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Performance of Smart Antennas and PCF
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  1. Performance of Smart Antennas and PCF Ari Alastalo, Steven Gray and Venkatesh Vadde Nokia Research Center Steven Gray, Nokia

  2. Introduction • IEEE802.11 will be an important method for providing high rate low mobility data services • While existing capacity may seem high, particularly for IEEE802.11a, the enterprise environment may experience capacity limits • Antenna technologies offer a means to boast IEEE802.11 capacity without changing the existing PHYs Steven Gray, Nokia

  3. STA AP STA What is a SMART Antenna? Antenna that adjusts its beam pattern based upon the channel and interference between AP and STA Steven Gray, Nokia

  4. Approach to Model Performance • Measure channel using a channel sounder to determine • Multipath power profile as a function of time • Signal-to-interference ratio • Simulate PHY to obtain PER information as a function of channel measurements and the number of packets delivered as a function of time • Use PER, number of packets delivered by the PHY as a function of time and models of real-time traffic to examine delay and throughput using a PHY with and without smart antennas • PCF is used for delivery of audio and video packets Steven Gray, Nokia

  5. PHY Simulation Parameters Steven Gray, Nokia

  6. PHY Channel Sounder Approach • 127 chip pn sequence is transmitted at 5.3 GHz with a bandpass bandwidth of 30 MHz • A 32 element array with 0.5 wavelength space is used to downconvert the transmitted pn sequence • Snap shots of the delay spread are written to memory and stored on a hard disk For further information see: Jarmo Kivinen, Timo O. Korhonen, Pauli Aikio, Ralf Gruber, Pertti Vainikainen, and Sven-Gustav Häggman, IEEE Transactions on Instrumentation and Measurement, vol. 48, no. 1, pp. 39-44, 1999. Steven Gray, Nokia

  7. Subcarrier OFDM Symbol Index FFT RF A/D STA STA FFT RF A/D Decoding Antenna Combining FFT RF A/D Weight Calculation STA STA IFFT RF D/A IFFT RF D/A Coding & Modulation Antenna Weighting IFFT RF D/A Smart Antenna Operation (5GHz) Uplink Operation Downlink Operation Steven Gray, Nokia

  8. average over TX positions 1 Ruoholahti Heikkiläntie Airport 0.99 Airport 2 0.98 0.97 amplitude correlation coefficient 0.96 0.95 0.94 0.93 0 50 100 150 200 250 300 350 400 450 time [ms] Channel Correlation • The figure to the right shows the correlation coefficient for different measurement environments • For smart antenna operation, the AP needs the ability to probe the channel between any STA and AP Note: Ruholahti and Heikkiläntie are Nokia office buildings Steven Gray, Nokia

  9. MAC Simulation Parameters Steven Gray, Nokia

  10. Traffic Model: • All traffic measured in data-units/slot-time • 1 data-unit takes 1 slot-time for transmission • Max traffic in network = 1.0 • Audio and video traffic originates from “calls” made by the user • Calls are Poisson distributed; once placed, each call generates periodic packet traffic • Mean inter-call-arrival-time controls load on the network Steven Gray, Nokia

  11. Real-time traffic load = apkt_sz: audio packet size vpkt_sz: video packet size int_call_arvl_time: mean intercall arrival time (8:1 ratio between audio and video calls) int_apkt_arvl_time, int_vpkt_arvl_time: arrival time between audio packets and video packets respectively acalls, vcalls: number of audio anc video calls respectively Traffic Model (cont.): Steven Gray, Nokia

  12. -1 10 1 No SDMA 3-channel SDMA 0.9 0.8 0.7 -2 10 PCF Latencies Normalized Througput 0.6 0.5 0.4 -3 10 0.3 0.2 No SDMA DCF:PCF = 1:1 DCF:PCF = 1:1 0.1 3-channel SDMA Audio Packets Audio Packets -4 0 10 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 4 4 x 10 x 10 Packets/Second Packets/Seconds Throughput & Latency Curves: Audio Packets Steven Gray, Nokia

  13. -1 10 1 No SDMA 0.9 3-channel SDMA 0.8 0.7 -2 10 PCF Latencies 0.6 Normalized Throughput 0.5 0.4 -3 10 0.3 0.2 No SDMA DCF:PCF = 1:3 DCF:PCF = 1:3 0.1 3-channel SDMA Audio Packets Aduio Packets -4 10 0 0 0.5 1 1.5 2 2.5 3 3.5 4 0 1 2 3 4 4 4 x 10 x 10 Packets/Second Packets/Seconds Throughput & Latency Curves: Audio Packets Steven Gray, Nokia

  14. -1 10 1 No SDMA No SDMA 0.9 3-channel SDMA 3-channel SDMA 0.8 0.7 -2 10 0.6 PCF Latencies Normalized Throughput 0.5 0.4 -3 10 0.3 0.2 DCF:PCF = 1:1 DCF:PCF = 1:1 0.1 Video Packets Video Packets -4 0 10 0 100 200 300 400 500 0 100 200 300 400 500 Packets/Second Packets/Seconds Throughput & Latency Curves: Video Packets Steven Gray, Nokia

  15. -1 1 10 No SDMA 0.9 3-channel SDMA 0.8 0.7 -2 10 0.6 PCF Latencies Normalized Throughput 0.5 0.4 -3 10 0.3 0.2 No SDMA DCF:PCF = 1:3 0.1 3-channel SDMA DCF:PCF = 1:3 Video Packets Video Packets 0 -4 0 100 200 300 400 500 600 700 800 10 0 200 400 600 800 Packets/Second Packets/Seconds Throughput & Latency Curves: Video Packets Steven Gray, Nokia

  16. Message Fields Frame Control RA TA FCS Duration New Control Frame Subtypes for Smart Antennas • AP to STA • A message requesting the STA to transmit a preamble (null frame) for channel estimation • Channel Id Request • STA to AP • A response to the above request • Channel Id Response Steven Gray, Nokia

  17. Remove Polling Dependence • The existing standards reads, "During each CFP, the PC shall issue polls to a subset of the STAs on the polling list in order by ascending AID value". Steven Gray, Nokia

  18. Conclusions • Particularly in a large enterprise environment, smart antennas can help boast capacity • Wireless office replacement for "wired" Ethernet • Public service networks such as airports • Changes to the existing MAC are minor to enable antenna technologies in IEEE802.11a networks • SDMA is not for all WLANS • Multiple antennas cost additional money for the AP that may not be required in homes and small businesses Steven Gray, Nokia