Question

(c) Consider a Frequency Hoping (FH) binary orthogonal FSK system carrying speech signal with a cut-off frequency of 3.4 kHz for transmission after undergoing analog- digital conversion process. The following chip sequences for 4 code-division multiple access (CDMA) transmitters are given: A: (-1-1 -1 +1 +1 -1+1+1) B:(-1-1 +1 -1 +1 +1+1-1) C: (-1 +1 -1 +1 +1 +1-1-1) D: (-1 +1 -1-1-1-1+1-1) Assuming A, B and C are simultaneously sending 0, determine the resulting chip sequence in the channel? (1) (11 [7 marks] Determine the bit sent by transmitter A if the data chip sequence in the channel is given as (-1 +1 -3 +1 -1 -3 +1 +1). [3 marks]

Answer 1

Let d1,d2,d3, d4 be the data sent by the 4 transmitter A,B,C,D.Then the chip sequence present in the cannel is given by DATA = d1.A+d2.B+d3.C+d4.D.

Also we can see the codes are orthogonal.(ie; Multiplication of the sequence with any other sequence is equal to zero);

A*B=1+1-1-1+1-1+1-1 =0;

Similary A*C,B*C,B*D etc =0.(THIS IS A BASIC PROPERTY OF CDMA SIGNALLING).

a.)A,B,C are trasmitting 0 and say D is transmitting 1 then the chip signal in the channel is

0*A+0*B+0*C +1*D=[-1 +1 -1 -1 -1 -1 +1 -1];

b.)given signal in channel=[-1 +1 -3 +1 -1 -3 +1 +1];

we know data = d1.A+d2.B+d3.C+d4.D.

MULTIPLY DATA by A we have;

DATA*A= d1.*A*A+0+0+0 as A*B=0,A*C=0 A*D=0;

Data *A =1-1+3+1-1+3+1+1=8;

A*A=8;

therefore  ;

d1 Data * A/A* A = 1

Data sent by A=1;

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