Abstract: A lineariser (100) for reproducing distortion present in the output of an amplifier (110) (or other signal handling device) generates a predistortion signal from the amplifier input. The predistortion signal is mixed into the amplifier input signal using, for example, a vector modulator (112). The predistortion signal may be derived in a quadrature format, the orthogonal components of the predistortion signal being mixed into separate mixers of the vector modulator. The predistortion signal is generated by multiplying the input signal with itself repeatedly to generate components of distortion which are susceptible of independent control. The predistortion signal is generated digitally using DSP (116). A multiplier or mixer may be used to square the sampled input signal to produce a reduced frequency signal which the DSP can use to gennerate the predistortion signal. Another lineariser mixes the predistortion signal into the input signal during up conversion.

Abstract: Phase shifting network (110) predistorts the input to non-linear amplifier (114) to counteract AM-PM distortion. The input signal is sampled at (116) and its amplitude is detected at (118). The amplitude signal is then used to index a lookup table (122) which contains corresponding data for controlling the phase shifting network (110). The input signal may be digital and the phase shifting process may be implemented in the digital domain.

Abstract: Phase shifting network (110) predistorts the input to non-linear amplifier (114) to counteract AM-PM distortion. The input signal is sampled at (116) and its amplitude is detected at (118). The amplitude signal is then used to index a lookup table (122) which contains corresponding data for controlling the phase shifting network (110). The input signal may be digital and the phase shifting process may be implemented in the digital domain.

Abstract: The pre-distorter controller determines at (210) if the residual distortion in the amplifier output is above a threshold. If so, the phase and amplitude of a pre-distortion signal combined with the amplifier input signal are adjusted sequentially to reduce the residual distortion below the threshold. When the residual distortion falls below the threshold, step (214) determines if the residual distortion is below a second, smaller, threshold. If not, step (218) makes small adjustments to the phase and amplitude of the pre-distortion signal sequentially and the residual distortion is again tested at (214) to determine if it is now below the second threshold. If it is, then further adjustments are not made unless the residual distortion level moves above the second threshold. The control process can also be used with a feed forward lineariser to minimise output distortion (FIG. 5) or unwanted input signal components in the signal led forward (FIG. 6).

Abstract: In a multi-directional orthogonal frequency division modulation (OFDM) communication system, for example, on a digital subscriber line, an uplink channel is provided by a first group of the OFDM sub-channels (sub-carriers), and a downlink channel is provided by a second group of the OFDM sub-channels (sub-carriers). In one aspect, communication efficiency is improved by controlling the relative number of sub-channels allocated to each group, and hence controlling the capacity of the channels dynamically. Preferably, the relative capacities are controlled in response to demand for channel capacity. In another aspect, the orthogonality of the sub-carriers generated by different transmitters is improved by providing a frequency and/or time synchronising signal for providing reference frequency and timing.

Abstract: In a multi-directional orthogonal frequency division modulation (OFDM) communication system, for example, on a digital subscriber line, an uplink channel is provided by a first group of the OFDM sub-channels (sub-carriers), and a downlink channel is provided by a second group of the OFDM sub-channels (sub-carriers). In one aspect, communication efficiency is improved by controlling the relative number of sub-channels allocated to each group, and hence controlling the capacity of the channels dynamically. Preferably, the relative capacities are controlled in response to demand for channel capacity. In another aspect, the orthogonality of the sub-carriers generated by different transmitters is improved by providing a frequency and/or time synchronizing signal for providing reference frequency and timing.

Abstract: An analogue to digital converter system for digitizing a stream of analogue symbols, the analogue to digital converter system includes an analogue to digital converter for sampling the symbols at predetermined sample timings and a feedback loop for adjusting the sample timings. The feedback loop includes an eye opening detector connected to an output of the analogue to digital converter and responsive to successive digitized symbol samples to determine eye opening signals. The eye opening detector includes a deviation detector for determining a deviation signal representative of a deviation of a digitized symbol sample value with respect to a mean sample value. Preferably, a variance measurement calculator calculates the variance of these deviations. A feedback control is responsive to successive eye opening signals to generate timing control signals for adjusting the sample timings.

Abstract: A radio frequency circuit for a TDMA mobile telephone system allows bulky and expensive IF filters to be saved by sharing common filters between transmit and receive channels by automatic switching of bidirectional SAW filters.