Title: ECSE 6592 Wireless Ad Hoc and Sensor Networks Spatial Diversity in Wireless Networks
1ECSE 6592 Wireless Ad Hoc and Sensor Networks
Spatial Diversity in Wireless Networks
2Introduction
- Main characteristic in wireless channels-
randomness in users transmission channels and
randomness in users geographical locations - Diversity- Convey information through multiple
independent instantiations of random attenuations - Spatial diversity- through multiple antennas or
multiple users
3Wireless Channel Characteristics
- Three kind of attenuations-path loss, shadowing
loss, fading loss - Path loss Signals attenuate due to distance
- Shadowing loss absorption of radio waves by
scattering structures - Fading loss constructive and destructive
interference of multiple reflected radio wave
paths
4Attenuation in Wireless Channels
5Wireless Channel Characteristics
- Key parameters of wireless channels-coherence
time, coherence bandwidth - If symbol periodgtcoherence time, the channel is
time selective - If symbol periodlt channel delay spread, the
channel is frequency selective
6MIMO Channel Model
H(kl) is the lth tap of the Mr x Mt channel
response matrix, z is noise vector
7Theoretical Consideration
- Information-Theoretic results for
multiple-antenna channels - Information-Theoretic results for multi-user
channels - Diversity order
- Design consideration
8Information-Theoretic results for multiple
antenna channels (1)
9Information-Theoretic results for multiple
antenna channels (2)
- Assume the receiver had access to perfect channel
state information through training or other
methods
10Information-Theoretic results for multiple
antenna channels (3)
- At high SNR the outage probability is the same as
frame error probability in terms of SNR exponent - For given rate, we can compare performance
through an outage analysis
11Information-Theoretic results for multi-user
channels
- Two types of topology- multiple access channel
and broadcast channel
12Information-Theoretic results for multi-user
channels
13Diversity Order and multiplexing gain
14Relation between Diversity Order and Multiplexing
Gain
15Relation between rate and SNR
16Design Consideration
- Space time code with low decoding complexity and
achieving maximum diversity order - Trade-off between diversity order and rate
- If system is delay-constrained, design with high
diversity order and lower data rate - Fairness for resource sharing between users
- Cross layer design
17Signal transmission
- Transmitter Techniques- spatial multiplexing,
space-time trellis code and block codes - Receiver techniques- joint equalization with
channel estimation, space-time code decoding
18Spatial Multiplexing (Bell Labs Layered
Space-Time Architecture, BLAST)
- Multiple transmitted data streams are separated
and detected successfully using a combination of
array processing (nulling) and multi-user
detection (interference cancellation) techniques - A broadband channel scenario using a MIMO
generalization of classical decision feedback
equalizer (DFE) - The nulling operation is performed as
feed-forward filter and the interference
cancellation operation is performed by the
feedback filter
19Spatial Multiplexing-continued
- May have error propagation
- The presence of antenna correlation and the lack
of scattering richness in the propagation
environment reduce the achievable rates of
spatial multiplexing techniques - Enhancement Use MMSE interference cancellation,
perform ML detection for first few streams
20Space time coding
- Improve downlink performance without requiring
multiple receive antennas - Easily combined with channel coding
- Do not require channel state information at the
transmitter - Robust against non-ideal operating conditions
21Space-time Trellis codes
- Maps information bit stream into Mt streams of
symbols - Decoding complexity increases exponentially as a
function of the diversity level and transmission
rate - Example
22Space time block codes
23Cons and pros of space time block codes
- Achieve full diversity at full transmission rate
for any signal constellation - Does not require CSI at the transmitter
- ML decoding involves only linear processing at
the receiver - Does not provide coding gain
- A rate-1 STBC cannot be constructed for any
complex signal constellation with more than two
transmit antennas - Simple decoding rule valid only for flat-fading
channel where channel gain is const over two
consecutive symbols
24Tradeoff between diversity and throughput
- BLAST achieves max spatial multiplexing with
small diversity gain - Space time codes achieves max diversity gain with
no multiplexing gain - Linear dispersion codes (LDC) achieve higher
rate with polynomial decoding complexity for a
wide SNR range - Build in the diversity into the modulation
