CDMA Based MAC Protocol for Wireless Ad Hoc Networks

1
CDMA Based MAC Protocol for Wireless Ad Hoc
Networks

• Alaa Mouquatash Marwan Krunz
• Presentation by
• Moheeb Abu-Rajab

2
Agenda

• Motivation
• Protocol Design
• Protocol Description
• Simulation
• Results Evaluation
• Conclusion and Future Work

3
Motivation

• Fundamental problem in MANETS is low throughput
• Contention-Based MAC protocols
• Harsh Characteristics of the radio channel
• Challenge
• How to increase throughput while maintaining low
  energy consumption?
• This paper focuses on improving the overall
  network throughput by means of a CDMA-based
  design of the MAC-protocol

4
CDMA Spread Spectrum Communication

• Bandwidth is one of the most scarce resources in
  wireless communication systems.
• Traditionally signals were transmitted using the
  least possible bandwidth
• M-ary Communication systems (eg QAM, 16-QAM,
  32-QAM etc.)
• TDMA and FDMA were the most prevalent schemes for
  sharing the bandwidth between multiple users.
• Attributed by low spectrum utilization.
• Low noise immunity.
• Wireless Communication researchers strive for a
  paradigm that can provide better spectrum
  utilization and lower energy requirements

5
CDMA SS (continued)

• Spread Spectrum follows a completely different
  approach
• Instead of limiting the Tx. Signal bandwidth SS
  spreads the bandwidth on a wider band.
• How it achieves better spectrum utilization, and
  better noise immunity?
• SS uses low correlated Pseudo Random codes (PN
  codes) to spread the original digital bit
  stream.
• Multiple users can share the same spectral band
  if they were spread using different codes
• The quality of the received signal is judged by
• Power constraints
• Correlation between spread codes

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CDMA System
Spread Digital Stream
Spread Digital Stream
Digital Output Stream
QPSK De-modulator
Digital Input Stream
QPSK Modulator
Output RF
PN-Sequence Generator
PN-Sequence Generator
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CDMA/MAC

• How can we exploit this feature of CDMA to
  enhance the throughput of traditional MAC
  protocols for MANETs?
• 802.11 uses CDMA as a modulation scheme to
  achieve better noise immunity
• Only uses a common code to spread the tx. Bit
  stream
• The basic approach to enable a CDMA/MAC is uses
  multiple codes rather than one code, thus
  allowing more than one user to share the comm.
  channel

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CDMA/MAC, Challenges

• Code Assignment
• We need to assign a unique and orthogonal code
  for each node to avoid interference
• Complicated for large networks with large number
  of nodes
• Not feasible for MANETs time-asynchronous
  systems
• Even unique Codes exhibit non-zero correlation
  creating Multi-Access Interference MAI at the
  Receiver
• Spreading Code Protocol
• We need to decide which codes will be used for
  packet transmission and which codes to use for
  monitoring the anticipation of signal

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CDMA/MAC, Challenges (continued)

• Spreading Code protocol
• Three different Approaches
• Receiver Based
• Transmitter based
• Hybrid of both

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Spreading Code Protocols

• Receiver Based
• Sender uses the code of the intended Rx. To
  spread the signal
• Idle node will monitor its own code only.
• Advantages
• Simple Rx.
• Disadvantage
• Primary collision can happen

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Primary Collision
A
B
C
Primary Collision
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Code Spreading Protocols (continued)

• Transmitter Based
• A different code is assigned to each node
• But, the receiving node must listen to all codes
• Advantages
• Avoids Primary Collision
• Simplified Broadcast
• Disadvantage
• Increased complexity of the Rx stage.

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Code Spreading Protocols (continued)

• Hybrid Protocols
• Prevalent Approach
• Fields of the packet are spread using a common
  code.
• Other fields are spread by a receiver or a
  transmitter based mechanism
• In the reservation based schemes
• a code is used for RTS/CTS
• Another code for data exchange
• Receivers will listen to the common code
• If a receiver was intended by the tx.
• Switch to own (or tx.) code to receive the signal
• Example RA-CDMA

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RA-CDMA/ MAC

• Reservation based.
• Uses hybrid code assignment scheme
• Based on Randomly accessing the channel starting
  with RTS and CTS
• With proper code assignment, it can avoid primary
  collision
• However, the non-zero cross-correlation between
  codes
• Increased MAI at Rx.
• Can lead to Secondary collision -gt Near Far
  Problem

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Near-Far Problem (Secondary Collision)
High-MAI
Far
Secondary Collision
Near
A
B
C
High-MAI
F
Secondary Collision
D
G
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Effect of Near-Far Problem
N 5
N 10
N 20
Expected Progress per Hop
Probability of Transmission
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The Proposed Protocol CA-CDMA

• Main Goals
• To provide a CDMA-based MAC solution that
  addresses the near-far problem
• A Protocol that can achieve enhanced throughput
  keeping the same energy requirement
• Basic idea
• a distributed admission and feedback among nodes

