Abstract: Frame synchronization for data transmitted using quadrature amplitude modulation is achieved by providing that one symbol per frame is chosen from a larger signal point constellation than the remaining symbols. Thus, bits are assembled into groups of unequal size to control the mapping between the data and the signal points. Where constellations having a number of points not equal to a power of two, these may be coded in groups; or the data may be converted into a mixed-base number the bases of which correspond to the number of points in the respective constellations.

Abstract: Data are transmitted using quadrature amplitude modulation to select for transmission symbols from two (or more) different signal point constellations; constellation switching being used to facilitate transmission of a non-integral average number of bits per symbol and/or frame synchronization. Trellis shaping (where redundancy is introduced and selection of constellation "regions" performed over a number of symbols so as to minimize transmitted power) is employed; switching is provided so that power information (from stores is available appropriate to the constellation in use for each respective symbol. Synchronization may alternatively be provided by substitution outside the power control loop, of a symbol from an outer region for one selected from an inner region of a constellation. In another aspect of the invention, constellation-switching is employed within frames of A symbols, where A is not a power of two.

Abstract: A block of output symbols is generated, using quadrature amplitude modulation, from a block of input bits. The input bits are processed, using one or more redundant codes having a Hamming distance greater than one, to produce digital words which control the in-phase components and digital words which control the quadrature components of the output symbols. A group of bits generated by a single coding step supplies bits for both types of digital word, thereby enabling that coding step to process simultaneously a larger number of bits than would be the case if separate coding were used for producing the two types of word.

Abstract: A duplex data transmission system in which a station can vary its transmission power to keep the signal-to-noise ratio the same in each direction or, in a system employing a different data rate in each direction, to keep the two ratios in a fixed relation to one another. In an initial training sequence, each station measures "talker echo" from its own transmission, and subsequently measures noise from the other station's transmission. The other station then makes the same measurements. The noise data thus collected is sent by both to a control circuit in one or other station which calculates from both sets of data an adjustment (e.g. a reduction) of the power to be transmitted. One or other stations then accordingly reduces power.

Abstract: A method of transmission between a first and a second station via a transmission link in at least part of which transmissions in two directions travel over a common path, transmissions by the second station occupying a bandwidth which is the same as or at least partially overlaps the bandwidth occupied by transmissions by the first station, in which the first station includes an echo canceller and which simultaneous transmission by the stations occur only during periods which immediately follow a period of transmission by the first station alone and are of short duration relative to the rate of change of the echo characteristics of the path. The bandwidth overlap may be intentional or may be the result of imperfect filtering in a frequency division duplex system.

Abstract: A trellis coder has a convolutional encoder which has n states and can progress from a current state to a follower state depending on an input. The state progressions are selected such that they can be represented by a diagram having 90.degree. rotational symmetry. A state transition produces via mapping means by quadrature modulation one of four output carrier signal phases, such that any three state sequence gives rise to a pair of signals having the same phase difference as the pair generated by a corresponding sequence having a position in the diagram rotated by 90.degree. from that of the sequence in question. Input means enable a single bit data input to control the coder state progression so that a given differential output phase always corresponds to the same input bit value.