1. A contribution to TG4a Dani Raphaeli InfoRange Inc
21 November 2018
Comparing CSMA and CDMA, Two Multiple Access Approaches, for Use in a
A contribution to TG4a
Dani Raphaeli
InfoRange Inc
Dani Raphaeli, InfoRange, Inc

2. Desired Attributes in 4a
21 November 2018
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.
21 November 2018
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.
21 November 2018
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
21 November 2018
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
21 November 2018
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
Dani Raphaeli, InfoRange, Inc

7. Complexity
21 November 2018
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
21 November 2018
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
21 November 2018
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
21 November 2018
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
21 November 2018
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
21 November 2018
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. 21 November 2018
Thank You!
Dani Raphaeli, InfoRange, Inc