November 2001

Throughput Maximization for CDMA
via Rate and Power Control
of Transceivers with Different Priorities

by V. Rodriguez and D. Goodman
{virgilio, dgoodman}@poly.edu

• Main problem: user-to-user interference

  • Under ideal conditions, users should not interfere one another.
  • A typical wireless channel is far from ideal.
    Particular impairments are:

    • Asynchronous transmission (lack of users coordination)
    • Multi-path (several delayed replicas of the transmitted signals are received)
  • These impairments lead to user-to-user interference
  • ``Near-far problem'' is created: a sufficiently powerful interferer could degrade the receiver's performance to an arbitrary degree
• Power control: of paramount importance because of above
• Typically sought: power level for each active user to optimize certain measure of network performance
• Sought in this work: Both optimal power levels AND optimal data rates
• Throughput (successfully transmitted bits/per time unit) is a ``natural'' measure of network performance for data transmission
• Each user's contribution to the throughput is proportional to the product its data rate by its frame success probability
• Bits may be weighted differently depending on the user's ``priority''
• Two key variables influence a transmitter frame-success probability:

  • Processing gain (A channel-determined constant divided by the user's data rate)
  • Ratio of user's transmit power to the sum of the powers of other active users
• We seek to find, for each user, an optimal power level AND an optimal data rate (determined indirectly through an optimal processing gain)
• Preliminary results are available for a two-user situation under negligible noise

  • One user should always be allowed to transmit at the highest permissible data rate (lowest permissible processing gain)
  • This user's optimal power ratio is determined by solving certain meaningful channel-determined equation
  • This automatically determines the other user's optimal power-ratio as well
  • Other user's data rate is determined so that its signal-to-interference ratio (product of its processing gain by its power ratio) equals a channel-determined constant
  • If maximum permissible transmit data rate is LOW enough, it becomes optimal for both users to transmit at this highest rate. Optimal power ratios are then determined by solving certain channel-determined equation
• Extending above results to an ``n-user'' situation appears feasible. Some results have already been extended.
• Working paper is available