January 2004

H. Sampath, PhD, Marvell Semiconductor Slide 1

doc.: IEEE 802.11-04/0075r0

Submission

Pros and Cons of Circular Delay Diversity Scheme for MIMO-OFDM System

Hemanth Sampath Ravi Narasimhan

Marvell Semiconductor, Inc.

January 2004

H. Sampath, PhD, Marvell Semiconductor Slide 2

doc.: IEEE 802.11-04/0075r0

Submission

Circular Delay Diversity Scheme

• 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(1) s(2) .... s(N)

s(1) s(2) ....

s(1) s(2) ....

s(N)

s(N)

t 2

tMT

1

2

MT

1

2

MR

N = FFT SizeMR x MT

January 2004

H. Sampath, PhD, Marvell Semiconductor Slide 3

doc.: IEEE 802.11-04/0075r0

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 tk f / N) + … + hN(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, PhD, Marvell Semiconductor Slide 4

doc.: IEEE 802.11-04/0075r0

Submission

Resultant Channel (E-LOS, 20 MHz, 64-pt FFT)

Delay Diversity leads to increase in frequency selectivity (proportional to circular delays!)

January 2004

H. Sampath, PhD, Marvell Semiconductor Slide 5

doc.: IEEE 802.11-04/0075r0

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.

• 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, PhD, Marvell Semiconductor Slide 6

doc.: IEEE 802.11-04/0075r0

Submission

Drawbacks of Delay diversity

• Sensitivity to K-factor: For LOS channels, 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 :

• More for higher K-factor

• More for larger circular delay.

• More for weaker FEC.

– Performance loss results in a constant shift in the BER vs. SNR plot.

January 2004

H. Sampath, PhD, Marvell Semiconductor Slide 7

doc.: IEEE 802.11-04/0075r0

Submission

Simulations• Packet Error Rate (PER) vs. SNR results for 1x2 & 1x1 system.

• 1x2 system employs 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 RF impairments.– 1000 byte packets, 20 MHz channelization, 64 point FFT.– Channel 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, PhD, Marvell Semiconductor Slide 8

doc.: IEEE 802.11-04/0075r0

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, PhD, Marvell Semiconductor Slide 9

doc.: IEEE 802.11-04/0075r0

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, PhD, Marvell Semiconductor Slide 10

doc.: IEEE 802.11-04/0075r0

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, PhD, Marvell Semiconductor Slide 11

doc.: IEEE 802.11-04/0075r0

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, PhD, Marvell Semiconductor Slide 12

doc.: IEEE 802.11-04/0075r0

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 handle 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, PhD, Marvell Semiconductor Slide 13

doc.: IEEE 802.11-04/0075r0

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 802.11g receivers, and scalable with number of antennas.

– Disadvantages: The delay parameter needs to be adjusted based on K-factor and delay spread, requiring feedback.