802.11n Specification and the use of Space-Time Wireless Channels

1. 802.11n Specification and the use ofSpace-Time Wireless Channels Shad Nygren April 27, 2006 Del Mar Electronics Show

2. Objectives • Discuss the history and present state of the 802.11n specification. • Discuss MIMO, Space-Time Wireless Channels and Space-Time Block Codes which are one of the most interesting aspects of the 802.11n specification. • Understand how the magic of MIMO and Space-Time Wireless Channels work.

3. About Me • Master’s Degree in Computer Science from University of Nevada, Reno • 24 years experience with computers, networking and wireless communications

4. 802.11n History • 1999, 802.11a/b standards ratified by IEEE • June 2003, 802.11g ratified by IEEE. • 802.11g was based on OFDM from 802.11a but using the 2.4GHz band and backwards compatible with 802.11b • January 2004, IEEE forms new 802.11 Task Group (TGn) to investigate higher data rates

5. 802.11n History Cont • Standards Process: From many proposals down to two • TGnSync • WWiSE • After much debate these two groups created a Joint Proposal • October 2005, the Enhanced Wireless Consortium (EWC) was founded by Intel, Broadcom, Marvell, Atheros and others

6. 802.11n Progress in 2006 • Jan 19, 2006, IEEE 802.11n task group approved the Joint Proposal’s specification based on EWC’s specification. • March 2006 IEEE 802.11 Working Group sent the 802.11n Draft to its first letter ballot. • Currently working its way thru the IEEE standards process. Hopefully a final standard will be in place in about a year.

7. 802.11n Goals • Investigate next generation wireless LAN technology capable of supporting multimedia applications • Provide higher data rates than 802.11b/g – At least 100Mbps at MAC layer • Backwards compatibility with 802.11b/g

8. 802.11n Physical Layer • Operates in 2.4GHz and/or 5GHz unlicensed bands • Uses OFDM like 802.11a/g • Backwards compatible and mixed mode interoperable with 802.11a/b/g • High Throughput (HT) and 40MHz modes • Optionally uses MIMO

9. 2.4GHz Unlicensed Band 802.11b Channel Frequency Map

10. 802.11g OFDM • 64 point FFT • 52 OFDM subcarriers • 48 Data Carriers • 4 Pilot Carriers • 12 unused carriers • Carrier Separation 0.3125MHz (20MHz/64) • Total Bandwidth 20MHz with occupied bandwidth of 16.6MHz • Symbol duration 4us with 0.8us guard interval

11. OFDM Carriers Source: International Engineering Consortium http://www.iec.org/online/tutorials/ofdm/topic04.html

12. 802.11a/g OFDM Rates250,000 Symbols per Sec

13. 802.11a/g OFDM Physical Layer • Divided into two elements • PLCP – Physical Layer Convergence Protocol prepares frames for transmission and directs the PMD to transmit and receive signals, change channels etc • PMD – Physical Medium Dependant layer provides actual transmission and reception over the wireless medium by modulating and demodulating the frame transmissions

14. Options for Increasing Data Rate • Double the Clock Rate – From 20MHz (250,000 Symbols per Second) to 40MHz (500,000 Symbols per Second) • Double the Number of Carriers – From 64 to 128, not increasing the bandwidth • Use Higher Order Modulation – From 64QAM (6 bits / symbol) to 4096QAM (12 bits / symbol)

15. Options for Increasing Data Rate • OFDM with Bit Loading – Different Modulation Per Carrier • Better Code – Turbo or Low Density Parity Check • MIMO – Multiple Input Multiple Output antennas for multiple data streams

16. Higher Data Rate Considerations

17. 802.11n OFDM • 20MHz High Throughput Mode • 56 OFDM subcarriers • 52 Data Carriers • 4 Pilot Carriers • 40MHz High Throughput Mode • 114 OFDM subcarriers (2 extra subcarriers) • 108 Data Carriers (4 extra data carriers) • 6 Pilot Carriers (2 less pilot carriers)

18. 802.11n Mandatory Features • Frame Aggregation • Block ACK • N-immediate ACK – Block ACK between two HT peers using an immediate Block Ack policy • Long NAV – Provides protection for a sequence of multiple PPDUs

19. NAVNetwork Allocation Vector • Counter resident at each station that represents the amount of time that the previous station needs to send its frame. • The NAV must be zero before a station can attempt to send a frame. • The transmitting station calculates the amount of time necessary to send the frame based on the frame’s length and data rate.

20. NAVNetwork Allocation Vector • The transmitting station places a value in the duration field in the header representing the time required to transmit the frame. • When stations receive a frame, they examine the duration field value and use it as the basis for setting their corresponding NAV. • This process reserves the medium for the sending stations.

21. 802.11n Optional Features • Advanced Coding – Using different coding per OFDM carrier • Green Field mode • Beamforming • Short Guard Interval – Reduce from 800ns (250,000 symbols per second) to 400ns and send 277,778 symbols per second • Space Time Block Coding

22. 802.11n Modes • Legacy Mode – packets are transmitted in the legacy 802.11a/g format • Mixed Mode – packets are transmitted with a preamble compatible with 802.11a/g so they can be decoded by legacy devices while the rest of the packet is transmitted in the new mode • Green Field – optional mode where the packets are transmitted without the legacy compatibility part

23. 802.11n for 20/40MHz operation • 40MHz comprised of two adjacent 20MHz channels • One Control Channel • One Extension Channel • Beacon is sent in legacy mode on the control channel only • A single BSS may include: • 20MHz-only capable stations • 20/40MHz capable stations • Legacy stations • Clear Channel Assessment will be done on the control channel and possibly on the extension channel. The results will then be combined.

