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Spread Spectrum Access Methods for Wireless Communications
Ryuji Kohno, Reuven Meidan, and Laurence B. Milstein
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.
- 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
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".
CDMA is all the rage in the media-access world. It has advantages over
TDMA/FDMA at the cost of implementation complexity.
- 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?
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