1. Physical Layer Approach for 802.11n
doc.: IEEE /0abcr0
July 2004
Physical Layer Approach for n
Mustafa Eroz, Feng-Wen Sun, Lin-Nan Lee &
Mallik Moturi
Hughes Network Systems
11717 Exploration Lane
Germantown, MD 20876
Mustafa Eroz, Hughes Network Systems
Mustafa Eroz, Hughes Network Systems

2. 802.11n Physical Layer Design Issues
July 2004
802.11n Physical Layer Design Issues
Achieving more than 100 Mbps information throughput in 20MHz channel without using additional power/bandwidth or accepting range compromise is very difficult
This is a n mandatory requirement
802.11a/g sends 54 Mbps at “top speed”, information throughput is about 60% of that.
Assuming similar MAC and Link Layer overhead, we need to be about 3-4 times more efficient than a/g.
This translates to about bits per channel use with channel coding and other overhead, comparable to sending 56 kbps information through a telephone line.
In the wireless channel, this kind of transmission efficiency can only be achieved with some form of Multi-Input/Multi-Output (MIMO) space-time diversity scheme with more than two transmit antenna.
Mustafa Eroz, Hughes Network Systems

3. Technical Challenges of MIMO Transmission
July 2004
Technical Challenges of MIMO Transmission
To meet safety and interference regulations, total transmit power must be the same as single transmit antenna case
Early MIMO schemes such as Blast, Space-Time codes, and their variations require very high SNR to achieve acceptable packet error rate performance, leading to very short range for required bandwidth efficiency
The required information throughput at reasonable transmit power can only be achieved in conjunction with very powerful error correction coding schemes
Near Shannon-limit codes have been standardized by the 3rd Generation Wireless (3GPP & 3GPP2) for mobile wireless application in 1999, and Digital Video Broadcast (DVB) project for direct broadcast via satellite recently (2003)
We expect near Shannon-limit codes redesigned for the MIMO architecture and adapted to the general framework to provide the only solution which meets this challenge.
Mustafa Eroz, Hughes Network Systems

4. Forward Error Correction (FEC) Coding as the Enabling Technology
doc.: IEEE /0abcr0
July 2004
Forward Error Correction (FEC) Coding as the Enabling Technology
For the past several decades, forward error correction coding has been responsible
for the most significant improvements for many communication systems.
Mustafa Eroz, Hughes Network Systems
Mustafa Eroz, Hughes Network Systems

5. July 2004
Applications of State-of-the-Art FEC Codes to Consumer Marketplace – Milestone Examples
Design of turbo coding techniques for mobile wireless channels and adoption of optimized codes, interleaver, and puncturing patterns for 3GPP and 3GPP2 standards (1999).
Major breakthrough since Viterbi decoding (1968)
These codes are implemented in more than 50 million cdma2000 and W-CDMA/UMTS cellular phones already
Adoption of high-performance low-density parity check (LDPC) codes for DVB-S2 standard (2003)
Shannon capacity for SISO channel has been reached for all practical purposes.
These codes have been implemented in silicon and will be deployed in next generation broadcast and interactive satellite networks shortly.
Mustafa Eroz, Hughes Network Systems

6. Turbo Codes for 3G Wireless
July 2004
Turbo Codes for 3G Wireless
Turbo codes provides substantial gain in third generation wireless systems
(v=30 kph)
Mustafa Eroz, Hughes Network Systems

7. July 2004
3G Turbo Code Design
Even though the turbo codes were known a few years earlier, key aspects must be designed
Constraint length and best generator polynomials for the constituent code
Best puncturing tables for all the required code rates
A flexible algorithmic turbo interleaving technique with guaranteed distance criteria for any interleaver sizes, avoiding separate interleaver table for each block size
All these aspects were optimized, specified, and adopted by 3GPP and 3GPP2
Mustafa Eroz, Hughes Network Systems

8. July 2004
LDPC Codes
Low density parity check (LDPC) codes offer a much richer platform for various
implementation techniques than turbo codes.
While any randomly chosen turbo code would have reasonably good performance,
large block size LDPC codes have a greater potential to approach Shannon
limit. But, a great deal of expertise is needed to design such LDPC codes.
Similarly, if the parity check matrix design of LDPC codes is done with no
restrictions, the resulting decoder complexity would be high.
DVB-S2 LDPC codes provide performance close to Shannon limit while
avoiding decoder complexity.
Mustafa Eroz, Hughes Network Systems

9. LDPC Advantages Excellent performance
July 2004
LDPC Advantages
Excellent performance
Very simple LDPC decoder implementation
More suitable to high speed applications since LDPC decoder
operations can be performed in parallel.
Adoptable to any higher-order modulation with simple mapping.
Mustafa Eroz, Hughes Network Systems

10. July 2004
What is LDPC Code ?
LDPC codes are linear block codes with sparse parity check matrix H(n-k)xn
An example LDPC code with n=8 and R=1/2
Bit nodes and check nodes communicate with each other iteratively
to find the transmitted values of bit nodes.
Mustafa Eroz, Hughes Network Systems

11. Simple LDPC Encoding Restrict the parity check matrix to where
July 2004
Simple LDPC Encoding
Restrict the parity check matrix to
where
Encode information block
into codeword
Using
and recursively solving for parity bits :
Solve
Mustafa Eroz, Hughes Network Systems

12. LDPC Decoder Algorithm
July 2004
LDPC Decoder Algorithm
Demod output:
where N is the block size
( log-likelihood ratio of received bits)
Initialization:
Mustafa Eroz, Hughes Network Systems

13. Check Node Update It can be shown that July 2004
Mustafa Eroz, Hughes Network Systems

14. Bit Node Update & Hard Decision
July 2004
Bit Node Update & Hard Decision
Bit Node Update:
Hard Decision:
Stop the iterations if hard decisions satisfy all the parity check equations
or if the maximum number of iterations is reached.
Mustafa Eroz, Hughes Network Systems

15. Performance in AWGN Channels
July 2004
Performance in AWGN Channels
Mustafa Eroz, Hughes Network Systems

16. Up to 40% Throughput Improvement over DVB-S1
July 2004
Up to 40% Throughput Improvement over DVB-S1
Mustafa Eroz, Hughes Network Systems

17. LDPC Codes for MIMO Channels
July 2004
LDPC Codes for MIMO Channels
Advanced LDPC codes bring the performance of practical communication system
very close to theoretical limits for single-input, single-output AWGN.
With clever customization and optimization, LDPC codes can approach Shannon
limit for MIMO fading channels as well.
We intend to submit a physical layer proposal based on a set of LDPC codes
highly optimized for n application before the next meeting.
Mustafa Eroz, Hughes Network Systems