Abstract: In a QAM radio transmission system, stuffing indications for a transmitted stuffed data signal are transmitted only once rather than repeatedly. The two possible states of each stuffing indication are represented by respective sets of signal points in the signal point constellation, with a separation between any two signal points in the different sets which is at least three times the signal point separation of the system. The reliability with which the stuffing indications are sent is much greater than for other transmitted information.

Abstract: A carrier lock detector for a quadrature amplitude modulation system includes Exclusive-OR gating circuits for determining when detected signal points occur within first areas centered on signal point positions in a phase plane diagram or second areas between the signal point positions, and for producing corresponding output signals. An integrated difference between these output signals is produced by an integrator and compared with a threshold level to provide a carrier lock detection signal. The arrangement is such that the integrated difference is substantially zero when the carrier is unlocked, so that the threshold level can be set to a low value to enable reliable operation of the detector at low signal-to-noise ratios.

Abstract: The present invention provides a method of detecting the clock rate and recovering the carrier in a TDMA (time division multiple access) signal. The quadrature baseband components are stored for a predetermined amount of time as sampled data and the clock phase error of the baseband samples are estimated. The clock phase of the baseband samples are corrected from the estimated clock phase error by using interpolation over the sampled quadrature components. The carrier phase error of the stored baseband time corrected quadrature samples are then estimated and the carrier phase is corrected according to the estimated phase error. The corrected clock and carrier signals are then forwarded to decision circuit means for providing demodulated data.

Abstract: A square array of signal points forming a QAM signal point constellation is modified by relocating signal points from corners of the square to positions extrapolated from the square array and at reduced distances from the origin, corresponding to reduced peak amplitude levels. The relocation is effected in a manner to maintain, at least for a majority of the relocated points, a Hamming distance of one. To this end for a constellation of 2.sup.2n points, with n=4 or more and points with I and Q amplitudes 1, 3, 5, . . . 2.sup.n -1 units in the square array, a plurality of points in each quadrant are relocated to positions, relative to positions which they would have in the square array, rotated through an angle of 180.degree. about predetermined reference points having I and Q amplitude co-ordinates of (2.sup.n,2.sup.n-1) and (2.sup.n-1,2.sup.n). Specific examples are described for 256 QAM (n=4) and 1024 QAM (n=5).