24. 802.11n Modulation and Coding per Spatial Stream

25. 802.11n Modulation and Coding Two Spatial Streams

26. 802.11n Modulation and Coding Four Spatial Streams

27. MIMO Any sufficiently advanced technology is indistinguishable from magic. Arthur C. Clarke

28. MIMO Magic • MIMO is not magic but is an advanced RF communications technology based on valid mathematical and scientific principals • MIMO does not violate Shannon’s Law • Pronounced “MyMoe” – This was standardized by a vote at an IEEE meeting.

29. Multiple Antennas • Well studied topic for the past few years • OFDM is very well suited for use with multiple antennas • Many existing 802.11 products already have 2 antennas, using switched diversity • Additional component required for exploiting full diversity is an additional RF front-end • Recent advances in RF technology will make this cost effective in the near future

30. Antenna Diversity • Space Diversity • Polarization Diversity • Pattern Diversity • Transmit Diversity

31. Temporal Diversity • Frequency Diversity • Code Diversity • Time Diversity

32. Diversity Reception • Idea from which MIMO arose • Several methods are possible • Selection Combining • Switched Combining • Equal Gain Combining • Maximum Ratio Combining

33. Maximum Ratio Combining (MRC) • A way of combining signals from diversity reception • The signals are weighted according to their Signal to Noise ratios and then combined

34. Diversity Gain Definition • Diversity Transmission - is a method for improving reception of a transmitted signal, by receiving and processing multiple versions of the same transmitted signal • Diversity Gain - is a value that quantifies the performance improvement by a diversity transmission scheme in a fading channel

35. Diversity Gain for Multiple Branches • The performance gain of a system can be quite dramatic • For example, with a system using QPSK requiring a maximum BER of 0.01 diversity gain is 13.9dB Source: Space-Time Wireless Channels by Durgin

36. Shannon Capacity for Conventional Systems • 1948 Claude Shannon’s Noisy Channel Coding Theorem describes maximum efficiency of error correcting codes • Shannon-Hartley Theorem describes what channel capacity is for finite bandwidth continuous time channel with Gaussian Noise • With Single Transmit and Single Receive Antenna • B is Bandwidth • SINR is Signal to Interference and Noise Ratio • C can be increased by increasing B or SINR

37. Shannon Capacity forConventional Multi-Antenna Systems • SINR ratio can be improved by using multiple antennas • Overall capacity can be improved because the SINR is improved • Multiple Transmit Antennas • Multiple Receive Antennas • Combination of multiple Transmit and Receive antennas

38. SINR withMultiple Receive Antennas • N antennas are used at the receiver • They receive N various faded copies of the signal • Which can be coherently combined to produce a N2 increase in power • There are also N sets of noise/interference that add together as well

39. Shannon Channel Capacity with Multiple Receive Antennas • With this N*SINR the channel capacity of the system becomes

40. SINR withMultiple Transmit Antennas • If M antennas are used at the transmitter the total power is divided into the M branches. • The power per transmitter antenna drops but signals may be phased so that they add coherently • Noise + interference is the same as SISO • The result is a M-fold increase in SINR

41. Shannon Channel Capacity with Multiple Transmit Antennas • With this M*SINR the channel capacity of the system becomes

42. SINR with Multiple Transmit and Multiple Receive Antennas • SINR is a combination of the MISO (multiple transmit antennas) SIMO (multiple receive antennas) cases

43. Shannon Capacity of a Single Channel with Multiple Transmit and Multiple Receive Antennas • With this M*N*SINR the channel capacity of the conventional system using multiple antennas becomes

44. Conventional Multi-Antenna Transmission • Conventionally it is not possible to send more than one simultaneous signal per frequency • Seemingly the best approach would be to weight the transmitter elements to maximize signal power at the receiver. Source: DATACOMMRESEARCH

45. Increasing Shannon capacity by using multiple spatial channels • A shift in perspective led to the development of truly multiple-input, multiple-output systems that have capacity greater than the best conventional single channel system. • Dramatic capacity increases are possible if we consider different signals sent thru each transmitter antenna.

46. Multi-Channel MIMO • Different signals are are sent thru each transmitter antenna Source: DATACOMMRESEARCH

47. Won’t the physical channels interfere with each other?I don’t believe this is possibleShow me the Math

48. MIMO Channel Matrix Model • y = received vector • x = transmitted vector • H = channel matrix • t = time, τ = delay

49. Processing the MIMO Signalat the Transmitter • At the transmitter a linear signal processing operation V is performed on the transmitted signal vector x and the result is Vx(t) • V is an M x M unitary matrix with the property VV† = I where I is the identity matrix and the † operator indicates the conjugate transpose or Hermitian operation • Unitary matrices do not change the geometrical length of vectors so no power is added or subtracted from the transmitted signal

50. Processing the MIMO signalat the Receiver • At the receiver a linear processing signal processing operation U† is performed on the received signal vector y • U† is an N x N unitary matrix where U†U = I • I is the identity matrix which means that no power is being added or subtracted from the received signal