spreading proportion depends on the number of bits used in the code.

b T

c T

Original Data

Pseudo-random

Bit

Transmitted Data

Time

0 0

00 0 0 0 0 0 01 1 1 1 1 1 1 1 1 1 1 1

00 1 1 0 0 1 110 0 0

1 1 1

1 1 10 0 0 0 0

Example of Direct Sequence Spread Spectrum



Spread Spectrum Methods (Cont.):

Frequency Hopping Spread Spectrum (FHSS):

At the transmitter, the original signal is broadcasted over a series of radio frequencies, hopping from different frequencies in a fixed pattern.

The receiver should use the same hopping pattern simultaneously

with the transmitter in order to receive the data correctly.

The spreading code specifies the sequence of channels and the receiver should use the same code to tune into the sequence of channels that are used by the sender.

f1 f2 f3 f4 f5 f6 f7 f8

f1f2f3f4f5f6f7f8

1

2

3

4

5

6

7

8

1 23 45 678

Example of Frequency Hoping Spread Spectrum

(a) Channel assignment (b) Channel use

Frequency

Frequency

Time

Energy




Chirp Spread Spectrum (CSS):

It is the first and simplest application of Multi Dimensional Multiple Access (MDMA) technology.

It encodes the information by using wideband linear frequency modulated chirp.

It uses the entire allocated bandwidth to broadcast the signal, which helps making it robust to channel noise.

Also, it is immunized from the multi-path fading, because it utilizes the spectrum broadband.

It finds the noise by relying on the linear nature of the chirp pulse.



Time Hopped Spread Spectrum (THSS):

The period and duty cycle of a pulsed radio frequency carrier are varied pseudo-randomly under the control of coded sequence similar to FHSS.

It consumes low power due to the very low duty cycles.

In THSS, the pseudo random number determines the sequence of how the burst will be transmitted.

Time-Hopping Spread Spectrum

Time

Frame

f T f 2T f 3T f 4T 0


Transmitted time slots (k bits)

t


Technologies based on CDMA:

WiFi (IEEE 802.11):

802.11b and 802.11g working in the 2.4 frequency band (Industry, Science and Medicine band or ISM).

DSSS and FHSS physical layer options have been designed specifically to overcome the interfering with other devices in this band.

Three Non-Overlapping DSSS Channels in the ISM Band

Channel 12.4120

Channel 62.4370

Channel 112.46202.4000

GHz2.4835

GHz

22 MHz


IEEE 802.11

2.4 GHz

FHSSS

2 Mbps4 GFSK

1 Mbps2 GFSK

DSSS

1 Mbps

DBPSK

2 MbpsDQPSK

IEEE 802.11 b

Extension

5.5 MbpsDQPSK – CCKBPSK - PBCC

11 MbpsDQPSK – CCKQPSK - PBCC

850 to 950 nm

Diffuse IR



Technologies based on CDMA (Cont.):

Bluetooth:

It operates in the unlicensed ISM band and it can connect up to eight devices in the same time.

It uses Frequency Hoping Spread Spectrum (FHSS) to make it rare for more than one device to transmit on the same time using the same frequency and to avoid any interference.

Each device will use 79 individual, randomly chosen frequencies within a designated range, changing from one to another on a regular basis.



Bluetooth (Cont.):

A Bluetooth channel is divided into time slots each has a length of 625 micro second and each channel has a wide of 1MHz.

The devices hop through the timeslots making 1600 hops per second.



Global Positioning System (GPS):

All GPS satellites broadcast at two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal).

The receiver can distinguish the source of the signal by using direct sequence spread spectrum (DSSS) modulation technique where the low-bit rate message data is encoded with a high-rate pseudo-random noise (PRN) sequence.



Global Positioning System (GPS) (Cont.):

DSSS provides the structure for the transmission of ranging signal and essential data such as satellite co-ordinates and health.

The satellite carrier frequencies are modulated using the ranging signal which is PRN code which is different between satellites.

The receiver uses the satellite PRN code to reconstruct the actual message data.

The data is transmitted at rate of 50 bits per second.



Global Positioning System (GPS) (Cont.):

There are two types of PRN ranging distinct codes used:

1. The short coarse/acquisition (C/A) code (also called Gold code) at 1.023 million chips per second.

2. The long precise (P) code at 10.23 million chips per second.


Performance analysis of IEEE 802.11 protocol:

There are several assumptions have been considered in presenting this analytical framework:

1. The effect of bit errors introduced by channel noise has been ignored. The only considered errors are the one which caused by collisions due to other simultaneous transmissions.

2. There are no hidden stations.

3. The propagation delays are not considered.

4. There are n stations in the network.


Performance analysis of IEEE 802.11 protocol (Cont.):

5. Every station always has a packet to send (saturated conditions).

6. The collision probability of a transmitted frame is constant and independent of the number of retransmissions that this frame has experienced in the past.

7. RTS/CTS access mechanism.

The sequence of events in a successful frame transmission using the RTS/CTS access method will be like the following:



RTS: Request To Send.SIFS: Short Inter-Frame Space.CTS: Clear To Send.DIFS: Distributed Coordination Function.


Source

Destination

t0

Other

SIFS CTS SIFS

DATA

SIFS

RTS: Request To Send.SIFS: Short Inter-Frame Space.CTS: Clear To Send.DIFS: Distributed Coordination Function.

NAV (RTS)

NAV (CTS)

NAV (DATA)

DIFS

RTS

ACK

Defer Access Back-off



Abbreviations:



For simplicity, I assumed that the value of τ will be known and we will start by calculating the value of p:

Because we have n stations and all of them will transmit with a probability of τ , then the value of will be:trP



The probability of successful transmission will be equal to the probability that one station is transmitting while the remaining stations stay silent:

If is the frame header, the average time delays for RTS/CTS will be:



The throughput will be equal to the time needed to transmit the payload information divided by the average length of slot time:


Major Development in CDMA:

Wideband Code Division Multiple Access (W-CDMA):

It has a radio channel of 5 MHz wide and a chip rate of 3.84 Mcps. It supports frequency division (FDD) and inter-cell asynchronous

operation. It employs coherent detection on both the uplink and downlink. It has a Variable mission on a 10 ms frame basis. It supports Multi-code transmission. It uses a direct sequence CDMA transmission technique.


Major Development in CDMA (Cont.):

Time-division – CDMA (TD-CDMA):

It uses a combined time division and code division multiple access scheme.

The signals from different users are separated in both time and code domain.

It has a frame length of 10 ms and it is divided into 15 time slots. Each slot has duration of 666 micro second and length of 2560 chips. It uses Quadrature Phase Shift Keying (QPSK) to modulate the data.


Major Development in CDMA (Cont.):

Time Division Synchronous Code Division Multiple Access (TD-SCDMA):

It is an alternative to W-CDMA in the mobile telecommunications networks.

It is better suited for densely populated areas. It uses time-division duplexing (TDD). It can accommodate asymmetric traffic with different data rates on

downlink and uplink. It uses both TDMA and CDMA.


Thank you for listening

cdma

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