Abstract: A frequency hopping radio communications system has a plurality of radios each incorporating a transmitter and a receiver. When a radio is not transmitting or receiving, a switch switches it into a mode in which it monitors the signal strength in each of the available hop channels so as to assess the level of interference, and selects a subset of the total number of channels, those having the lowest interference level. The identities of this subset of channels are stored in a store. When the radio is to transmit a message, it first transmits the identities of the subset of channels stored in the store--using the full number of hop channels (the information being transmitted with sufficient redundancy to overcome likely interference). The receiving radios receive the identities of the subset of channels. The transmitting radio then transmits, pseudo-randomly, on this subset only and each of the receivers receives on this subset only.

Abstract: A personal portable telephone (11) allows the user access to the cellular radio network via a number of different gateways including a communal unit (15, 16), user's mobile telephone set (9), home telephone (13) or office PABX (14). The telephone (11) can also be used as a paging unit for use within the cellular radio network. The communal radio unit (15) is a multi-channel arrangement capable of servicing a large number of personal telephones within its service area which is small compared to that of a normal cellular radio cell. Communal units (15) may be transportable and located in public areas such as railway stations, airports and on trains or coaches. The telephone (11) automatically transmits its identification number to a gateway (9, 13, 14, 15, 16) when it enters its service area enabling the subscriber to be accessed by callers without their knowing his whereabouts.

Abstract: A syncrhonization arrangement is disclosed for synchronizing a receiver with a transmitter whose frequency is hopping through a sequence of predetermined frequencies arranged pseudo-randomly. The transmitter includes a time base circuit in the form of a string of dividers operated from a stable oscillator. One divider produces (in this example) a 28 bit synchronization signal which changes state once every sixty four hop frequencies. The 28 bits control a keystream generator which produces a pseudo-random output sequence controlling the transmitting frequencies. This 28 bit signal is formatted by repeating it several times successively, with its bits partially re-circulated to a predetermined extent each time, masking bits are added, and the result transmitted to the receiver on one of the hop frequencies.

Abstract: An input signal is mixed with a reference frequency and with a frequency 90.degree. phase-displaced from the reference frequency, so as to produce resultant waveforms whose slopes are dependent on the respective frequency differences detected. Both waveforms are differentiated to produce respective slope signals. Two comparisons are then carried out: one comparison compares the sign of one slope signal with the sign of the waveform producing the other slope signal, and the second comparison compares the sign of the other slope signal with the sign of the waveform producing the first slope signal. The two comparisons produce nominally identical intermediate signals having values dependent on the difference between the input and reference frequencies. Control means operative in dependence on the relative magnitudes of the two waveforms then selects the intermediate signals alternately, each selected intermediate signal constituting the output signal of the circuit.

Abstract: The invention relates to the correction of errors in received data, such as binary data. The system described monitors the actual level of signal representing each received data bit in a received data block so as to produce for each bit a first signal output representing the particular binary value of that bit and a second signal output representing the degree of probability of the bit having that particular value. The system generates, in response to the received block of data and using a very fast algorithm, a plurality of further, mutually different, blocks of data in each of which up to a predetermined number (the same for all the generated blocks) of the bits differ in binary value from the corresponding bits in the received data block.

Abstract: A frequency synthesizing arrangement is disclosed employing a phase-locked loop. The phase detector of the phase-locked loop receives an input frequency via a divider having a division factor N, and compares this divided frequency with the frequency received from a voltage controlled oscillator via a divider having a division factor N-b. Any difference is eliminated by the control signal from the phase detector which is connected to adjust the VCO frequency. Therefore, the phase-locked loop multiplies the divided input frequency by the factor (N-b). By making b very much smaller than N, the minimum step change in output frequency is approximately F.b/N.sup.2 (where F is the input frequency to the divider). In this way, the minimum step change in frequency can be made very small.