Patent ID: 8094611

Claim:
A method for operating a second transmitter in a communication system in which a first transmitter transmits a first signal via a plurality of orthogonal frequency division multiplexing (OFDM) subcarriers within a frequency band of the system, wherein the first signal contains data that is encoded and modulated with a QAM or PSK modulation scheme, and is transmitted with a time frame structure that includes a plurality of frames, wherein each frame contains a plurality of subframes, each subframe contains a plurality of time slots, each time slot contains a plurality of OFDM symbols, and each OFDM symbol contains a cyclic prefix, the method comprising: transmitting with the second transmitter a second signal within the frequency band of the system, the second signal overlapping in time with at least a portion of the first signal, wherein: the second signal is formed by adding guard period(s) to either or both end(s) of a time-domain sequence, wherein the time-domain sequence has an autocorrelation with a high peak-to-sidelobe ratio and has a low peak-to-average ratio in the frequency domain, and wherein the second signal is aligned with a slot boundary of the first signal when the second signal is synchronized with the first signal, or the second signal partially overlaps in time with the first signal when the second signal is not synchronized with the first signal; the guard period(s) of the second signal is sufficient to account for time alignment uncertainty between the second signal and the first signal; and the second signal has a duration of at least several OFDM symbol periods of the first signal and the second signal has a high spreading gain for enhanced detectability against noise and interference from the first signal; the second signal occupies a bandwidth that is narrower than the bandwidth of the frequency band of the system and some frequency components within the occupied bandwidth are not energized in order to reduce interference between the second signal and the first signal; and the transmission power of the second signal is controlled to reduce interference with the first signal.