7/30/2019 Unit 4 CDMA

http://slidepdf.com/reader/full/unit-4-cdma 1/8

UNIT 4 SATELLITE ACCESS:

CODE-DIVISION MULTIPLE ACCESS (CDMA)

Code-division multiple access (CDMA) is scheme in which a number of users can occupy all of the

transponder bandwidth all the time. Therefore, individual carriers may be present simultaneously

within the same rf bandwidth, but each carrier carries a unique code waveform (in addition to the

information signal) that helps to differentiate them at the receiver.

  SSMA

In CDMA, the carrier is modulated in the normal way by the information waveform and then it is

again modulated by the code waveform. The second modulation is done to spread the spectrum

over the available rf bandwidth. Therefore, CDMA are also known as spread-spectrum multiple

access (SSMA).

  Waveforms Used:

CDMA can be used with analog as well as digital signals. Let the information signal is a polarNRZ waveform which is shown in Fig 1 and the elements are called bits.

Fig 1: The information signal (Polar NRZ)

Let the code waveform   is also a polar NRZ signal, as sketched in Fig.2. The pulses are called

chips and they vary randomly between and . This randomness is an essential feature inspread-spectrum systems. The modulation used here is binary phase shift keying (BPSK).

Fig 2: The code signal (Polar NRZ)

  Implementation:

The commonly used methods to spread the spectrum i.e., to implement CDMA are

i.  Direct sequence spread spectrum (DS/SS)

ii.  Frequency Hoping spread spectrum (FH/SS)

7/30/2019 Unit 4 CDMA

http://slidepdf.com/reader/full/unit-4-cdma 2/8

 

DIRECT- SEQUENCE SPREAD SPECTRUM

In Fig. 3,  is an NRZ binary information signal, and  is a NRZ binary code signal. These two

signals form the inputs to a multiplier (balanced modulator), the output of which is proportional to

the product  .

Fig 3: A basic CDMA system

This product signal is applied to a second balanced modulator, the output of which is a BPSK signal

at the carrier frequency. It is assumed that the carrier is the uplink frequency, and hence the uplink 

carrier is described by

-------- (1) The corresponding downlink carrier is

  -------- (2) At the receiver, an identical generator is synchronized to the of the downlink carrier. This

synchronization is carried out in the acquisition and tracking block in the receiver side. The

transmitted is a polar NRZ type waveform, and it is synchronized with the locally generated

transmitted . Therefore  

Now, the output of the multiplier is

    -------- (3) 

This gives the BPSK signal which is transmitted.

THE CODE SIGNAL  

A binary code is carried by the code signal It has special properties needed for the

implementation of CDMA. The binary symbols in the codes are known as chips.

  Clock control:

A clock controls the chip generation. The speed of this clock gives the chip rate in chips per second.

7/30/2019 Unit 4 CDMA

http://slidepdf.com/reader/full/unit-4-cdma 3/8

Denoting the clock speed by , the chip period is the reciprocal of the clock speed:

-------- (4)

Since the waveform is periodic, the sequence of N chips will be repeated for each period. The

periodic time for the waveform is

-------- (5) 

  Code generation:

The codes are generated using binary shift registers and associated linear logic circuits. The circuit 

for a three-stage shift register that generates a sequence of N= 7 chips is shown in Fig.4 a.

Fig 4: Code generation 

Fig shown is a 3-stage shift register circuit that generates a sequence of N=7 chips. The exclusive OR

is given feedback occurs from stages and it provides input to the shift register. All the chips are

clocked at the clock rate . The generator starts with all stages holding binary 1s, and the

following states are as shown in the table in Fig. 4 a. and the code waveform generated is shown in

Fig. 4.b.

  Maximal Sequence Codes

Since the generated codes are of maximum length that can be generated they are known maximal

sequence or m-sequence codes. In fig 4.b, the maximal length sequence is 7chips. Therefore, the

maximum sequence that can be generated by a code generator employing an n-stage shift register

is,

-------- (6)

  Pseudo-Noise codes (PN codes):

The code exhibits noiselike properties since binary 1s and 0s are randomly distributed in that code.

7/30/2019 Unit 4 CDMA

http://slidepdf.com/reader/full/unit-4-cdma 4/8

Therefore, the codes are also known as PN codes. The important features of codes are given below,

1.  The number of binary 1s is given by

-------- (7)

and the number of binary 0s is given by

-------- (8) 

2.  The dc offset determines the carrier level relative to the peak value;

   -------- (9)

where {  

3.  The total number of maximal sequences that can be generated by an n-stage shift register

is given by ------- (10) 

where ,   is known as Euler’s -function,

The Euler’s –function is given as,

-------- (11) 

where ,…, are the prime factors of N.

