MultiHop Cellular AdHoc CDMA Networks

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Multi-Hop Cellular Ad-Hoc CDMA Networks

• Elvino S. Sousa, (Mark de Faria)
• Bell University Labs Chair in Wireless
  CommunicationsDept. of Electrical and Computer
  EngineeringUniversity of Toronto

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Introduction

• Structure of a Multi-hop Cellular Network
• Previous Research
• System Model
• Use of Timeslots
• Routing in a Multi-hop Cellular Network
• Power Control
• Simulation Results
• Conclusions

3
Traditional Cellular Networks

• Mobile terminals transmit all packets to a base
  station
• For a call to be possible, mobile terminals must
  be within transmission range of the base station
  in a single hop

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Ad-hoc Networks

• Does not rely on any expensive fixed
  infrastructure for transmission between terminals
• Mobile terminals communicate directly with one
  another if within transmission range
• A mobile terminal can communicate with other
  terminals outside its range by allowing other
  terminals to forward its packets

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Multiple Hop Cellular Networks

• Combines properties from these two networks.
• Maximum transmission power of mobile terminals is
  lower than in a traditional cellular network
• Therefore, mobile terminals will not necessarily
  be within transmission range of the base station.

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Multiple Hop Cellular Networks

• Mobile terminals will have to forward packets for
  other terminals towards the base station so that
  communication with the base is possible
• Potential for lower power consumption than a
  traditional cellular network with a high capacity

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Comparison of the Networks
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Difficulties

• Since mobile terminals are not within
  transmission range of the base station, routing
  paths need to be calculated from each mobile
  terminal to the base station
• Mobile terminals will have to transmit and
  receive from mobile terminals as well as the base
  station

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Bottleneck of the System
Base station
Mobile Terminal
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Previous Research

• Lin and Hsu investigated the multi-hop cellular
  network in 2000
• Using a Request to Send/Clear to Send (RTS/CTS)
  multiple access method, and using the base
  station only to forward intercellular calls, they
  showed that overall capacity was higher than a
  traditional cellular network when traffic
  locality was high

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Previous Research

• Further research was performed by Vojcic using
  CDMA as the multiple access method
• Base station was used to forward all calls
• Studied a balanced SIR/Power Routing Algorithm
  similar to Dijkstras algorithm
• Introduced a mechanism of mobile terminals
  transmitting at a certain time and receiving at a
  certain time and a similar mechanism will be used
  in this research

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System Model

• CDMA will be used as the multiple access
  technique
• FDD will be used to separate transmissions on the
  forward and reverse link
• All transmissions on the reverse link will be
  forwarded to the base station
• Base stations will be designed to receive packets
  on the low band as in CDMA2000
• Mobile terminals will be designed to transmit and
  receive packets on both the high frequency band
  and the low frequency band

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System Model

• Mobile terminals will be able to simultaneously
  receive and transmit to multiple terminals
• Mobile terminals will be able to simultaneously
  transmit multiple packets to one receiver
• Mobile terminals will not be able to transmit and
  receive simultaneously on the same frequency band
• Non-orthogonal spreading codes will be used on
  the forward and the reverse links

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Transmission in Timeslots

• Mobile terminals have to transmit and receive
  packets from other mobile terminals as well as
  the base station
• Therefore, terminals will have to make
  transmissions as well as receive transmissions on
  multiple frequency bands
• It is not practical to have transmissions and
  receptions on the same frequency band at the same
  time

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Transmissions in Timeslots

• Mobile terminals will be separated into two
  groups, assigning transmissions of each group in
  one of two timeslots
• Terminals in one group will be transmitting on
  the first timeslot and receiving on the second
• Terminals will be grouped so that they will only
  be transmitting to terminals that are currently
  receiving

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Transmissions in Timeslots

• We will define the layer number of a mobile
  terminal as the number of hops needed to reach
  the base station

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Transmission in Timeslots

• Any terminal in an even numbered layer will be
  transmitting on even valued timeslots and
  receiving on odd valued timeslots
• Any terminal in an odd numbered layer will be
  transmitting on odd valued timeslots and
  receiving on even valued timeslots
• This will ensure that terminals will never be
  transmitting to another terminal that is
  transmitting on the same frequency band

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Example of use of Timeslots
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Routing in a Multi-hop Cellular Network

• For transmissions from source to destination to
  be made within the network, routing paths from
  all mobile terminals to a base station need to be
  found
• Also a mechanism needs to be constructed for
  routing paths to be updated in case of a change
  in network topology

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Route Determination Algorithm

• Discovery of routing paths will begin at the base
  station by transmitting a packet to any terminal
  within its range
• Any terminal that is within transmission range of
  the base station will become part of layer 1 and
  its parent will be the base station
• Mobile terminals in layer 1 will reply to the
  base station using a mechanism similar to
  unslotted ALOHA
• The base station will acknowledge the receipt of
  each received packet by assigning a spreading
  code and transmitting a packet to each terminal
  in layer 1 for which it received a reply.

