Random Packet-CDMA: Reducing Delay and Increasing Throughput of WLAN Systems

1. Random Packet-CDMA: Reducing Delay and IncreasingThroughput of WLAN Systems Roland Kempter and Behrouz Farhang-Boroujeny kempter@ece.utah.edu farhang@ece.utah.edu

2. Organization • Introduction: Scheduled vs. Random Channel Access • Random Packet-CDMA (RP-CDMA) • RP-CDMA: Header Detection • RP-CDMA: Payload Detection • Conclusions

3. Handshakew/ TX Transmitter (TX) Handshake w/ Base Base Tell about resources Packet in queue Ask for resources Thinking about it! TX Data (IP) traffic is bursty! Introduction:scheduling-based access • Advantages • Base knows everything: • packet collisions can be avoided • service can be guaranteed (great for voice) • Disadvantages • Resources need to be negotiated: • OK in circuit switched systems • large overhead (or low efficiency) with packetized traffic

4. TX Base TX Transmitter (TX) Send Acknowledgement Check medium (sometimes) Packet in queue Introduction:random channel access • Advantages • self regulating (good for bursty traffic) • load adaptive • simplifies infrastructure • Disadvantages • Collisions, low stable throughput • degrades heavily as load increases • delay variations

5. Advances in (Random) Channel Access No collisions Know SS for detection handshaking Not Random Protocol Max. throughput of Spread Aloha (18% of a fully coordinated system) S = G RP-CDMA: combination of Spread Aloha with CDMA technology 1970 1980 1990 1997 2000+ 1976 CDMA:Concurrent Transmissions Aloha Ethernet IEEE 802.3 CSMA Spread Aloha IEEE 802.11 Introduction:background on random channel access

6. System Characteristics Header Channel: low-traffic SpreadALOHA channel Data Channel: CDMA, possibly large number of TX RP-CDMA packet structure [KemAmiFar06, SchKemKot06] [KemAmiFar06] R. Kempter, P. Amini and B. Farhang-Boroujeny, “Throughput and Stability of RP-CDMA and Spread Aloha in Multipacket Capture Channels", submitted to IEEE/ACM Transactions on Networking, October 2006 [SchKemKot06] C. Schlegel, R. Kempter and P. Kota, “A Novel Random Wireless Packet Multiple Access Method Using CDMA”,IEEE Trans. Wireless Communications, p. 1362-1370, Vol. 5, No. 6, June 2006

7. Lh<Ld and concurrent TX:feature of RP-CDMA header detection likely (h/d)interference limited Note If header detection limited by (h/d)interference:  CSMA/CA on the header unlikely to improve performance RP-CDMA Performance: interference in the header channel System Effect Packet format separates TX into a virtual header and virtual data channel

8. Collisions: notnecessarily As long as no overlap at chip level: still separated by Spreading Gain N. RP-CDMA Performance: collisions in the header channel

9. Assume • Header detected successfully iff after receiver, SNIR > header detection threshold • No chip-level collisions happened • Equal power scenario Packet timingestablished during header recovery: Matched Filter for header detection • Solve: Matched Filter [TseHan99] • Check for chip-level collisions RP-CDMA Header Detection: results

10. RP-CDMA: header detection, spread aloha, equal powers (a) SNRh/SNRd=5 dB (b) Nh=Nd=20 (c) Ld / Lh=25

11. Interference limitation Small systems Interference can be suppressed Large systems Interference can only be suppressed up to a certain load Collision limitation Point f (Nh, Nd, K, Ph/Pd, pathloss, , header technology, data technology) RP-CDMA: performance • Conclusions from the header process • After a certain point which is f(N,SNR),increasing Ld / Lh • does not improve performance

12. Assume Data frame is detected successfullyiff after receiver, SNIR  >threshold . SNIR  is multi-user receiver specific RP-CDMA: data channel performance

13. [KemSch05] R. Kempter and C. Schlegel, “Packet Random Access in CDMA Radio Networks”,in Proceedings of Allerton Conference 2005. [ShiSchKem06] Z. Shi, C. Schlegel and R. Kempter, “On the Performance of Partitioned-Spreading CDMA”,Conference on Information Sciences and Systems, CISS 2006, Princeton, 2006 RP-CDMA Data Detection • Matched Filter [TseHan99] • Decorrelator [TseHan99] • MMSE [TseHan99] • Partitioned Spreading [KemSch05, ShiSchKem06]: iterative demodulation, resolves (virtually) allmultiuser interference

14. RP-CDMA: network simulations • IP packet sizes are trimodally distributed [Inet2]: • Pr(L=50 bytes) = 0.5 • Pr(L=500 bytes) = 0.4 • Pr(L=1500 bytes) = 0.1 • Also • RP-CDMA header size: Lh= 50 bits  E[Ld / Lh] = 60 • Payload and header spreading gains: N=32 • Header/Payload SNR: 5 dB • Header and data detection thresholds: 3 dB • Power control • Nodes transmit to a central base station

15. RP-CDMA: equal power, central base station

16. Software processes, payload decoding technology can be adopted f(load) Laid out for max. # users RP-CDMA receiver: SOFTWARE DEFINED RADIO

17. Conclusions 1/2 • We demonstrated the performance in the RP-CDMA • header channel by modeling it as a Spread Aloha packet. • After a point, RP-CDMA becomes interferencerather than collision limited: • f (Nh, Nd, K, Ph/Pd, pathloss, , h. technology, d. technology) • For the data channel, we compared: • - Matched Filter - Decorrelator - MMSE • - Partitioned Spreading • in base station centric networksw/ power control

18. Conclusions 2/2 • RP-CDMA is determined by header process: • improvingheaderdirectly is hard (non-linear in parameters): • BUT: with Partitioned Spreading, for a targeted data rate, Pd can be low, increasingPh / Pd •  pushing the interference limit • We presented the general block-diagram of an RP-CDMA • software defined receiver • RP-CDMA allows to adopt to different load situationsvia software changes • low load:power save w/ matched filter • high load: partitioned spreading

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