1. copyright 2005 Macrodiversity Packet Combining for the IEEE 802.11a Uplink Shi Cheng and Matthew C. Valenti Lane Dept. of Comp. Sci. & Elect. Eng. West Virginia University Morgantown, WV mvalenti@wvu.edu This work was supported in part by Cisco through the University Research Program This presentation does not necessarily represent the views of Cisco.

2. © 2005 Outline Macrodiversity vs. Microdiversity.  Protocol for packet-level macrodiversity. Application to 802.11a  Throughput analysis  Effect of imperfect ACK signaling  Effect of channel estimation

3. © 2005 Microdiversity With a conventional array, then elements are closely spaced (~ /2) and connected through high bandwidth cabling.  Microdiversity.  Signals undergo different small-scale fading, but same large- scale effects (path-loss and shadowing). Receiver Transmitter

4. © 2005 Macrodiversity With a distributed array, the antennas are widely separated (e.g. different base stations) and connected through a moderate bandwidth backbone.  Macrodiversity.  Antennas far enough apart to provide independent channels.  Provides robustness against not only small-scale fading, but also large-scale effects. Receiver #2 Transmitter Receiver #1 Backbone Network

5. © 2005 Network Topology K access points (APs) along a ring of radius r. Master AP is the one closest to the mobile terminal.  Conventional system: Only master AP receives uplink signal.  Macrodiversity system: All K AP’s receive uplink signal. The other AP’s are supplemental.

6. © 2005 Packet-Level Macrodiversity Protocol Mobile transmission received by all K AP’s  Packets are encoded with CRC code.  Used by each AP to determine if packet correct.

7. © 2005 Packet-Level Macrodiversity Protocol If a supplemental AP correctly receives a packet, it forwards it over the backbone to the master AP. If the master AP does not receive the mobile’s transmission correctly, then it will use one of the packets forwarded by the supplemental APs (if any). Therefore, the packet is accepted if it is correct at any of the K access points. Overall packet error probability:

8. © 2005 Packet-Level Macrodiversity Protocol The master AP sends an ACK back to the mobile.

9. © 2005 Propagation Model Quasi-static frequency selective Rayleigh fading.  Impulse response:  L is the number of taps  T is the tap spacing T = 50 nsec in 802.11a   ’s are complex Gaussian with variance   rms is the RMS delay spread.

10. © 2005 Details of IEEE 802.11a OFDM K=7 Convolutional Code. Adaptive modulation and coding Rate OptionModulationCode Rate 6 MbpsBPSK1/2 9 MbpsBPSK3/4 12 MbpsQPSK1/2 18 MbpsQPSK3/4 24 Mbps16-QAM1/2 36 Mbps16-QAM3/4 48 Mbps64-QAM2/3 54 Mbps64-QAM3/4

11. Centrally located mobile. 6 Mbps rate option 1500 byte packet 50 nsec rms delay spread Packet Error Rate: Central Location at 6 Mbps

12. Influence of Rate Option Centrally located mobile. 1500 byte packet 50 nsec rms delay spread

13. Receiver CSI available Exponential path loss: Influence of Mobile Location 6 Mbps rate option 1500 byte packet

14. MAC Protocol SIFS 16  sec DIFS 34  sec DIFS Data Packet ACK Distributed Coordination Function ACK is sent only by the master AP 14 bytes

15. Throughput of 6 and 12 Mbps options Centrally located mobile. No receive CSI. 1500 byte packet 50 nsec rms delay spread

16. Throughput of 36 and 54 Mbps options Centrally located mobile. No receive CSI. 1500 byte packet 50 nsec rms delay spread

17. Throughput of with Randomly Positioned Mobile 6 Mbps option. Uniform location. No receive CSI. 1500 byte packet 50 nsec rms delay spread

18. © 2005 Conclusions Benefits of macrodiversity combining:  Mobile station requires less transmit power.  Improves coverage in hard to reach locations.  Does not require complex MRC combining. Disadvantages:  Supplemental AP must signal quickly over the backbone.  AP might use different channels for different sectors.