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

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

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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).

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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.

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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.

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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)
• F4GHz, BW500MHz, Tx-16dbm (assuming 2dB
  flatness)
• Eb/N010dB, NF7dB, antenna gain 3dbi
• -16-6-(-17410710logR)20logfd92 -gt
• R1K -gt d1.1Km
• R1M -gt d35m

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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)

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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.

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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,
• BW500M, f4g
• In band interferer 40dbm at the receiver
• -16-3-(20log(fd)92)-(-40)10log(500M/R)10
• R1K -gt d16m
• R1M -gt d0.5m

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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.

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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

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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.

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Thank You!