Multiple Access in Wireless Digital Networks

One-line summary: Spread ALOHA is equivalnt to CDMA with a common spreading code for all users, so ALOHA techniques can be carried over to spread-spectrum systems.

Overview/Main Points

• Advantage to using a single code for all transmitters in a CDMA network, esp. for small cell sizes and multiple access satellite apps
• Choice of a multiple-access protocol depends on traffic characteristics and state of the technology at deployment time:
  • DAMA (demand assigned multiple access): users request on a separate control channel; request protocol introduces delay and just moves the multiple-access problem to the (lower-bandwidth) request channel
  • DAMA w/random access: e.g. INMARSAT uses pure ALOHA for request channel. OK since allocation tends to be long-lived; no good if transmissions are bursty or short-lived
• Multiple access protocols
  • Slotted or pure ALOHA. Efficiency (r): eff. channel capacity divided by capacity of a continuous channel with same power & bandwidth. For ALOHA, r=.18, asymptotically optimal for the special case of small values of thruput and S/N ratio.
  • Spread spectrum: max channel capacity in bits per Nyquist sample: C = .5 log (1+P/N) based on Shannon & Nyquist relations. "Spread spectrum" means C << 1.
  • CDMA. Multiply channel signals by orthogonal set of spreading signals; multiply by cx conjugate at receiver. Requires multiple receivers at CDMA BS to demodulate received signal.
  • Qualcomm CDMA (IS-95 std). Spreading code is dynamically assigned via separate ALOHA channel when call request is made. Up to 64 codes can be active at once.
• We can choose same spreading code for all CDMA users and the channel will still have multiple-access capability (spread ALOHA):
  • Each subchannel's bits will be offset by a constant amount g from the previous subchannel's bits within the frame
  • With k subchannels, prob. that 2 bits will not overlap is (1-1/g)^k; then total traffic G = k/g.
  • Get the noise-immunity of spread-spectrum with the nice queueing properties of slotted ALOHA.
• Previous studies: no compelling evidence that there is a clear advantage for multiple-code CDMA systems, despite their complexity

Relevance

Flaws