This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: Dec 2014

DS-CDMA

In this modulation of a carrier is being done by a digital code sequence whose bit rate is much higher than the information signal band width.

DS-CDMA

Here the output of several DS spread spectrum modulators, say N are using their own codes pi(t) where i=1,2... N. After combining these together these are transmitted. It should be noted that each of the user is transmitting data at the carrier frequency and each pseudo-random frequency pi(t) has the same chip rate. The data rate for each user is also the same Fb.

It is clear that multiplication of m1(t) produces a signal whose spectrum is the convolution of the spectrum of s1(t) with the spectrum of p1(t). Thus assuming that the signal m1(t) is relatively narrow band compared with the code or spreading signal g1(t), the product p1(t)*m1(t) will be approximately the bandwidth of p1(t).

RECEIVER:

At the receiver side arrangements are being made to receive the message from the desired group. Here the corresponding p1(t) is generated at the receiver and is perfectly synchronised with the received signal from the corresponding user. The first step in the receiving process is to multiply the incoming signal by p1(t). Suppose that signal of p1(t) is to be received, than at the receiver side the incoming signal would be multiplied by p1(t). The RF signal is first down converted to IF and then the pi(t) is multiplied to this IF signal and then the product is compared with the received IF signal in the correlator. The function of the correlator is to compare the two signals and recover the original signal data. Essentially the correlator subtracts the recovered PSK carrier + chip code from the received PSK carrier + chip code + data. The resultant is data. In fact the output of correlator which is the desired signal occupies the information bandwidth centred at the intermediate frequency. It is applied to conventional demodulator with bandwidth just wide enough to accommodate the desperado signal. The conventional demodulator has been represented by unit bit decision.

FH-CDMA

FH-CDMA

With frequency hopping, each earth station within a CDMA network is assigned a different frequency hopping pattern. Each transmitter switches from one frequency band to the next according to their preassigned pattern.

TRANSMISSION: Here in first sequence, frequency band F2 transmits in time slot t1

F4 in time slot t2

F3 in time slot t3

F5 in time slot t4

F1 in time slot t5

F2 in time slot t6

The resultant pattern is called frequency hopping pattern. Actually these FH pattern is determined by a binary code and each station uses a different code sequence.

It uses a FSK modulator. Thus FH spread spectrum is FM or FSK technique. The code pattern generator consists of PN code generator and a frequency synthesiser capable of responding to the coded output from the code generator. It must be remembered that the signal to be frequency hopped is usually a BPSK signal although M-ary FSK, MSK may be employed. Supposing that BFSK has been used then the signal before FHSS is used can be expressed as

RECEIVER:

A simple block diagram of FH receiver is shown in the figure. Here a local frequency synthesizer is switched with a synchronised replica of the transmitted PN code and the resultant signal is multiplied to the received FHSS signal. This multiplication removes the frequency hops on the received signal and thus the original modulated signals remains untouched. This signal is then applied to the conventional demodulator to get the orthogonal data/information.

Advantages of CDMA:

1. It is simple to operate since it does not require any transmission synchronisation between stations. The only synchronisation is that of the receiver to the sequence of the received carrier.
2. It offers useful protection properties against interference from other systems and interference due to multiple paths; this makes it attractive for networks of small stations with large antenna beamwidth and for satellite communication with mobiles.
3. It is highly secure.