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Teletraffic Performance Analysis of Multiclass OFDMTDMA Systems with AMC

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Title: Teletraffic Performance Analysis of Multiclass OFDMTDMA Systems with AMC


1
Teletraffic Performance Analysis of Multi-class
OFDM-TDMA Systems with AMC
  • Hua Wang, Villy B. Iversen

2
Outline
  • Introduction
  • Previous work
  • OFDM Transmission with AMC
  • Traffic model analytical model
  • Numerical examples
  • Conclusion

3
Introduction
  • OFDM technology is immune to inter-symbol
    interference and frequency selective fading
  • Many standards have adopted OFDM as the air
    interface such as IEEE 802.16
  • The economical usefulness of a system is usually
    measured by the Erlang capacity
  • Erlang-B formula cant be used in wireless
    networks due to the time-varying nature of
    wireless links
  • AMC is proposed to enhance the spectrum
    efficiency
  • With AMC, the bandwidth allocation to each user
    is no longer deterministic, but rather dynamic

4
Previous work
  • Qingwen L., Shengli Z., and Georgious B. Queuing
    With Adaptive Modulation and Coding Over Wireless
    Links Cross-Layer Analysis and Design
  • A finite-length queue is coupled with AMC
  • Concentrate on a single-user case
  • Tarhini, C., and Chahed, T. System capacity in
    OFDMA-based WiMAX
  • Erlang capacity calculation with fixed modulation
    scheme
  • Two traffic streams are considered
  • Hua Wang, Villy B. Iversen Erlang Capacity of
    Multi-class TDMA Systems with Adaptive Modulation
    and Coding
  • Evaluate the Erlang capacity of multi-class TDMA
    systems with AMC by separating the calculation of
    blocking and outage probabilities

5
OFDM Transmission with AMC
  • An OFDM-TDMA system with M subchannels
  • A frame is divided into K time slots
  • Users transmit in the assigned time slots over
    all subchannels
  • Channel quality of each subchannel of each user
    is independent identically distributed
  • Each subchannel follows a Rayleigh fading
  • Received SNR on subchannel m is a random variable
    with pdf

6
OFDM Transmission with AMC (cont.)
  • AMC scheme is applied to adjust transmission mode
    according to channel conditions
  • N total number of transmission mode (N5)
  • Partition the SNR range into N1 intervals
  • Algorithm for determining the boundary of each
    interval with prescribed PER is in Ref. 1
  • Transmission mode n on subchannel m is chosen
    when
  • Transmission mode n on subchannel m will be
    chosen with probability

7
OFDM Transmission with AMC (cont.)
  • Rm number of bits can be transmitted over
    subchannel m in one time slot
  • R number of bits can be transmitted over all
    subchannels in one time slot

8
Definition of Unit Channel in Multi-class Systems
  • L types of service classes
  • Each service class has a bit rate requirement of
    r_i bits per frame
  • Specify a common channel bandwidth which has
    constant bit rate of r_unit bits per frame
  • Number of unit channels required by service class
    i
  • Number of time slots occupied by a unit channel
    is a random variable

9
Calculation of Outage Probability
  • In AMC, the number of time slots allocated to
    each user is varying frame by frame
  • Outage is defined when the total number of
    required time slots exceed the available time
    slots
  • Outage probability in state x (x unit channels
    are occupied)
  • where

10
State Dependent Acceptance Probability
  • A new connection attempt will be accepted with
    certain prob. under the condition that the
    acceptance of the new connection will not cause
    the system outage prob. Above the threshold
  • Acceptance prob. of a single-channel call in
    state x
  • Acceptance prob. of a d-channel call in state x

11
Traffic model
  • BPP traffic model
  • Binomial Zlt1, Engset case
  • Poisson Z1, Poisson arrival process
  • Pascal Zgt1, Pascal arrival process
  • Traffic parameters
  • Offered traffic A (the average number of call
    attempts per mean service time)
  • Peakedness Z (the burstyness of the arrival
    process)
  • Number of unit channels needed d

12
Algorithms for Calculating Global State
Probabilities
  • The connection-level characteristics of
    multi-class OFDM-TDMA systems with AMC can be
    modelled by a multi-rate loss system with
    state-dependent blocking

13
Algorithms for Calculating Global State
Probabilities (cont.)
  • Delbrouck 7 developed a general algorithm for
    calculating the global state probabilities for
    multi-rate loss system with BPP traffic
  • Reference 5 extends the Delbroucks algorithm
    to include state-dependent blocking

14
Performance Measures
  • Time Congestion
  • Traffic Congestion
  • Call Congestion

15
Numerical Results
  • Consider an OFDM-TDMA system with 5 subchannels
  • Total number of time slots for data transmission
    within a frame is 100, each of which contains 4
    OFDM symbols
  • Number of transmission mode is set to be 5 with
    target PER at 10e-4
  • Target outage probability is 2
  • Three types of service classes are considered

16
Numerical Results (cont.)
17
Conclusion
  • Developed an analytical model of multi-rate loss
    system with state-dependent blocking to evaluate
    the performance of multi-class OFDM-TDMA with AMC
  • The blocking prob. of a call attempt depends both
    on the state of the system and the bandwidth
    requirement
  • Due to reversibility, we have local balance and
    may calculate the global state probabilities by
    an effective algorithm

18
  • Thank you
  • Any questions?
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