Optimal Sequence Allocation and Multi-rate CDMA Systems

1
Optimal Sequence Allocation and Multi-rate CDMA
Systems

• Krishna Kiran Mukkavilli, Sridhar Rajagopal,
  Tarik Muharemovic, Vikram Kanodia

2
Motivation

• 3rd Generation Comm. Systems
• Multimedia(Data, Voice, Video)
• Multiple Rate Comm.
• Multi-Rate Detection
• Users entering/leaving the system
• Optimal Sequence Allocation to achieve Capacity.

3
Outline

• Conventional CDMA multiuser system
• Discussion of multirate systems
• Methods of multirate CDMA access
• Performance of multiuser detectors
• Interference avoidance
• Application to variable number of users

4
Multi-rate CDMA systems

• Multi code access (MC)
• Give more Codes
• Variable spreading length (VSL)
• Change Spreading Length
• Variable chip rate(VCR)
• Change Chip Frequency

5
Multi code (MC)

• Higher Rate users assigned more codes
• Data transmitted in parallel
• Virtual User Concept
• Same Spreading for all users.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

6
Multi Code
Code 1
Code 2
User Rate R
User Rate 2R
T
Code 3
7
Variable spreading length(VSL)

• Higher Rate Users allocated smaller spreading
  lengths
• For detection, rate of slowest user is
  considered.
• More bits of higher rate users detected per bit
  of lower rate users
• For detection, put 0s

8
Variable Spreading Length
User Rate R
T
User Rate 2R
2T
9
Variable Chip Rate(VCR)

• User allocated different chip rates
• Larger Bandwidth required
• Requires more RF hardware
• Oscillators
• Not practical for implementation

10
Variable Chip Rate
User Rate R
T
User Rate 2R
2T
11
Implementation Aspects

• VSL and VCR have a sparse correlation matrix
• VCR requires larger bandwidth
• MC requires more codes
• VSL proposed for next generation systems

12
Multiuser Detectors

• Maximum likelihood detector (MLD)
• Conventional single user detector (SUD)
• MMSE detector
• Decorrelating detector

13
Simulations

• Four users
• 2 users at rate R
• 2 users at rate 2R
• Random Codes
• Spread length 32 for low rate user
• 10000 bits
• Channels
• AWGN
• Fading - Jakes Model

14
Investigate...

• Performance of multiuser detectors
• Near far problem in detectors
• Performance of high rate and low rate users in MC
  and VSL systems
• All users with equal power
• Users with unequal power

15
BER comparison for different detectors in multi
code system
0
10
MLD
MMSE
Decorrelator
Single user detector
-1
10
-2
10
BER
-3
10
-4
10
2
3
4
5
6
7
8
9
10
11
12
SNR
16
BER comparison for detectors with unequal powers
0
10
MLD Equal Power
MLD Unequal Power
SUD Equal Power
SUD Unequal Power
-1
10
-2
10
BER
-3
10
-4
10
2
3
4
5
6
7
8
9
10
11
12
SNR
17
Comparison of Different Rate Users in MC and VSL
0
10
High rate MC
High rate VSL
Low rate MC
Low rate VSL
-1
10
-2
10
BER
-3
10
-4
10
2
3
4
5
6
7
8
9
10
11
12
SNR
18
VSL System

• Virtual user from high rate user
• lower spreading length
• lower interference (other virtual users are
  orthogonal)
• High rate user
• interference from same number of virtual users
  with lower spread length

19
Variable Spreading Length
User Rate R
T
User Rate 2R
2T
20
Near Far effect for Different Rate Users in MC
and VSL
0
10
Low rate MC
Low rate VSL
High rate MC
High rate VSL
-1
10
-2
10
BER
-3
10
-4
10
2
3
4
5
6
7
8
9
10
11
12
SNR
21
Results

• Multi Code
• High rate and low rate users have same
  performance (both BER and NFR)
• VSL
• Low rate users have bad BER and NFR
• High rate users performance is similar to
  multicode access system.

22
Interference Avoidance in Wireless Multiuser
Systems

• Interference Avoidance ? send where there is less
  noise
• Fixed modulation - traditional approach
• TDMA
• FDMA
• CDMA
• CWMA
• Future wireless systems - dynamically adapt to
  the changing interference pattern

23
Preliminaries for Multiuser Systems
(class notes pg. 5-12)

• System model
• Capacity region

24
Total Square Correlation vs Iteration Number
9.5
Total Square Correlation
Optimum Lower Bound
9
8.5
Total Square Correlation
8
7.5
7
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
Iteration Number
25
Preliminaries for Multiuser Systems
(class notes pg. 5-12)

• Sum Capacity
• W channel bandwidth
• Pi power of i-th user
• N0 noise power spectrum

26
Multiuser Spread Spectrum Systems

• System model

27
Multiuser Spread Spectrum Systems

• Sum Capacity
• Optimum sequences maximize Sum Capacity
• Total Square Correlation (TSC)
• Max. Sum Capacity ? Min. TSC

28
Eigen-Algorithm

• Iterative reduction of TSC
• User k updates his spreading sequence
• Rayleigh quotient
• Choose sk to be eigenvector with smallest
  eigenvalue

29
Performance comparison of optimal codes with
random codes
0
10
Random Code Allocation
Optimal Code Allocation
-1
10
-2
10
BER
-3
10
-4
10
8
10
12
14
16
18
20
22
SNR
30
BER Performance with an Incoming User
0
10
Random Code to new user
Iteration for new user only
Optimal Code Allocation
-1
10
BER
-2
10
-3
10
6
7
8
9
10
11
12
13
14
15
16
SNR
31
Conclusions

• Significant improvement in performance with
  optimal codes
• Iterative algorithm compatible with user dynamics
• Good sub-optimal schemes for user addition
• Can be combined with the multi-rate schemes