In this chapter, we investigate a pilot-channel aided SIC scheme for uplink WCDMA
systems. The scheme alleviates the interference from traffic-channel as well as pilot-channel signals of other users. Also, we discuss three SICs with different ordering method, and compare their corresponding architectures as well as processing delays and computational complexities. In addition to showing the superiority over the conventional RAKE receiver, the influence of ordering method, pilot-to-traffic amplitude ratio, grouping interval, power distribution ratio, channel and timing estimation errors on the BER are jointly examined and discussed. It is found that ordering based on average power (SIC I) requires the least computational complexity at the expense of BER when grouping interval is small, and it seems to be suitable for AWGN channels with moderate loading. Ordering based on RAKE outputs after each cancellation of grouping-interval bits of one user (SIC II) outperforms ordering based on RAKE outputs at initial stage in each grouping interval (SIC III) when grouping interval is small. But SIC II has the highest computational complexity and the largest processing delay among all SICs. SIC III is proposed to be a better choice over multipath fading channels when BER and computational complexity are jointly concerned.
The pilot-channel aided SIC scheme for multirate WCDMA systems in the uplink is also presented. The scheme alleviates interference resulting from data-channel signals as well as pilot-channel signals of other users in the received signals. The cancellation order is decided by ranking the average of RAKE output strength at the 1st stage of SIC over the grouping interval, and properly chosen grouping interval help to reduce the BER of users with relatively large spreading gain depends on the Doppler shift of channel.
Table 3-1 Characteristics of Three SICs per G Bits per K Users
SIC Reordering Frequency (denoted as RF)
Throughput (Bits/Step)
Latency (Steps)
Hardware Required for Minimum Delay
Computational Complexity C(.) BASE=PCSR+CE +DS+RAKE+DR+AD I <<fd G/5 5K+3 K*C(BASE)+K*C(OR)* GfdTb/RF
II K/(GTb) G/6 6K+3 K*C(BASE)+K*C(FM)+K( K-1)/2*C(RAKE)
III 1/(GTb) G/5 5K+5
As shown in block diagrams in Fig. 3-7
~ Fig. 3-9 K*C(BASE)+K*C(FM)+( K-1)*C(RAKE) PCSR: Pilot Channel Signal Removal; CE: Channel Estimation; DS: Decision; DR: Data Respread; AD: Adder; FM: Find
Max; OR: Average Power
Table 3-2 Simulation Parameters
Symbol Quantity
1/Tc Chip Rate 3.84Mhz
fc Carrier Frequency 2 GHz
SF Spreading Factor 16
1/Tb Bit Rate 240 Kbps
N Scramble Sequence Length 38400 Chips
NTc Frame Period 10 ms
AWGN 7/15 βc Pilot-to-Traffic Amplitude Ratio
Fading Channels 10/15
K User Number 8
AWGN 15 dB SNR Average Eb/N0
Fading Channels 20 dB Multipath Fading Channel Conditions See Table 3-3
Table 3-3 Propagation Conditions for Multipath Fading Environments [71].
Case 1, speed 3km/h Case 2, speed 3 km/h Case 3, 120 km/h Case 4, 250 km/h Relative
Delay [ns]
Average Power [dB]
Relative Delay [ns]
Average Power [dB]
Relative Delay [ns]
Average Power [dB]
Relative Delay [ns]
Average Power [dB]
0 0 0 0 0 0 0 0 976 -10 976 0 260 -3 260 -3
20000 0 521 -6 521 -6
781 -9 781 -9
Table 3-4 Simulation Parameters for Multirate Systems
Chip Rate (1/Tc) 3.84Mhz
Carrier Frequency 2GHz
Scramble Sequence Length (Nc) 38400Chips
Pilot-to-Traffic Gain Ratio (βc) 2/3
User Number (K) 6 Spreading Factor (SFk) Randomly Chosen from {8,16,32}
Chip Energy to Noise Ratio (Ec/N0) -3 dB Moving Average Window for Channel Estimation(Wc) 2048Chips
Multipath Fading Channel Conditions Case 3 in Table 3-3
Fig. 3-1 The received signal timing and data detection group (a) τk,p;J,f ≥0 and (b) τk,p;J,f <0 where τk,p;J,f = τk,p - τJ,f.
j ( )
r t
j
j 1
2βp bT
1 2βp bT
1 2βp bT
1 W
1 W 1 W
) (
,
ˆavn;JF
α )
( )
( J,F J* J,F
pilot t C t
A −τ −τ
) (
)
( J,2 J* J,2
pilot t C t
A −τ −τ
) (
)
( J,1 J* J,1
pilot t C t
A −τ −τ
) (
2 ,
ˆavn;J
α
) (
1 ,
ˆavn;J
α
Fig. 3-2 Channel estimation of user J with F paths
ˆpilot k; ( )
C t
jβp
*
( ,1)
k k
C t−τ αˆk( ),1n′
*
( ,2)
k k
C t−τ αˆk( ),2n′
) (
ˆk,nF′
α )
( ,
*
F k k t C −τ
Fig. 3-3 Structure of the pilot respread of the k-th user with F paths
ˆdata k; ( )
C t
βd ( )
ˆk,1n
α ′
( )
ˆk,2n
α ′
n＇can be n and/or n-1 and/or n+1
... ...
