An Introduction To Code Division Multiple Access 01

An Introduction to Code Division Multiple Access [CDMA] S. Suresh Mohan , M. Vinoth Kumar Pre final year, Bachelor of Engineering, Department of Electronics and Communication Engineering, Thanthai Periyar Government Institute of Technology, Bagayam,Vellore-632002. ‘[email protected]’, ‘[email protected]’ ABSTRACT CDMA is a form of multiplexing which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. It refers to protocols used in second and third generation wireless communication. It employs analog to digital conversion in combination with spread spectrum technology. Audio input is digitized into binary element. The transmitted frequency is varied to a defined pattern code so it can be intercepted only by a receiver whose frequency response is programmed with transmitted code. Many codes occupy same channel but only user associated with particular code can understand each other. It is used in high frequency cellular telephone system in the 800MHZ and 1.9GHZ bands. CDMA has been used in military applications such as Antijamming, ranging, secure communication. Introduction: CDMA (Code-Division Multiple Access) refers to any of several protocols used in so-called secondgeneration (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission

An analogy to the problem of multiple

channel, optimizing the use of available

access is a room (channel) in which

bandwidth. The technology is used in

people wish to communicate with each

ultra-high-frequency (UHF) cellular

other. To avoid confusion, people could

telephone systems in the 800-MHz and

take turns speaking (time division),

1.9-GHz bands.

speak at different pitches (frequency division), or speak in different languages

(code division). CDMA is analogous to

than Tb, the bandwidth of the spread

the last example where people speaking

spectrum signal is much larger than the

the same language can understand each

bandwidth of the original signal. The

other, but not other people. Similarly, in

ratio Tb / Tc is called spreading factor or

radio CDMA, each group of users is

processing gain and determines to

given a shared code. Many codes occupy

certain extent the upper limit of total

the same channel, but only users

number of users supported

associated with a particular code can

simultaneously by a base station.

understand each other. CDMA is a spread spectrum multiple access technique. A spread spectrum technique is one which spreads the bandwidth of the data uniformly for the same transmitted power. Spreading code

Each user in a CDMA system uses a

is a pseudo-random code which has a

different code to modulate their signal.

narrow Ambiguity function unlike other

Choosing the codes used to modulate the

narrow pulse codes. In CDMA a locally

signal

generated code runs at a much higher

performance of CDMA systems. The

rate than the data to be transmitted. Data

best performance will occur when there

for transmission is simply logically XOR

is good separation between the signal of

(exclusive OR) added with the faster

a desired user and the signals of other

code. The figure shows how spread

users. The separation of the signals is

spectrum signal is generated. The data

made by correlating the received signal

signal with pulse duration of Tb is XOR

with the locally generated code of the

added with the code signal with pulse

desired user. If the signal matches the

duration of Tc. bandwidth is proportional

desired user's code then the correlation

to 1 / T where T = bit time Therefore, the

function will be high and the system can

bandwidth of the data signal is 1 / Tb and

extract that signal. If the desired user's

the bandwidth of the spread spectrum

code has nothing in common with the

signal is 1 / Tc. Since Tc is much smaller

signal the correlation should be as close

is

very

important

in

the

to zero as possible (thus eliminating the

Each user is associated with a different

signal); this is referred to as cross

code, say v. If the data to be transmitted

correlation.

is a digital zero, then the actual bits transmitted will be –v, and if the data to

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

be transmitted is a digital one, then the actual bits transmitted will be v. For example, if v=(1,–1), and the data that the user wishes to transmit is (1, 0, 1, 1) this would correspond to (v, –v, v, v) which is then constructed in binary as ((1,–1),(–1,1),(1,–1),(1,–1)). For the

Synchronous CDMA orthogonality

purposes of this article, we call this

exploits mathematical properties of

constructed vector the transmitted

between vectors representing the data

vector.

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. Each user in synchronous CDMA uses a code orthogonal to the others' codes to modulate their signal. An

If sender0 has code (1,–1) and data (1,0,1,1), and sender1 has code (1,1) and data (0,0,1,1), and both senders transmit simultaneously, then this table describes the coding steps: Ste p 0

example of four mutually orthogonal digital signals is shown in the figure. Orthogonal codes have a crosscorrelation equal to zero. An example of

1 2

four mutually orthogonal digital signals. 3

Encode sender0

Encode sender1

vector0=(1,–1),

vector1=(1,1),

data0=(1,0,1,1)=( data1=(0,0,1,1)=( v,–v,v,v) –v,–v,v,v) encode0=vector0. encode1=vector1. data0 data1 encode0=(1,–1). encode1=(1,1).(– (1,–1,1,1) 1,–1,1,1) encode0=((1,–1), encode1=((–1,–1), (–1,1),(1,–1),(1,– (–1,–1),(1,1), 1))

