Spread Spectrum Access Methods for Wireless Communications

One-line summary: CDMA combats multipath and interference effects by spreading a signal over a wide frequency band; CDMA has advantages and disadvantages over TDMA and FDMA - whether or not they balance out is an open question.

Overview/Main Points

• Fundamentals: a signal is spread into a larger frequency range than is needed to represent it - the redundancy gives error resilience, and the wideband frequency combats multipath effects because of frequency diversity. Cell-reuse patterns (as in TDMA and FDMA) no longer are strictly necessary; CDMA is described as having a universal one-cell reuse pattern.
  • Direct sequence (DS) modulates the signal with a code word chosen from an orthogonal set; the signal is chipped at a higher frequency and modulated with the code word - processing gain Gp is defined as Bspread / R, where Bspead is chipping/code rate, and R is source information rate. FDD is used to separate upstream and downstream signals.
  • Frequency hopping (FH) is where the signal is modulated onto carriers whose frequency is selected from a sequence or alternatively randomly. Slow FH is where the hopping rate is less than the information rate, and fast FH is the converse (multiple hops per source bit.) Bit interleaving is used as well as error-correcting codes to overcome interference caused fades. The TDMA summary treats FH more rigorously.
• Near-far problem: strong DS signals damp out weaker DS signals - complex BS coordinated power control is needed. ("Cocktail party" analogy.)
• Soft handoff: universal reuse pattern plus RAKE receiver can be used to make soft-handoffs (make before break) possible between cells and between sectors within a cell. (This seems independent of CDMA..)
• Performance:
  where Eb is received energy-per-bit, No is the noise spectral density, G is processing gain, M is # users/cell, alpha is voice activity factor, K is intercell interference to intracell interference ratio (cell spillover factor - varies with 1/N, where N is power low exponent).
• Coherent vs. non-coherent detection: Qualcomm IS-95 uses a pilot tone on the downstream path (BS->user) for coherent detection, but uses non-coherent detection on upstream. Qualcomm IS-95 is also narrowband, with 1.25MHz bandwidth (compared with 10MHz BCDMA, coherent on both paths, overlayed on current analog signals).
• Coherence bandwidth: the coherence bandwidth is the frequency range across which fading properties are correlated. The CDMA frequency spread should exceed the coherence bandwidth. 2-5Mhz for indoors, +10MHz for small rooms, +1MHz for outdoors and +10MHz for sattelite links. Coherence bandwidth is proportional to 1/(delay spread).
• Power control: high signal latency impedes the strict power control needed for CDMA. High velocities also impede power control, since multipath fades will be rapid.
• Diversity: spatial diversity through multiple BS antennae and multiple BS. Multiple BS enables soft handoff. Temporal diversity through bit interleaving, convolutional codes, and TDMA techniques.
• ISM band: orignally prohibited telecommunications, but CDMA is now allowed in this band. (FCC loophole?)
• Comparisons: as system capacity is approached, all users' performance degrades equally - contrast with TDMA/FDMA systems. CDMA advantageous for uniform user spatial distribution, while TDMA/FDMA better for "hot spots".

Relevance

Flaws

• Although the features of DS were discussed at length, DS was never really adequately defined.
• The capacity of CDMA systems was not well presented. The effective (Eb/No) formula demonstrates the interference-limited nature of the system, but more than one factor in that formula is affected by the number of users, making it hard to gauge how performance degrades as a function of users.
• The advantages of CDMA were qualitatively and not quantitatively presented. Is the added complexity worth it?