) (
)
( k,F *k k,F
O t C t
C −τ −τ
) (
)
( k,2 k* k,2
O t C t
C −τ −τ
) (
)
( k,1 k* k,1
O t C t
C −τ −τ
] ˆ n[ bk
) (
ˆk,nF′
α
Fig. 3-4 Structure of the data respread of the k-th user with F paths
( )
ˆav k fn; ,
α
Pilot Respread
Pilot Respread
Pilot
Respread
+
-...
pilot( )
A t
ˆ( ) r t Channel Estimates
( ) r t
Fig. 3-5 Block diagram of pilot-channel signal regenerator and remover, 1≤ k ≤ K, 1 ≤ f ≤ P
) (
;
ˆ n u
YSIC< >
] ˆ [
; n
bSIC<u>
)
; (t
Cdata<u>
) ˆ (
; t
rSICu
) ˆ (
1
; t
rSICu+
Fig. 3-6 Generalized SIC structure at the u-th stage
Fig. 3-7 Block diagram of SIC I with PCSR (1≤ f ≤ F)
Fig. 3-8 Block diagram of SIC II with PCSR (1≤ f ≤ F).
Fig. 3-9 Block diagram of SIC III with PCSR (1≤ f ≤ F).
0 5 10 15 20 25 30 35 40 10-2
10-1
SNR
MSE
Sim.; βc=1/3; W= 64 Ana.; βc=1/3; W= 64 Sim.; βc=2/3; W= 64 Ana.; βc=2/3; W= 64 Sim.; βc=3/3; W= 64 Ana.; βc=3/3; W= 64 Sim.; βc=1/3; W= 128 Ana.; βc=1/3; W= 128 Sim.; βc=2/3; W= 128 Ana.; βc=2/3; W= 128 Sim.; βc=3/3; W= 128 Ana.; βc=3/3; W= 128
Fig. 3-10 MSE of channel estimates with various SNRs, flat Rayleigh fading channel, PDR=1.0, G=1.
1 1.2 1.4 1.6 1.8 2 10-4
10-3 10-2 10-1 100
1 1.2 1.4 1.6 1.8 2 10-4
10-3 10-2 10-1 100
1 1.2 1.4 1.6 1.8 2 10-4
10-3 10-2 10-1 100
PDR
BER
Solid Line: Simulated
Dotted Line: Analytical MF: Matched Filter
PDR PDR
MF G= 1 G= 4 G= 16 G= 50 G= 2400
(a) (b) (c)
Fig. 3-11 BER versus PDR with different grouping interval G for (a) SIC I, (b) SIC II, (c) SIC III; AWGN, know channel parameters, with PCSR.
2 4 6 8 10 12 14 16 10-8
10-7 10-6 10-5 10-4 10-3 10-2 10-1
USER NUMBER
BER
1 2 3 4 5 6 7 8
10-6 10-5 10-4 10-3 10-2 10-1
USER IN DIFFERENT CANCELLATION ORDER
Sim; MF
Sim; SIC I, G: 1 Sim; SIC II, G: 1 Sim; SIC III, G: 1
Sim; SIC I, G: 2400, PDR:1.3 Ana; MF
Ana; SIC I, G: 1 Ana; SIC II, G: 1 Ana; SIC III, G: 1
Ana; SIC I, G: 2400, PDR:1.3
(a) (b)
Sim :simulated Ana: Analytical
Fig. 3-12 Simulated and analytical results of SICs with PCSR (a) individual BER in an eight-user system, (b) average BER.
1 1.2 1.4 1.6 1.8 2 10-6
10-5 10-4 10-3 10-2 10-1
1 1.2 1.4 1.6 1.8 2 10-6
10-5 10-4 10-3 10-2 10-1
1 1.2 1.4 1.6 1.8 2 10-6
10-5 10-4 10-3 10-2 10-1
1 1.2 1.4 1.6 1.8 2 10-6
10-5 10-4 10-3 10-2 10-1
BER
Solid Line: Analytical Dotted Line: Simulated
PDR PDR PDR PDR
SNR= 5dB, Without PCSR SNR= 5dB, With PCSR SNR= 15dB, Without PCSR SNR= 15dB, With PCSR SNR= 25dB, Without PCSR SNR= 25dB, With PCSR
(a) (b) (c) (d)
Fig. 3-13 BER comparison with different PDRs and SNRs with/without PCSR for (a) SIC I with G=1, (b) SIC II with G=1, (c) SIC III with G=1, (d) SIC I with G=2400; AWGN, channel estimation with W=128.
1/15 5/15 9/15 13/15 17/15 21/15
x,+: PDR=1.3(8 users),PDR=1.1(16 users) Solid Line: Simulated system; AWGN, channel estimation with W=128.
1 1.5 2
PDR PDR PDR PDR PDR
PDR PDR PDR PDR PDR
PDR
Fig. 3-15 BER versus PDR for the three SICs in multipath fading channels (a) Case 1, (b) Case 2, (c) Case 3, (d) Case 4; with PCSR, known channel parameters.
