CATV Return Path Characterization for Reliable Communications
Charles A. Eldering, Nageen Himayat, Floyd M. Gardner
IEEE Communications Magazine, August 1995
Return path noise in CATV networks is dominated by "noise funneling"
(addition at cascade amps) and noise sources inside subcriber homes that
can be only partially attenuated, and will be reduced if <500 subscribers
are on one segment. Return path characteristics suggest that FDMA or FDMA/TDMA
hybrid will be better choice than CDMA or pure TDMA for MAC on this shared
Sources of noise:
- Noise funneling: return path usually engineered for unity gain, thus
attenuation occurs as you go up the tree (toward the head end) and noise
adds logarithmically. Noise can be correlated or uncorrelated (in/out of
MAC layer recommendations (TDMA, FDMA, TDMA+FDMA, CDMA)
- Narrowband signals: dynamic interference-cancellation algorithms (but
can't protect against malicious interferers). Can result in loss of a wide
channel depending on distribution.
- Impulse noise: use FEC
- Common mode distortion: keep equipment clean
- Location-specific, usu. in subscriber's home: "search and
- Return path filtering: filter high-freq (>15Mhz) noise as it leaves
- Power backoff: Need to keep average return-channel power constant;
easy with TDMA, hard with anything else.
- Line echo: can cause intersymbol interference for single-carrier
TDMA, but CDMA and N-FDMA (narrowband) are virtually immune.
- Narrowband interferers: TDMA hurt; SS-CDMA not much help if
inteferers unifrmly distributed, some help if they are not.
- Impulse noise: TDMA can use FEC; CDMA spreads the misery across
many users, esp. if SS; FDMA spreads impulse over many frequency slots,
but then a strong enough impulse can wipe out all the slots.
- Capacity: CDMA requires perfect synchronization to achieve
- Recommendation: FDMA or TDMA+FDMA.
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