GB2101453A - Radio communications link - Google Patents

Abstract

This invention relates to communications where multipath reception is expected. A transmitter (10) and a receiver (11) are synchronously switched between a Training mode and a Traffic mode by reference clocks (18, 21). In Training mode the reception pattern of the communications link is established by measuring the time delays in multiple reception of a transmitted pulse. In the Traffic mode, window waveforms enable gates (25, 26, 27) in accordance with the reception pattern so that multiple receptions of a transmitted pulse may be combined (28). By providing overlaid waveforms for each transmitted pulse, the problem of limiting transmission repetition rate to the time delay of the longest transmission path is overcome. <IMAGE>

Description

SPECIFICATION Improvements in or relating to communications This invention relates to communications and in particular to the transfer of signals via a communications link in which multipath propagation is expected. Such multipath propagation occurs for example in communications by radio frequency link involving skywave propagation due to variation in height and reflection mechanism of ionospheric layers.

The precise path, and therefore the time delay, between transmission and distant reception of any particular signal cannot be predicted.

Propagation is very likely to occur via a plurality of different paths, each introducing a different time delay resulting in multiple reception of the signal delayed by various amounts. These receptions occur at the receiver to destroy the wanted signal.

In data transmission coherence of a received signal will be destroyed if a data pulse is transmitted before the time for reception of the previous data pulse via all propagating paths has passed. Hence multipath propagation limits data rate resulting in low bandwidth. Faster data rates can be achieved only at the expense of multiplexing over several such low bandwidth channels.

According to the present invention a method for the transfer of signals via a communications link includes determination of the reception pattern of the communications link and identification of signals received in accordance with the reception pattern.

The reception pattern of a communications link is the pattern of time delays applied to each reception of a transmitted signal by propagation.

It has been discovered that such a pattern can be determined for conditions existing over the link and that the stability of the pattern is such that signals received in accordance with the pattern is redetermined at intervals to maintain reception coherence.

Preferably the reception pattern is determined by transmission during a Training period, in which the time delay of each reception is recorded, and signal transfer occurs during a subsequent Traffic period. Advantageously transmission during the Training period is pulsed and therinterval between pulses is greater than the time taken for propagation via the longest propagation path, so that the time delay from transmission of each reception may be recorded.

According to another aspect of the present invention communications apparatus includes a transmitter for signal transmission, means for introducing to the transmitter a reference signal during a training period, and a receiver having means for measuring delays in reception of the reference signal to determine the reception pattern of the transmitter-receiver communications link and means for identifying signals received in accordance with the reception pattern.

In order that features and advantages of the present invention may be fully appreciated an embodiment will now be described with reference to the accompanying diagrammatic drawings, of which: Fig. 1 represents communications apparatus in accordance with the present invention.

Fig. 2 represents a signal pulse as transmitted and as received, and Fig. 3 represents waveforms as an example of the operation of the apparatus of Fig. 1.

Communications apparatus (Fig. 1) includes a transmitter 10 and a distant receiver 11. In accordance with the present invention the reception pattern of the transmitter/receiver link is determined by transmitting a pulse from the transmitter and measuring the arrival time of receptions via the various propagation paths. A transmitter pulse 12 (Fig. 2) is received at the receiver 11 after delays d1, d2, and d3 resulting from propagation by three different paths. The delays d1, d2 and d3 are measured at the receiver to determine the reception pattern of the communications link. The received signal is gated at gate 14 to a detector 1 5 which identifies pulse reception.The detector 1 5 triggers a processor 1 6 to record the arrival time with respect to the processor internal clock 1 7 of each reception, and the resulting reception pattern is stored by the processor 1 6. The transmission of the pulse is controlled by a reference clock 1 8 at the transmitter triggering a pulse generator 1 9 and gating the resulting pulse via gate 20 for transmission. The reference clock 1 8 is crystal controlled to ensure accuracy as a reference standard, an identical reference clock 21 is provided at the receiver and controls gate 14.A signal from the reference clock is fed to processor 1 6 so that the recorded time delays may be related to the actual time of transmission. Hence after pulse transmission has occurred and a time elapsed which is greater than the propagation time of the longest path the reception pattern of time delays applied to propagation paths over the communications link is stored at the processor 1 6. Another pulse may now be transmitted to record another measurement of the reception pattern. by integrating the many such records an accurate determination of the reception pattern may be made. This is achieved by periodically transmitting pulses, the interval between pulse transmissions being greater than the time delay for the longest propagation path, and integrating the results in the processor 1 6.Operation of the apparatus in the way described above will hereinafter be referred to as the Training Mode.

After operating for a predetermined time in training mode gates 14 and 20 are switched out of this mode by reference clocks 21 and 1 8 respectively. Operation is now as follows, and will be referred to as the Traffic Mode.

A binary data stream applied to Data Input 22 is transmitted by transmitter 1 0. The data stream may be a series of marks (pulse) and spaces (no pulse) at a rate referenced to the clock 18, for example marks A, B and D and space C (Fig. 3(a)).