1 1.5 2
PDR PDR PDR
PDR
Fig. 3-16 BER versus PDR for the three SICs in multipath fading channels (a) Case 1, (b) Case 2, (c) Case 3, (d) Case 4; channel estimation with W=128 and timing estimation error with variance 2 samples at 1/32 chips resolution.
100 101 102 103 The 1st Detected User
The 3rd Detected User The 5th Detected User The 6th Detected User The 8th Detected User Average BER
(a) (b) (c)
Fig. 3-17 BER versus grouping interval G for user in different detection order for (a) SIC I with PDR=1.3, (b) SIC II with PDR=1.0, (c) SIC III with PDR=1.0; channel case 3, known channel parameter, with PCSR.
1 5 9 13 17 21 10-3
10-2 10-1
1 5 9 13 17 21 10-3
10-2 10-1
1 5 9 13 17 21 10-3
10-2 10-1
1 5 9 13 17 21 10-3
10-2 10-1
βc
BER
Solid Line: with PCSR Dotted Line: without PCSR
x1/15
x1/15 βc βc x1/15 x1/15
βc
RAKE Receiver SIC I; PDR=1.3 SIC II; PDR=1.0 SIC III; PDR=1.0
(a) (b) (c) (d)
Fig. 3-18 BER versus βc for multipath fading channels (a) Case 1, (b) Case 2, (c) Case 3, (d) Case 4; channel estimation with W=128.
100 101 102 103 10-4
10-3 10-2 10-1
100 101 102 103 10-4
10-3 10-2 10-1
100 101 102 103 10-4
10-3 10-2 10-1
100 101 102 103 10-4
10-3 10-2 10-1
G
BER
Solid Line: Known Channel Parameters Dotted Line: Channel Estimation with W=128
G G
(bits) (bits) (bits) G (bits)
RAKE ; PDR=1.0
SIC I; Solid Line: PDR=(a)1.6, (b)1.3, (c)1.6, (d)1.6; Dotted Line: PDR=(a)1.3, (b)1.2, (c)1.3, (d)1.3 SIC II; Solid Line: PDR=(a)1.0, (b)1.0, (c)1.5, (d)1.4; Dotted Line: PDR=1.0
SIC III; Solid Line: PDR=(a)1.0, (b)1.0, (c)1.5, (d)1.4; Dotted Line: PDR=1.0
(a) (b) (c) (d)
Fig. 3-19 BER versus grouping interval G for multipath fading channels (a) Case 1, (b) Case 2, (c) Case 3, (d) Case 4; with PCSR
102 103 104 10-3
10-2 10-1 100
102 103 104 10-3
10-2 10-1 100
GROUPING INTERVAL (Gc)
BER
BER vs. Gc RAKE; SF= 8 users RAKE; SF=16 users RAKE; SF=32 users SIC ; SF= 8 users SIC ; SF=16 users SIC ; SF=32 users
Fig. 3-20 BER vs. grouping interval Gc, fd =222Hz for (a) multirate systems and (b) single rate systems.
101 102 103
102 103 104 105
Doppler shift (Hz)
GROUPING INTERVAL Gc
Gc for the minimum BER vs. fd
SF=16 users SF=32 users
Fig. 3-21 Grouping interval Gc for the minimum BER vs. Doppler shift
Chapter 4
Advanced Techniques for Pilot-Channel Aided Interference Cancellation
4.1 Overview
In the uplink of WCDMA systems, traffic-channel signal and pilot-channel signal are multiplexed in I and Q channel, respectively. In the former chapter, pilot-channel signal are removed before data detection is performed to acquire better performance. In this chapter, the channel estimation accuracy is further improved. Additionally, we find that the SIC outperforms PIC under fading channels but faces the problem of power reordering and longer processing delay. In this chapter, we propose techniques to overcome this drawback.
In the first part of this chapter, we propose a pipelined structure to reduce the latency, i.e.
a pilot-channel aided pipeline scheme for interference cancellation (IC) in uplink wideband DS/CDMA system is proposed. Generally speaking, pipelined implementation is inherent in SIC but not in channel estimation. This scheme combines channel estimation and user data detection into sequential type with low complexity and leads to pipeline implementation.
Besides, interference between data channel signals and pilot channel signals under multipath fading channel are also taken into consideration. Compared with conventional channel estimation using correlator output and SIC without pilot channel signal removal, the proposed scheme shows better quality both on channel estimation and user data detection.
In the second part of this chapter, we further propose an adaptable scheme which has the ability to perform better than SIC discussed in Chapter 3 as well as adapt its structure
according to the environment and channel condition. The pilot-channel aided adaptable IC scheme combines successive (SIC) and parallel (PIC) interference cancellation to adapt to different services under different circumstances for uplink wideband CDMA system. The processing delay and computational complexity can be adjusted based on system loading and required performance. The interference between data channel signals and pilot channel signals under multipath fading channel are also taken into consideration. This results in better quality both on channel parameter estimation and user data detection. Compared with SIC and PIC, the proposed scheme shows better performance with reasonable hardware while it needs shorter processing delay than SIC.