25Build Diversity into modulation
26Receiver techniques
- Coherent and non-coherent techniques
- Coherent technique require channel state
information by channel estimation or training
sequences and feed this to joint
equalization/decoding algorithm - Non-coherent techniques does not require CSI and
more suitable for rapidly time-varying channels
27Joint Equalization/Decoding techniques
- M-BCJR algorithm at each trellis step, only M
active states associated with the highest metrics
are retained - Significant reduction in the number of
equalizer/decoder states
28Sphere decoder
- Suitable for codes with lattice structures
- Perform ML search with low computation complexity
29Joint Equalization/Decoding of space time Block
codes
- Eliminate inter-antenna interference using a low
complexity linear combiner - Single-carrier frequency domain equalizer (SC-FDE)
30Performance of SC-FDE
31Non-coherent techniques
- Does not require channel estimation
- Include blind identification and detection
schemes - Exploit channel structure (finite impulse
response), input constellation (finite alphabet),
output (cyclostationarity) to eliminate training
symbols - Use ML receiver which assumes statistics about
channel state but not knowledge of the state
itself
32Summary in signal transmission
- Mitigate fading effect by using space diversity
- Use MIMO to realize spatial rate multiplexing
gains - Use equalization techniques (ex M-BCJR, SC-FDE)
to mitigate channel frequency selectivity - Use channel estimation and tracking, adaptive
filtering, differential transmission/detection to
mitigate time selectivity
33Networking issues
- Medium sharing resource allocation
- Mobility and routing
- Hybrid networks
34Resource allocation
- Allocation criteria rate-based criteria and
job-based criteria - Rate-based criteria provide average data rates to
users which satisfy certain properties - Job-based criteria schedule data delivery in
order to optimize various QoS guarantees based on
the job requests
35Resource allocation-Rate-Based QoS criteria
- Utilize the multi-user diversity inherently
available in wireless channels - Schedule users when their channel state is close
to peak rate it can support - gt inherent unfairness
- Keep track of the average throughput Tk(t) and
rate Rk(t), transmit the user with the largest
Rk(t)/ Tk(t) among the active users - If channel is slow time-varying, introduce random
phase rotations between the antennas to simulate
fast fading
36Impact of spatial diversity
- Multi-antenna diversity provide greater
reliability by smoothening channel variations - Multi-user diversity utilize the channel
variability across users to increase throughput - Choose diversity techniques according to channel
conditions, mobility and application constraints - For example, low delay-applications with high
reliability requirement may use multi-antenna
diversity with space time codes
37Hybrid Networks
- Two approaches to increasing TCP efficiency in
hybrid networks - Reduce error rate in wireless channel by using
more sophisticated coding schemes, such as
space-time codes - Use explicit loss notification (ELN) to inform
the sender that the packet loss occurred due to
wireless link failure rather than congestion in
wired part
38Space time code and TCP throughput
- STBC-enhanced 802.11a achieves a particular
throughput value at a much lower SNR value than
the standard 802.11a - STBC modify the SNR region under which a
particular transmission rate should be chosen - STBC increase the transmission range and improve
robustness of WLANs
39STBC-enhanced 802.11a
- The difference between STBC 802.11a and 802.11a
becomes smaller when channel quality is
sufficiently good - STBC-802.11a can switch to faster transmission
mode at much lower SNR values
40Conclusion
- In wireless networks, power and spectral
bandwidth are limited - Limitation on signal processing at terminal and
requirement of sophisticated resource allocation
techniques due to variation in capacity - Spatial diversity improves data rates and
reliability of individual links - Space time codes improves link capacity and
system capacity through resource allocation
41Future works
- Space time code design
- Implementation issues-low-cost multiple RF chains
and low-power parallelizable implementation of
STC receiver signal processing algorithm - Receiver signal processing-the development of
practical adaptive algorithm that can track rapid
variation of large number of taps in MIMO channel
and/or equalizer - Standardization activities
42Reference
- 1S. N. Diggavi, N. Al-Dhahir, A. Stamoulis, and
A.R. Calderbank, Great Expectations The Value
of Spatial Diversity in Wireless Networks,
Proceeding of The IEEE, Vol. 92, No. 2,
pp219-270, Feb 2004 - 2 Sergio Verdu, Multiuser Detection,
Cambridge University Press, 1998