(1,1))

4

signal0=(1,–1,–

signal1=(–1,–1,–

Further, after decoding, all values

1,1,1,–1,1,–1)

1,–1,1,1,1,1)

greater than 0 are interpreted as 1 while

Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal: (1,–1,–1,1,1,–1,1,–1) + (–1,–1,–1,– 1,1,1,1,1) = (0,–2,–2,0,2,0,2,0) This raw signal is called an interference pattern. The receiver then extracts an intelligible signal for any known sender by combining the sender's code with the interference pattern, the receiver combines it with the codes of the senders. The following table explains how this works and shows that the

all values less than zero are interpreted as 0. For example, after decoding, data0 is (2,–2,2,2), but the receiver interprets this as (1,0,1,1). We can also consider what would happen if a receiver tries to decode a signal when the user has not sent any information. Assume signal0=(1,-1,1,1,1,-1,1,-1) is transmitted alone. The following table shows the decode at the receiver: Ste p

signals do not interfere with one another: 0 Ste p 0

1

Decode sender0

Decode sender1

vector0=(1,–1),

vector1=(1,1),

pattern=(0,–2,–

pattern=(0,–2,–

2,0,2,0,2,0) 2,0,2,0,2,0) decode0=pattern. decode1=pattern.v vector0 ector1 decode0=((0,–2), decode1=((0,–2),

2

(–2,0),(2,0),(2,0)). (–2,0),(2,0),(2,0)).

3

(1,–1) (1,1) decode0=((0+2), decode1=((0–2),(– (–2+0),(2+0), 2+0),(2+0),(2+0)) (2+0)) data0=(2,– data1=(–2,–

4

2,2,2)=(1,0,1,1)

2,2,2)=(0,0,1,1)

1

2

3

4

Decode sender0

Decode sender1

vector0=(1,–1),

vector1=(1,1),

pattern=(1,-1,-

pattern=(1,-1,-

1,1,1,-1,1,-1) 1,1,1,-1,1,-1) decode0=pattern. decode1=pattern.v vector0 ector1 decode0=((1,–1), decode1=((1,–1), (–1,1),(1,-1),(1,- (–1,1),(1,-1),(1,1)).(1,–1) 1)).(1,1) decode0=((1+1), decode1=((1–1),(– (–1-1),(1+1), 1+1),(1-1),(1-1)) (1+1)) data0=(2,– 2,2,2)=(1,0,1,1)

data1=(0,0,0,0)

When the receiver attempts to decode

exactly the same time. Thus, this

the signal using sender1’s code, the data

technique finds use in base-to-mobile

is all zeros, therefore the cross

links, where all of the transmissions

correlation is equal to zero and it is clear

originate from the same transmitter and

that sender1 did not transmit any data.

can be perfectly coordinated. 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

In CDMA, each phone's data has a unique code. Asynchronous CDMA:

the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems.

The previous example of orthogonal

These systems were designed using

Walsh sequences describes how 2 users

spread spectrum because of its security

can be multiplexed together in a

and resistance to jamming.

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

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.

Direct Sequence Spread Sprectrum DSSS CDMA : In this method, the direct sequence(input data) which is spread over a limited bandwidth is multiplied with a code or spreading sequence (a pseudorandom sequence also known as PN sequence) which will spread the input data over the

CDMA Demodulation:

entire bandwidth of the communication channel.



CDMA's

spread

spectrum

The power density is also reduced and is

technique

spread over the frequency spectrum and

transmission on the same carrier

hence is known as spread spectrum

frequency by assigning a unique

method. The modulation part of DSSS is

code to each conversation.

as shown below.



overlaps

every

The signal is spread at two levels

first using a Walsh Code and then using a PN Code. The number of bits in either of the two codes is known as the "chip rate," and each bit in the spreading signal is called a "chip". One bit from each conversation (baseband signal) is multiplied with the Walsh code and then the PN code CDMA Modulation:

by the spreading techniques giving

The modulated signal is transmitted over

the receiving side an enormous

the channel and all users can receive it

amount of data it can average just to

but only the user which knows the

determine the value of one bit.

correct code can decode the message.



This is depicted in the figure below.

assigns spreading code to each call

Base station is the one that

when a mobile requests for a call (unique

Walsh

code

for

each

conversation and a same PN code for each call in a cell sector). In the analysis henceforth we discuss the dynamic allocation of these spread codes

in

accordance

with

the

required QoS. 

Multipath Fading : In a mobile environment, a mobile station will receive one direct signal from the base station and multiple signals

which

are

reflected

from

obstructions like buildings and towers.

