Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Comparing CSMA and CDMA, Two Multiple Access Approaches for Use in 802.15.4a ] Date Submitted: [13 July, 2004] Source: [Dani Raphaeli] Company [Inforange] E-Mail: ［danr@eng.tau.ac.il] Re: [] Abstract: [] Purpose: [Providing technical contributions to IEEE 802.15.4a. ] Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15. Sato, Zhang, Sanada & Nakagawa, MMAC

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 Sato, Zhang, Sanada & Nakagawa, MMAC

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). Sato, Zhang, Sanada & Nakagawa, MMAC

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. Sato, Zhang, Sanada & Nakagawa, MMAC

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. Sato, Zhang, Sanada & Nakagawa, MMAC

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 Sato, Zhang, Sanada & Nakagawa, MMAC

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) Sato, Zhang, Sanada & Nakagawa, MMAC

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. Sato, Zhang, Sanada & Nakagawa, MMAC

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 Sato, Zhang, Sanada & Nakagawa, MMAC

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. Sato, Zhang, Sanada & Nakagawa, MMAC

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 Sato, Zhang, Sanada & Nakagawa, MMAC

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. Sato, Zhang, Sanada & Nakagawa, MMAC

13. Thank You! Sato, Zhang, Sanada & Nakagawa, MMAC