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Route Determination Algorithm

• Each terminal in layer 1 will transmit a message
  to any mobile terminal in its range
• Any terminal that is not in layer 1, that
  receives this message will become part of layer 2
• Each mobile in layer 2 will choose one of the
  terminals in layer 1, from which it received a
  transmission to be its parent and will send a
  reply to that terminal
• Two possible choices for parents will be
  investigated in this research and will be
  discussed later.

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Route Determination Algorithm

• The parent of the mobile terminal will forward
  the packet to the base station
• The base station will assign a spreading code to
  the terminal in layer 2 and this information will
  be forwarded to the newly discovered terminal
  through its parent
• This process will continue to discover mobile
  terminals in higher layers until no new terminals
  can be found and the algorithm will be said to
  have converged

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Choice of Parent in the Route Determination
Algorithm

• Maximum Distance Criteria
• - Pick terminal whose transmission
  has the lowest received SNIR
• Minimum Distance Criteria
• - Pick terminal whose transmission has
  the highest received SNIR above a
  certain threshold

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Example of Route Determination
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Properties of Routing Algorithm

• Provides routes to the base station for any
  terminal within transmission range (directly or
  indirectly)
• Provides routes to the base station with a
  minimum number of hops, given the maximum
  transmission range of the mobile terminals and
  the locations of the terminals

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Route Update Algorithm

• Interference between terminals is constantly
  changing, and locations of terminals may change
  as well
• Mechanism to update routing paths is required in
  case the network topology changes

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Route Update Algorithm

• If the mobile terminal does not receive a reply,
  it will transmit a packet to all terminals within
  its range to attempt to find a new parent
• All terminals that receive the packet will reply
  to the message
• The parentless terminal will choose to
  acknowledge one terminal with a small layer
  number
• The new parent will forward this information
  towards the base station. The base station will
  forward this information to the old parent so
  that it can update its records
• The terminal that used to be parentless will also
  transmit this information to its children so that
  they can update their layer numbers

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Example of Route Update Algorithm
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Power Control in a Multi-hop Cellular Network

• Transmission power should be high enough that the
  SNIR at the receiver is sufficiently high
• Transmission power should be not too high that
  excess interference is being created for other
  users
• Transmission power will need to be adjusted
  because mobile terminals may be moving and
  interference will be changing

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Power Control

• Closed loop power control will be used
• Increments of 1.0 dB of power will be used
• Power control is different in a multi-hop
  cellular network than in a traditional cellular
  network because power control now has to be done
  between a mobile terminal and its parent

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Simulation Model

• Radius of the cell is 100
• Number of terminals is 200
• Positions of terminals will be randomly
  distributed within the cell and all simulations
  are averaged upon 100 different realizations of
  positions of the mobile terminals
• Packet arrivals will be Poisson distributed with
  an arrival of rate of 0.1 packets per timeslot
  for each mobile terminal
• Path loss will be inversely proportional to d4
  and shadowing will be modeled as a lognormal
  random variable with standard deviation 6 dB
• Processing gain will be equal to 128
• Routing paths are found using the Route
  Determination Algorithm and Power Control is used

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Simulation Results

• Route determination algorithm for the multi-hop
  cellular network will be investigated based on
  various QoS parameters
• Parameters include Efficiency, Delay, Power
  Consumption, and Network Lifetime
• Maximum Transmission range for each mobile
  terminal will be reduced by a factor of K of the
  maximum transmission range in a traditional
  cellular system

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Efficiency (capacity, users supported)
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Delay
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Delay
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Power Consumption
A power savings, B diff for max distance vs
min distance
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Fairness (Lifetime timeslots for a terminal to
spend its energy)

A network lifetime, B
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Conclusions

• Using the route determination algorithm, a
  savings of up to 18dB of power can be obtained
• The introduction of multiple hops causes only a
  very small reduction in SNIR
• Small transmission ranges exhibit better
  performance than large transmission ranges
• Minimum distance criteria performed better than
  the maximum distance criteria in terms of
  capacity and power consumption. Maximum distance
  criteria performed better in terms of network
  lifetime

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Future Research

• A load-balancing mechanism could potentially
  allow the forwarding of packets to be performed
  by a greater number of mobile terminals and
  should be investigated
• Performance of the multi-hop cellular network in
  the presence of bursty traffic should be
  considered
• Introduction of multi-path routing could
  potentially increase the network lifetime and
  minimize the number of times that routing paths
  would need to be recalculated
• Performance of multi-hop cellular network in the
  presence of asymmetric traffic should be
  investigated

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Summary

• In this thesis, a minimum hop routing algorithm
  was designed and optimized for the multi-hop
  cellular network
• Mechanisms to update routing paths in case of a
  topology change were also developed
• Physical layer of mobile terminals was modified
  so that transmissions were made within timeslots
• A power control mechanism for the multi-hop
  cellular network was also outlined.