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Comparing CSMA and CDMA, Two Multiple Access Approaches, for Use in 802.15.4a

Comparing CSMA and CDMA, Two Multiple Access Approaches, for Use in 802.15.4a. A contribution to TG4a Dani Raphaeli InfoRange Inc. Desired Attributes in 4a. Low link rate, but high aggregated throughput Large distance Low complexity, low cost Low power Interference resistance Mobility

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Comparing CSMA and CDMA, Two Multiple Access Approaches, for Use in 802.15.4a

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  1. Comparing CSMA and CDMA, Two Multiple Access Approaches, for Use in 802.15.4a A contribution to TG4a Dani Raphaeli InfoRange Inc Dani Raphaeli, InfoRange, Inc

  2. Desired Attributes in 4a • Low link rate, but high aggregated throughput • Large distance • Low complexity, low cost • Low power • Interference resistance • Mobility • Mesh network (not a cellular design) • Large number of nodes in the same area Dani Raphaeli, InfoRange, Inc

  3. What Is CSMA? • Time axis divided between users. • Avoiding collisions as much as possible by carrier sensing and backoff. • To achieve high number of (active) users each user transmit short packets using high bit rate. • The channel bit rate is shared between the users in some inefficiency (collisions, backoff). Dani Raphaeli, InfoRange, Inc

  4. CDMA Principles • Each user sends low bit rate over the air. • Packets of multiple users may overlap. • No synchronization between users is required (although such sync can improve the total capacity). • Higher bit rate still possible by each user acting like many. • Many kinds of spread spectrum methods: FH, DS, IR. Dani Raphaeli, InfoRange, Inc

  5. System Design Aspects • According to the TRD, CSMA shall use 1mbps payload and CDMA 1kbps payload. • We can afford to define two classes of nodes: RFD and FFD. • In TDMA, all equal (in terms of PHY). • In CDMA, the RFD may support receiving only one or few users at a time, but FFD can receive many overlapping users. • In the following slides we will show several benefits and disadvantages of CDMA vs CSMA. Dani Raphaeli, InfoRange, Inc

  6. Distance • Tx power is limited by FCC. Assuming both systems using same BW they got same TX power. • Link budget example for UWB (free space): • F=4GHz, BW=500MHz, Tx=-16dbm (assuming 2dB flatness) • Eb/N0=10dB, NF=7dB, antenna gain –3dbi • -16-6-(-174+10+7+10logR)=20logfd+92 -> • R=1K -> d=1.1Km • R=1M -> d=35m Dani Raphaeli, InfoRange, Inc

  7. Complexity Just general comments since need to compare actual proposals for both methods CDMA benefits: • Slow operation mean lower power consumption and lower cost hardware • Many tasks can be performed in software • FFD more capable, RFD more simple TDMA benefits • Can support easily data transfer in high rate from node to node (e.G. WLAN) Dani Raphaeli, InfoRange, Inc

  8. Tx Power • Advantage of CSMA: Due to the high rate the CSMA node spends little time in transmitting. • Point in favor of the CDMA: since we transmit low power, more power drawn by the circuit than actually transmitted through antenna – and circuit work slower in the CDMA, somewhat balancing the disadvantage. • Nodes are spending time waiting to receive packets much more than for transmitting. Dani Raphaeli, InfoRange, Inc

  9. Interference Resistance • CDMA has larger processing gain, so interference is rejected more easily (either foreign or other channels) Link budget example: • Tx=-16dbm, antenna –3dbi, • BW=500M, f=4g • In band interferer –40dbm at the receiver • -16-3-(20log(fd)+92)-(-40)+10log(500M/R)=10 • R=1K -> d=16m • R=1M -> d=0.5m Dani Raphaeli, InfoRange, Inc

  10. Mesh Network, Mobile Network • Hard to maintain accurate synchronization in a mobile network in harsh environments. • All nodes are battery operated in the typical mesh applications. • Assuming the CDMA nodes use much lower power when active, the duty cycle of operation is much higher than in the CSMA node. • Users that will be using very low duty cycle, once loosing connection the association will be extremely slow, since both sides use low duty cycle. • Hidden nodes in mesh networks makes CSMA (even if RTS-CTS would have been used) inefficient and unfair with unpredictable latency. Dani Raphaeli, InfoRange, Inc

  11. Summery of CDMA Advantages • Range • Interference robustness • Easier Piconets separation • Faster association, robust operation, efficient and controlled latency in large mobile mesh networks • Slightly less complex Dani Raphaeli, InfoRange, Inc

  12. Summery of CDMA Disadvantages • Large energy used per each transmitted packet (well, this is where the distance gain comes from). • Limit on the burst rate of a single node. • Latency can get large for 1kbps rate. • Receiver for 1kbps starts to be slow to synchronize, and clocks accuracies getting tougher. Suggestion: • Use combination of CDMA and TDMA at the rate of 10-50kbps over the air. Dani Raphaeli, InfoRange, Inc

  13. Thank You! Dani Raphaeli, InfoRange, Inc

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