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CA-CDMA
High-MAI
Far
Secondary Collision
Near
A
B
C
D
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Design Goals

• Asynchronous, distributed, scalable solution for
  large Networks (Matches MANET environment)
• Receiver stage shouldnt be overly complex (Rx.
  Based spreading code)
• Adapt to channel characteristics and mobility
  patterns
• Able to coupe with incorrect code assignment
  code assignment is left to the upper layers

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Design Architecture

• Two Separate Frequency channels (FDM-like
  partitioning) - one for the RTS/CTS and the other
  for data exchange
• Common Spreading Code for the control channel
• Receiver Based spreading codes for the data
  channel
• Codes are not assumed to be orthogonal
• Control and data channels are completely
  orthogonal

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Design Architecture (Continued)
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Assumptions

• Control and data channels are completely
  orthogonal
• Channel gain is stationary for the duration of
  the control and data packet Tx. Period.
• Gain is same in both directions
• Data packets between pair of terminals observe
  similar gain
• The radio stage can provide a feedback to the
  upper MAC layer (about the interference level)
  both ways

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Protocol Description

• Contention based
• Uses a variant of RTS/CTS reservation scheme
• RTS and CTS are spread using a common code and
  transmitted over the control channel using fixed
  power Pmax
• RTS and CTS are heard by potentially interfering
  nodes, however, these nodes are allowed to
  transmit based on some constraints
• For the Data channel, Receive and Transmitter
  should agree on
• Spreading Code code assignment is dealt with at
  upper layers
• Transmit Power
• Choice of power is critical and represents a
  trade-off between link quality and (Max.
  Allowable Interference)

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Protocol Description (continued)

• In addition, the protocol incorporates an
  Interference Margin into the power computation
• This margin Allows nodes at some distance from a
  Rx. to start new transmissions in the future.
• Nodes exploit the knowledge of the power level of
  the overheard RTS and CTS transmissions to
  compute this Margin
• Distributed Admission Control A transmitter can
  decide when and at what power it can transmit
  without disturbing ongoing transmissions in its
  surrounding and at the same time ensuring enough
  power at the receiver given the current MAI at
  the receiver.
• Distribute Feedback to neighbors, through the CTS
  messages.

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The Interference Margin

• Allows nodes at some distance from a Rx. to start
  new transmissions in the future.
• Highly dependent on the network load which is
  Expressed in terms of the noise rise

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Interference Margin (continued)

• Min Power required at the receiver
• Is the min required power level required at
  the Rx level to achieve the min (Eb/N)
• Higher load -gt Higher noise rise -gt need more Tx.
  Power to achieve required Eb/N
• Increasing the Transmit power is constrained by
  two factors
• Energy Consumptions restrictions in MANETs
• FCC regulations

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Interference Margin (continued)

• To coupe with the first constraint
• Choose the noise rise to the so that the needed
  Tx. Power to cover the same max distance covered
  by the 802.11.
• To coupe with the second constraint
• Choose the noise rise so that the required
  energy per bit is the same as for 802.11
• Given that the rate used by CA-CDMA for data is
  less than 802.11 then for a a control channel
  that occupies 20 of the total available BW. -gt
  6dB max. allowable noise rise !!

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Distributed Admission Control

• How the channel is accessed?
• Goal
• Allow only Transmissions that cause neither
  primary nor secondary collisions
• RTS and CTS packets are used to
• Allow nodes to estimate the channel gains between
  the Tx. and the Rx.
• A receiver uses CTS to notify its neighbors of
  the additional interference noise allowable
  noise rise it can accept without impacting its
  current reception
• Each node keeps listening to the control channel
  regardless of the signal destination in order to
  keep track of the number of active nodes and the
  interference levels

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Admission Control (continued)
K
If (P(AB)min.gt P(AB)allowed.) MAI gtgtgt Suspend
As Tx. Else respond with ve CTS
P(AB)min M(Pth MAI)/G(AB) . P(AB)allowed
NR MPth/G(AB) .
If (P(AB)allowed.lt P(AB)Max. allowed.)
transmit data at P(AB)Max. allowed
G(AB) P(A)Rx./Pmax.
L
RTS, Pmax
A
B
P(AB)allowed
RTS, P(A)map
K
F
Allowable Noise Rise
L
G
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Simulation

• Event Based CSIM simulation tool

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Simulation Results
CA-CDMA
Network Throughput
802.11
Packet Generation Rate
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Conclusion Future Work

• Conclusion
• CA-CDMA is a distributed power control CDMA
  based MAC protocol.
• CA-CDMA provides an enhancement for the
  throughput in MANETs through addressing the near
  far problem
• Future Work
• Combine CA-CDMA with other capacity optimization
  schemes -gt directional antennas for example
• Multi-rate support is also another opportunity
  for capacity optimization
• Devise better schemes for access control over the
  control channel

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• THANKS !!