Power Control: As the propagation losses between BS and MS's are different according to individual communication distances, the

Each signal would have traveled a

received levels at the base station are

different length and would be displaced

different from each other when all

in time. Due to this, when they are

mobile stations transmit their signals at

combined at the mobile handset, it will

the same power.

cause interference resulting in poor signal quality. This is known as fading.

Moreover, the received level fluctuates quickly due to fading. In order to

This problem is handled in a very good

maintain the strength of received signal

way in CDMA. Here, the phase of the

level at BS, power control technique

multiple signals is modified such that

must be employed in CDMA systems.

only positive interference (addition) takes place and the overall signal strength increases. A receiver that implements the above principle is known as a RAKE receiver as shown in the figure below. Power control can be implemented in

two ways: open loop power control and

implemented in TM is to enhance the

closed loop power control.

current PSTN network and is targeted to serve customers in rural areas. TM's CDMA Network consists of 3 main elements: •

3 Mobile Switching Centres (MSC)

•

10 Central Base Station Controllers (CBSC)

•

257 Base Transceiver Stations (BTS)

Frequency used:

Effect of Power Control: Power

control

is

capable

of

•

compensating the fading fluctuation. Received powers from all MS are

869Mhz – 894 MHz •

controlled to be equal. 　 Near-Far problem is mitigated by the power

Base Station transmission: Base Station reception: 824 MHz – 849 MHz

•

control.

Currently 2 carriers being used within the above frequency band.

•

CDMA channels currently in use are (centre frequency):

Channel 1: 878.49 / 833.49 MHz (Base Tx/Rx) Channel 2: 877.23 / 832.23 MHz (Base Tx/Rx) Benefits: The channel bandwidth is 1.25 MHz. CDMA is a mobile technology that has been implemented by TM as a Fixed Wireless service. The CDMA network

Features:

i) Access Line •

•

Call forwarding on busy

•

Call forwarding on no reply

Fixed Wireless Telephone (CDMA) will provide basic

CDMA vs. GSM? – Which is more

telecommunications applications

ubiquitous? Well, the obvious answer is

of a normal fixed telephone,

GSM, but this answer may not be that

which support incoming and

obvious few years down the line –

outgoing PSTN voice calls.

CDMA is quickly catching up with GSM; and the subscriber base (for

ii) Data (Internet) •

CDMA 95B System-Internet services in the circuit switch environment

•

CDMA) in India has already surpassed 50 million fixed and mobile device users. As per the report from CDG,

CDMA 1X System-Internet Access up to 144 Kbps

•

50 million subscriber growth reached this milestone only four

iii) Facsimile

years after the technology’s introduction to the market, while

iv) Traffic Minutes/Calls

it it took GSM more than ten

The connection will support outgoing

years to reach the same number.

PSTN calls of all types: •

CDMA growth rate exceeds than

•

Geographic calls

that of GSM on a monthly basis

•

Non-geographic calls, 1800,

(5% vs. 4,1%).

1300

•

With up to 2.01 million net

•

Emergency calls, e.g. 999, 994

subscriber additions in June

•

Equal access

2007, CDMA2000’s 5 percent

•

Operator assistance

growth rate exceeded that of

•

Directory assistance

GSM, at 4.1 percent,.

•

Enhanced features

•

Call waiting

•

Call forwarding

•

Reliance Communications and Tata Teleservices, are among

the top 20 fastest-growing operators in the world. •

CDMA2000 is quickly becoming the technology of choice for emerging markets.

•

Reliance has embarked on one of the largest CDMA2000 network expansions on the planet—with plans to reach more than 20,000

Conclusion:

towns and 300,000 villages.  CDMA provides an advanced Key reasons behind this growth are: •

technology for cellular

Availability of very low-end (VLE) handsets – There are currently 45 VLE CDMA2000 devices

from

available

14

below

suppliers US$50

in

Rapid

 It provides high-quality service to a large number of users.  It is a system that has been extensively tested and it will be deployed later this year in

wholesale price. •

applications.

expansion

of

the

CDMA2000 networks into the rural areas of India to deliver state-of-the-art

telephone

and

precommercial applications. Commercial service is scheduled to begin in 1994. References:

broadband Internet access •

Price gap between 2G GSM low-

[1] G. K. Rushforth, “Transmitted-

end

handsets

CDMA2000

and

3G

reference techniques for random

handsets

has

orunknown channels,” IEEE Trans.

narrowed to only $4 USD

Inform. Theory, vol. IT-10, pp. 39– 42,Jan. 1964

.2. J.G. Proakis, Digital Communications, Second Edition, McGraw-Hill Book Co., 1989.