Pulse A will be multiply received at the receiver 11 as pulses A,, A2, and A3 (Fig. 3(b)). Since the time of transmission of pulse A was referenced to clock 18, the time delays in the reception of pulses A1, A2 and A3 may be predicted from the reception pattern previously acquired during training mode. The processor 1 6 sets up a window waveform W having windows W1, W2, W3 (Fig. 3(c)) in accordance with the stored reception pattern record. The window waveforms control gates, in gate circuitry 23, to which the received signal is applied. The window waveform causes the gates to open at the predicted times of arrival of pulses A1, A2 and A3, and a signal arriving in accordance with the reception pattern is thereby identified, received pulses appearing on output lines 24.The pulses may be weighted by setting, under processor 1 6 control, coefficients C1,C2 and C2 of multipliers 25, 26 and 27 respectively in accordance with an assessment of the reliability of transmission via any particular path. Such an assessment may be made, for example, by transmission of a reference data stream. The weighted pulses are combined in a combining circuit 28 and the resulting pulse level is applied to threshold circuitry 29, which produces a mark or space output at data output 30. The threshold level of circuitry 29 may be set in accordance with the background noise level in the communications link, via input 31.

Similarly a window waveform is set up for receptions of pulse B (Fig. 3(d)) and pulse C (Fig.

3(e)) and a mark or space output produced. Pulse B will be multiply received as pulses B1, B2, and B3 (Fig. 3(6)). If pulse B is transmitted before the last reception of pulse A, there will be pulse overlap.

However, it will be appreciated that this does not corrupt reception since windows for each pulse are set up only at the predicted time of arrival of each reception in accordance with the reception pattern record stored by processor 1 6. For certain time delays, reception will be ambiguous due to coincident windows being set up for different pulses, such as the windows for pulses A3 and C2.

Where sufficient unambiguous reception are available ambiguity may be resolved by majority logic in the combining circuitry 29. Alternatively an ambiguous reception can be given a zero weighting by setting its multiplier coefficient accordingly. As another alternative, ambiguous windows may be suppressed-resulting in only correctly received pulses being onwardly transmitted by gate circuitry 23.

When the apparatus has been in Traffic mode for a predetermined time, reference clocks 1 8 and 21 control the return to Training mode so that the reception pattern may be re-determined. In this way account may be taken of changing propagation conditions.

In one proposed embodiment of the present invention transmitted pulses are raised co-sine energy packets of 50,uS duration and the transmitter and receiver bandwidth 20 kHz. In the Traffic mode the data rate is 2.4 Kilobits per second. The Training mode is entered every 53 seconds, and 256 pulses are then transmitted at an interval of 3.33 mS. Reception time is quantised into 1 6 possible window positions, and the window width is 50,uS. For greatest resistance to changes in reception pattern during Traffic mode, the reception peak within the window may be determined, and any timing error fed back to the receiver clock 1 7 to compensate for small changes in path delays. A reference clock accuracy of 10 mS over the transmission period is sufficient for such an embodiment.

It will be appreciated by those skilled in that art that since pulse overlap does not corrupt communication apparatus operating in accordance with the present invention the data rate is not limited thereby, as in prior art arrangements. It will further be appreciated that the multiple reception of pulses is used to advantage, in that errors occurring in propagation over a path may be corrected by reference to other receptions. The invention provides apparatus with good resistance to interference by multiple reception, since reception by each path may be weighted in accordance with the degree of interference encountered.

Claims (14)

Claims The matter for which the applicant seeks protection is:

1. A method for the transfer of signals via a communications link including determination of the reception pattern of the communications link and identification of signals received in accordance with the reception pattern.

2. A method as claimed in claim 1 and including re-determination of the reception pattern at intervals.

3. A method as claimed in claim 1 or claim 2 and including reception pattern determination by transmission during a training period in which the time delay of each reception is recorded, and signal transfer during a subsequent traffic period.

4. A method as claimed in claim 3 and including pulsed transmission during the training period, the interval between pulses being greater than the longest propagation path.

5. Communications apparatus including a transmitter for signal transmission, means for introducing to the transmitter a reference signal during a training period, and a receiver having means for measuring delays in reception of the reference signal to determine the reception pattern of th e the transmitter-receiver communications link and means for identifying signals received in accordance with the reception pattern.

6. Communications apparatus as claimed in claim 5 and wherein the reference signal includes a pulse, and the receiver includes means for establishing a record of the time delay in pulse receptions to determine the reception pattern.

7. Communications apparatus as claimed in claim 6 and wherein the reception pattern is determined by integration of several records.

8. Communications apparatus as claimed in claim 5, 6 or 7 and wherein data is transferred during a traffic period by the presence or absence of pulses at periodic intervals.

9. Communications apparatus as claimed in claim 8 and wherein the receiver includes means for setting up a window waveform having windows corresponding to the reception pattern of a pulse at each interval and means for detecting the presence or absence of a pulse in each window.

10. Communications apparatus as claimed in claim 9-and wherein detection of a pulse in each window depends upon coefficients set in accordance with an assessment of reliability of transmission.

11. Communication apparatus as claimed in claim 9 or claim 10 and wherein the receiver includes majority logic to resolve inconsistent receptions.

12. Communications apparatus as claimed in claim 9, 10 or 11 and wherein the receiver includes means for setting up overlaid window waveforms corresponding to reception at consecutive intervals.

13. Communications apparatus as claimed in claim 1 2 and wherein windows corresponding to ambiguous reception are suppressed.

14. Communications apparatus as claimed in any of claims 9 to 13 and wherein window positions are quantized.

1 5. A method for the transfer of signals via a communications link substantially as herein described with reference to the drawings.

1 6. Communications apparatus substantially as hereindescribed with reference to the drawings.