Question

In a CDMA system, four stations use the following chip sequences:

station A: (-1 -1 -1 +1 +1 -1 +1 +1)

station B: (-1 -1 +1 -1 +1 +1 +1 -1)

station C: (-1 +1 -1 +1 +1 +1 -1 -1)

station D: (-1 +1 -1 -1 -1 -1 +1 -1)

a station receives sequence (-1 +1 -3 +1 -1 -3 +1 +1). Which of the four stations have transmitted? For the each of the stations that have transmitted, what binary value did they transmit?

Answer 1

Given

station A: (-1 -1 -1 +1 +1 -1 +1 +1)

station B: (-1 -1 +1 -1 +1 +1 +1 -1)

station C: (-1 +1 -1 +1 +1 +1 -1 -1)

station D: (-1 +1 -1 -1 -1 -1 +1 -1)

a station receives sequence (-1 +1 -3 +1 -1 -3 +1 +1).

As we know CDMA system works on concept of orthogonal codes

For station A

((-1 -1 -1 +1 +1 -1 +1 +1) . (-1 +1 -3 +1 -1 -3 +1 +1) ) /8 = 8/8 = 1

For Station B

((-1 -1 +1 -1 +1 +1 +1 -1).(-1 +1 -3 +1 -1 -3 +1 +1))/8 = -8/8 = -1

For Station C

((-1 +1 -1 +1 +1 +1 -1 -1).(-1 +1 -3 +1 -1 -3 +1 +1))/8 =0/8= 0

For Station D

((-1 +1 -1 -1 -1 -1 +1 -1).(-1 +1 -3 +1 -1 -3 +1 +1))/8 = 8/8 =1

Note : Here were performed one to one product.

From this we can conclude

Station A : Transmitted 1

Station B : Transmitted 0

Station C : Silent

Station D: Transmitted 1