Conference Ocean's 2008/MTS IEEE. Held at Kobe Conference center. Kobe Japan.

1. Karim OUERTANI, Samir SAOUDI, Mahmoud AMMAR
institut Telecom / Telecom Bretagne, Signal & Communications
Department
Technopˆle de Brest-Iroise, CS 83818 - 29238 Brest Cedex, FRANCE
o
E-mail: karim.ouertani@telecom-bretagne.eu
5. Simulation Results
Summary— In this work a novel channel estimation scheme is proposed for a RAKE
receiver operating in a time varying multi-path channel. The approach is an extension
0
of the well known nonlinear interpolation channel estimator, which is based on inter- 10
RAKE CE
polating the channel estimates from pilot symbol sequence. The proposed technique RAKE+Lagrange CE
RAKE PKC
RAKE+Lagrange PKC
manages to combine the obtained samples over one chip duration using a Lagrange 10
−1
interpolation filter, and thereby enhances the signal-to-noise ratio and improves the
quality of channel estimates. We also investigate optimal power assignment for the pi-
lot and data channels. Simulation results allowed us to pinpoint optimum pilot-to-data 10
−2
channel power ratio for the best bit error performance.
BER
−3
10
1. Correlation based channel estimation
−4
10
−5
10
−30 −25 −20 −15 −10 −5 0
SNR(dB)
Effect of imperfect channel estimation - K = 3 users.
In the figure :
Coherent RAKE block diagram with correlation based channel estimation. • ”PKC” refers to the Perfectly Known Channel simulation case.
• ”CE” refers to the Channel Estimation simulation case.
• Conventional correlation based channel estimation with a RAKE receiver. 10
0
RAKE K=5
• The CDMA signal is spread to the chip rate with an SF-long Walsh code. RAKE K=3
RAKE+Lagrange K=5
RAKE+Lagrange K=3
• The spread signal is oversampled by an oversampling factor Ns = 4. RAKE+Lagrange K=1
−1
10
• The signal is transmitted through a multipath Rayleigh fading channel, with a channel
response :
L
BER
−2
Gk (i) = gk,l(i)δ(iT − τk,l) (1) 10
l=1
−3
10
2. Channel estimation with Lagrange prefiltering
−4
10
rd
• The Ns = 4 samples corresponding to one chip are input to a 3 order Lagrange −30 −25 −20 −15
SNR(dB)
−10 −5 0
interpolation filter [1],[2] to get an interpolated chip value estimates. BER Vs SNR for conventional channel estimation (’RAKE’) and proposed
• Despreading process is performed with the interpolated chip estimates. channel estimation (’RAKE+Lagrange’) - K=1, 3 and 5 users.
• The Lagrange interpolation filters are widely used in numerous applications : sampling
rate conversion, digital communications, FIR filters design, etc.
6. Power Allocation
0
10
SNR = −16dB 0
10
SNR = −12dB 50% of Power Applied to Pilot Symbol
60% of Power Applied to Pilot Symbol
−1
−1 10
10
BER
BER
−2
10
Desired chip value interpolation by a 3rd order Lagrange filter. −2
10
• The filter coefficients are obtained as follows, 10
−3
−3
10
−4
0 10 20 30 40 50 60 70 80 90 10
−30 −28 −26 −24 −22 −20 −18 −16 −14 −12 −10
% of Power Applied to Pilot Symbol SNR(dB)
N d−k
hd(p) = f or p = 0, 1, 2, ..., Ns (2) BER vs percentage of power applied to BER performance with optimum
k=0 p − k pilot channel. pilot power allocation.
• N is the filter order, N = 3. • The amount of power applied to the pilot signal was varried from 1 % to 90 %.
• d is the delay to be fractionally • The optimum power allocation between the pilot channel and the data channel was
approximated, D = 4 . Tc investigated under the assumption of constant total transmit power.
• The lowest bit error rate is obtained for 60 % of the signal power applied to the pilot
• Structure of the interpolation scheme from the oversampled received signal frame : channel.
chip 0 chip n chip SF−1
Pilot 7. References
[1] T. I. Laakso, V. Valimaki, M. Karjalainen, and U. K. Laine. Splitting the Unit
r(n) r(n+1) r(n+2) r(n+3) Delay, in IEEE Signal Processing Magazine, pages: 30-60, January 1996.
h(0) h(1) h(2) h(3) [2] E. Simona Lohan, M. Renfors Performance Analysis of the RAKE Receiver in
the Presence of Multipath Delay Estimation Errors and Rician Fading Chan-
nels, in European transactions on telecommunications, vol. 14, pages: 435-447, July
2003.
[3] M. Meyr, M. Moeneclaey, and S. A. Fechtel. Digital Communication Receivers
: synchronization, channel estimation, and signal processing, Wiley series in
r(n+d) telecommunications and signal processing, Jhon Wiley & sons, 1998.
A 3rd order Lagrange interpolation filter.