doc.: ieee 802.11-04/0075r1 submission january 2004 h. sampath, r. narasimhan, marvell...
TRANSCRIPT
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 1
doc.: IEEE 802.11-04/0075r1
Submission
Advantages and Drawbacks of Circular Delay Diversity for MIMO-OFDM
Hemanth Sampath Ravi Narasimhan
[email protected]@marvell.com
Marvell Semiconductor, Inc.
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 2
doc.: IEEE 802.11-04/0075r1
Submission
Circular Delay Diversity
• Signal on the kth Tx antenna is circularly delayed by tk
samples.• To obtain full Tx-diversity, we must have tk > effective channel delay spread [Gore & Sandhu -2002]
s(0) s(1) .... s(N-1)
s(0) s(1) ....
s(0) s(1) ....
s(N-1)
s(N-1)
t 2
tMT
1
2
MT
1
2
MR
N = FFT Size
MR x MT
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 3
doc.: IEEE 802.11-04/0075r1
Submission
Frequency Domain Representation
• MR x MT channel H(f) collapses to a MR x 1 channel hr(f) at the receiver.
• hr (f) = h1(f) + h2(f) exp(-j2 t2 f / N) + … + hMT(f) exp(-j2tMT f/ N )where hi(f) is the MR x 1 channel from ith transmit antenna.
1
2
MT
1
2
MR
X (f )
X (f ) exp (-j2 t f / N ) 2
X (f ) exp (-j2 t f / N )MT
H ( f )
H ( f ) : MR x MT channel at tone f
f = 0,1,2, ..... , N-1
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 4
doc.: IEEE 802.11-04/0075r1
Submission
Resultant Channel (E-LOS, 20 MHz, 64-pt FFT)
Circular Delay Diversity leads to increase in frequency selectivity (proportional to circular delays!)
Tone Index
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 5
doc.: IEEE 802.11-04/0075r1
Submission
Advantages of Delay Diversity
• Provides transmit diversity gain in NLOS fading channels if
circular delay > channel delay spread.– Stronger FEC Higher gain.
– Diversity gain improves the slope of BER vs. SNR plots.
– Note: Introducing high circular delay >> channel delay spread can lead to performance loss due to limited FEC correction capability.
• Scalable to number of transmit antennas– Orthogonal ST block codes (e.g. Alamouti) are not scalable with number of
antennas.
• Backwards compatible with legacy 802.11 systems.
– Does not require an increase in number of PHY preambles, unlike Orthogonal ST block codes.
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 6
doc.: IEEE 802.11-04/0075r1
Submission
Drawbacks of Delay Diversity
• Sensitivity to K-factor: For LOS channels with high K-factor, delay-diversity converts static channel to a channel with increased frequency-domain nulls.
– Leads to performance loss w.r.t legacy systems
(Example: 1x2 has worse performance compared to 1x1).
– Performance loss :
• Greater for higher K-factor
• Greater for larger circular delay.
• Greater for weaker FEC.
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 7
doc.: IEEE 802.11-04/0075r1
Submission
Simulations• Packet Error Rate (PER) vs. SNR results for 1x2 & 1x1 system.
• 1x2 system employs cyclic delay diversity. – 2nd antenna has delay of 2 samples w.r.t 1st antenna. – Notation: 1x2 - [0, 2]– 1 sample = 50 nsec.
• Assumptions:– Perfect channel estimation, perfect synchronization, no phase noise, no
IQ imbalance, no nonlinearities in RF front-end.– 1000 byte packets, 20 MHz channelization, 64 point FFT.– Channels generated using Laurent Schumacher v3.2 Matlab code.– Unit transmit power per OFDM data tone.– Channel realizations for each Tx-Rx antenna pair has average power
(across all realizations) of unity.
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 8
doc.: IEEE 802.11-04/0075r1
Submission
12 Mbps in B-NLOS (15 nsec RMS delay spread & K= -100 dB)
At 10% PER, gain of 1x2-[0,1] is 0.5 dB; gain of 1x2-[0,32] is 1 dB !
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 9
doc.: IEEE 802.11-04/0075r1
Submission
54 Mbps in B-NLOS channel (15 nsec RMS delay spread & K= -100 dB)
At 10% PER, gain of 1x2-[0,1] is 0 dB; loss of 1x2-[0,32] is 2.5 dB !
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 10
doc.: IEEE 802.11-04/0075r1
Submission
54 Mbps in E-LOS channel (100 nsec RMS delay spread & K=6 dB)
At 10% PER, loss of 1x2-[0,1] is 2 dB; and loss of 1x2-[0,32] is 4.5 dB
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 11
doc.: IEEE 802.11-04/0075r1
Submission
12 Mbps in E-LOS channel (100 nsec RMS delay spread & K=6 dB)
At 10% PER, loss of 1x2 is 1.0 dB
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 12
doc.: IEEE 802.11-04/0075r1
Submission
Optimum Choice of Circular Delay Parameters (k)
• High K-Factor Low k
• Low K-Factor and low delay spread Low k
• Low K-Factor and high delay spread High k
• Weaker FEC Lower k
– E.g: Rate 3/4 code cannot exploit high frequency selectivity.1. Delay parameters needs to be optimized on a per-user basis, depending on coding rate, K-factor and delay-spread !
2. Requires (coarse) estimation / feedback of K-factor and delay spread!
January 2004
H. Sampath, R. Narasimhan, Marvell Semiconductor Slide 13
doc.: IEEE 802.11-04/0075r1
Submission
Conclusions
• Delay diversity provides transmit diversity gain for NLOS fading channels, if delays > effective channel delay spread.
• Delay diversity leads to performance loss in channels with non-zero K-factor.
• Implementation Issues:
– Advantages: The scheme is backwards compatible with 802.11a/g receivers, and scalable with number of antennas.
– Disadvantages: The delay parameter needs to be optimized using feedback of K-factor and delay spread.