1. February 2004
doc.: IEEE /080r0
July 2004
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: [Comparing CSMA and CDMA, Two Multiple Access Approaches for Use in a ]
Date Submitted: [13 July, 2004]
Source: [Dani Raphaeli]
Company [Inforange]
Re: [ ]
Abstract: []
Purpose: [Providing technical contributions to IEEE a. ]
Notice: This document has been prepared to assist the IEEE P 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 P
Sato, Zhang, Sanada & Nakagawa, MMAC
Welborn, Motorola

2. Desired Attributes in 4a
July 2004
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.
July 2004
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.
July 2004
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. July 2004
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. July 2004
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-( logR)=20logfd+92 ->
R=1K -> d=1.1Km
R=1M -> d=35m
Sato, Zhang, Sanada & Nakagawa, MMAC

7. July 2004
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. July 2004
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
July 2004
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
July 2004
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
July 2004
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
July 2004
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! February 2004 doc.: IEEE 802.15-04/080r0 July 2004
Sato, Zhang, Sanada & Nakagawa, MMAC
Welborn, Motorola