Testing CATV networks with direct sequence spread spectrum signals
View Patent ↗Use of direct sequence spread spectrum test signals injected at the same time and frequency of active network services to perform non-interfering measurements in the forward and reverse path of a CATV plant. Ideally, a plurality of transmitters, each transmitting a test signal with a unique spreading code, whereby multiple overlapping test signals are received simultaneously at the receiver (CDMA).
1. A method for measuring the frequency response of a transmission link, having a noise floor level, in an active CATV network transmitting signal traffic, having occupied wavelengths, to and from customers over the transmission link, the method comprising:
a) generating a first test signal;
b) modulating the first test signal using direct sequence spread spectrum with a chip rate, a data rate, and a spreading code length forming a direct sequence spread spectrum test signal;
c) transmitting the direct sequence spread spectrum test data signal into the transmission link at a transmitted power level below the noise floor level to limit interference with the signal traffic;
during step b) or c), selecting a shorter spreading code length for a wavelength of the direct sequence spread spectrum test signal that will not overlap with an occupied wavelength of the signal traffic than the spreading code length selected for a wavelength of the direct sequence spread spectrum test signal that will overlap with an occupied wavelength of the signal traffic;
d) demodulating the direct sequence spread spectrum test signal at a receiver in accordance with the selected shorter and longer spreading code lengths in order to measure the signal level of the first test signal; and
e) monitoring the received data signal power level at a receiver, thereby measuring a frequency response of the transmission link.
2. The method according to claim 1 , wherein step a) further comprises transmitting a plurality of test signals from a plurality of transmitters, and repeating steps b) to e) for each of the plurality of test signals.
3. The method according to claim 1 , wherein the spreading code length for wavelengths occupied by signal traffic is 20 to 200 Kchips/bit, and the spreading code length for wavelengths unoccupied by signal traffic is 8 to 200 chips/bit.
4. The method according to claim 2 , wherein each transmitter has a unique spreading code, whereby multiple overlapping test signals are received simultaneously at the receiver.
5. The method according to claim 1 , further comprising transmitting the frequency response back to a desired location over the transmission link.
6. The method according to claim 1 , wherein the direct sequence spread spectrum test signal has a coding gain of at least 40 dB.