Multi User Detection for CDMA

1
Multi User Detection for CDMA

• Group Members
• -Bhushan G. Jagyasi
• -Himanshu Soni

2
Multiple Access Interference(MAI)

• Conventional Detector

Modula- tion
Modula- tion
b1(.)
b1(.)
Decision
AWGN
g1
g1
b2(.)
b2(.)
Decision
g2
g2
r(t)
Noisy Channel
bn(.)
Decision
bn(.)
gn
gn
Reciever / Detector
Received signal , r(t) b1g1f1 b2g2f2
bngnfn n(t)
After detector y(t)b1(t) b1.g1.g1. f1
b2.g2.g1.f2 bn.gn.g1.fn no.g1
y(t) b1 f1 MAI zk
3
Actual Problem

• y(t) b1. f1 MAI zk
• Total Interference (Includes interference due
  to other cells also)
• I IMAI f IMAI
• where f Ratio of other cell MAI / IMAI
• Near Far Problem in CDMA
• Difficulty to implement more sophisticated algo
  at receiver because of limitations of size, cost,
  weight of handset.
• Solutions to all such problems is Multi-user
  detection

0
4
Genre of Multi-user detctors

• NonBlind Multiuser detection algorithm
• Detectors Available
• Linear Detector (LMS based)
• Subtractive interference canceller
• Requires following information
• Spreading waveform of desired signal
• Spreading waveform of interfering signal
• Timing (pd) of desired signal
• Timing (pd) of interfering signal
• Training data for every active user
• Drawback
• More Computational complexity.
• Requires much more information.

5
Genre of Multi-user detctors

• Blind multiuser detection algorithm
• Detectors Available
• Kalman based
• Requires following information
• Spreading waveform of desired signal
• Timing (pd) of desired signal
• Turbo MUD
• Joint channel decoding and MUD using iterative
  exchange of information between two processes.