For example, for  

The prime factors of 255 are 3, 5, and 17, and hence

() (

) ()  

The total number of maximal sequences that can be generated by an eight-stage code

generator is therefore

 

As a somewhat simpler example, consider the case when n 3. In this instance, N7.

There is only one prime factor, 7 itself, and therefore

 

And  

In this case there are only two distinct maximal sequences.

7/30/2019 Unit 4 CDMA

http://slidepdf.com/reader/full/unit-4-cdma 5/8

4.  Autocorrelation property of  

The autocorrelation function is a measure of comparison between the time-shifted version

and the unshifted version of  .

Fig 5: a) Autocorrelation function generator

b) Autocorrelation waveform

Autocorrelation function generator is shown in Fig. 5a , in that the waveform and its a

shifted version are multiplied and the output is averaged by the integrator. The

integrator removes the time-t dependence and thus the output is independent of time, but it 

will depend on the time lead or lag introduced by .

The autocorrelation function waveform obtained from the circuit is shown in Fig 5.b. When

, the two waveforms are coincide and the output is maximum. If there is any shift in

time from , position either advance or delay, it will decrease the output voltage. In a m-

sequence code waveform, the autocorrelation function decreases linearly from the maximum

value (unity in this case) to a negative level 1/N, as shown in Fig. 5.b.

 ACQUISITION AND TRACKING:

   ACQUISITION OF A CARRIER 

The acquisition circuit is shown in Fig. 6. This circuit utilizes the autocorrelation function.

⇒  Multiplier

The output from the first multiplier is given by,

[]  -------- (12) 

where information modulation(BPSK)

Fig 6: Acquisition of a carrier in a CDMA system

7/30/2019 Unit 4 CDMA

http://slidepdf.com/reader/full/unit-4-cdma 6/8

 

⇒  BPF 

The BPF which is present next to the multiplier has a passband centered on . It does the

amplitude averaging function for the code signal product by rejecting high-frequency contents.

-------- (13)

⇒  Envelope detector and τ shift  

  TRACKING CIRCUIT 

The delay lock loop circuit which is a form of tracking circuit is shown in Fig 7.

Fig 7: Delay lock loop and the waveform at the adder

7/30/2019 Unit 4 CDMA

http://slidepdf.com/reader/full/unit-4-cdma 7/8

 

SPECTRUM SPREADING AND DESPREADING

Consider BPSK spectrum with as the bit rate. The main lobe of the power-density spectrum

occupies a bandwidth extending from to   .This is sketched in Fig. 8a. If  is the

modulation signal, then the power density spectrum is extended by   to   as

sketched in Fig. 8b. Since the code signal has periodicity, its spectrum density is a line function.

Therefore, the signal occupies wider bandwidth for constant carrier powers. Hence its spectrum

density will be reduced. In direct-sequence spread-spectrum systems, the chip rate determines the

bandwidth. They have a chip rate which is much greater than the information bit rate i.e.,  

. Therefore power density is spread over the bandwidth determined and it is reduced approximately

in the ratio of  .

Fig 8 : Spectrum for a BPSK signal : (a) without spreading (b) with spreading

Despreading function is a function used to store the spectrum of the wanted signal after the

spreading operation in the transmitter. This function is performed by the code signal . By using

this despreading function; interference is reduced in the spread spectrum technique. Fig 9 shows

the spectrum of two signals on which despreading function is to be performed. Here, the interfering

signal which is not spread is an unwanted signal and the desired DS/SS signal is the signal to be

restored.

7/30/2019 Unit 4 CDMA

http://slidepdf.com/reader/full/unit-4-cdma 8/8

Fig 9 : (a) Spectrum of an interfering, nonspread signal along with the spread desired signal; (b) the

effect of the de-spreading operation on the desired signal resulting in spread of the interfer

 ADVANTAGES OF CDMA:

CDMA offers several advantages for satellite networking, especially

1.  CDMA is highly resistant to interference and therefore satellite spacing can be reduced

considerably without reducing the signal quality.

2.  Spread spectrum systems are resistant to multipath noise.

3.  Synchronization between stations in the system is not required in the CDMA system.

Therefore, a station can access the system at any time.

4.  Small antennas can be employed without any interference problem from adjacent satellites.

5.  Additional traffic could be accommodated with acceptable reduction in performance.

6.  This technique offers a highly secure form of communication.

DISADVANTAGES  

The main disadvantage is the low throughput efficiency.