Testing CATV networks with direct sequence spread spectrum signals
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.