GB2126844A - Multipath correction in radio systems - Google Patents

Abstract

A digital radio system has means to compensate for dominant path changes during transmission. The input data stream is divided into notional frames and synchronisation signals are inserted between the frames in the transmitter. At the receiver each frame of the input data stream is read into a FIFO store (30) and read out therefrom after a time delay T0-t1 where T0 is a constant and t1 is a transmission delay of the signal over its transmission path. If the dominant path changes during transmission, the corresponding change in t1 will be sensed by detecting the next synchronisation signal after the path change. This variable time delay in reading out data from the FIFO store 30 compensates for the jump in the data stream that would normally be caused by the path change. <IMAGE>

Description

SPECIFICATION Radio systems The present invention relates to radio systems for carrying digital information.

In a radio transmission between a transmitter and receiver it is possible for there to be a number of different paths which the radio signal can take to the receiver. The paths will typically have different path lengths and attenuations. Therefore the receiver will be presented with a number of incoming signals of various intensities and timeshifted relative to one another on account of the different lengths of the paths over which they have been transmitted. These various incoming signals will interfere with one another. This problem is particularly encountered with HF transmissions when differential delays of a few milliseconds are common. These different paths may be caused by the terrain or by propagation effects, e.g. different reflecting "layers" in the ionosphere for HF signals.

Normally, the signal transmitted along one of the paths will be "dominant" and the receiver will accordingly be captured by this "dominant" signal in preference to the rest. How readily a receiver is able to be captured by the signal transmitted along the "dominant" path can depend on the type of modulation being employed. It has been found that frequency shift keying (FSK) is particularly suitable in enabling the receiver to be captured by a "dominant" signal which is only a few dB stronger than the other time-shifted signals from different paths. Which path is dominant at any one time is dependent on a number of factors and it is possible for the "dominant" path to change during the course of a transmission.This may arise as a result of the movement of either the transmitter or receiver, a change in the carrier radio frequency of the transmission as in a frequency-hopping radio system, or other changes e.g. in the propagation conditions. Whilst the dominant path is changing over, there may be a period of time in which no one path is dominant and this normally results in a large number of errors in the received data as the receiver switches between two or more incoming signals from different paths. The paths are usually of different lengths thus causing the received data stream to appear to jump either forward or backward as a result of the different transmission time for the signal over the new path when the receiver changes from one path to another.Such jumps lead to synchronisation errors as some data bits may be repeated in the received data stream, if the new path length is longer, or missed if the new path is shorter.

The present invention provides a radio system comprising a radio transmitter for transmitting digital signals and having means for periodically inserting synchronisation information into a data stream, a radio receiver comprising means for detecting said synchronisation information, and buffer means for receiving and storing the data received and reading out said data with a time delay dependent on a time lapse between an expected time of arrival of the synchronisation information preceding that data and its actual time of arrival.

Preferably, the data is read out at a lower rate from the buffer than the rate at which it is received so that the output data stream is substantially continuous.

Advantageously, the transmitter and receiver may further comprise means for modifying the data in accordance with an error correction system.

The invention further provides a method of transmitting a data stream, comprising the steps of dividing the data stream into frames, inserting a synchronisation signal between the frames, transmitting and receiving the data and synchronisation signals, detecting the receipt of each synchronisation signal, storing the frame of data received after each synchronisation signal, comparing the time of receipt of the synchronisation signal relative to the time of receipt of the previous synchronisation signal so as to detect any change in the transmission time due to a change in dominant transmission path, and reading out each stored frame or data after a time delay which is dependent on and compensates for any said detected change in transmission time.

The method and system of the invention may advantageously be used when the data stream is derived from a vocoder and/or in conjunction with a frequency-hopping system.

A radio system embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a block diagram of a radio transmitter in the system; Figure 2 is a block diagram of a radio receiver in the system; and Figures 3A to 3D are relative timing diagrams to illustrate the operation of the receiver and transmitter of Figures 1 and 2.

The radio system to be described comprises a transmitter 2 and receiver 4 as illustrated in Figures 1 and 2 respectively. The data stream to be transmitted comprises a series of output "frames" from a vocoder (not shown) which is used to turn an analogue speech signal into a digital output. The radio system is a frequencyhopping radio system although a constant carrier frequency could alternatively be used. The transmitter may employ any type of modulation to encode the digital data stream to be transmitted although FSK is preferred.

The transmitter comprises a first-in first-out (FIFO) store 6 to which is fed the input data stream along line 8. Synchronisation signals are generated periodically at block 10 and inserted into the data stream via a switch 12 at regular intervals under the control of a timing circuit 14.

The synchronisation signals may each consist of a maximal length pseudo random sequence or any other suitable framing signal.

The timing circuit 14 also controls the FIFO 6 so that it reads out a 'frame' of the input data stream at a faster rate than that at which the data was fed into the store from the line 8. In this way sufficient space is created between consecutive 'frames' of data to allow for the insertion of the synchronisation signals. The "frames" created by the insertion of the synchronisation signals need not necessarily coincide with "frames" generated by the vocoder though preferably each of the "frames" defined by the synchronisation signals includes an integral number of vocoder "frames" and starts at the beginning of a vocoder "frame".

The data stream together with the synchronisation signals is fed along line 1 6 from switch 12 to a modem 18 and thence to a radio transmitter 20 connected to an aerial 22 from which the suitably modulated data stream is transmitted.

The receiver comprises an aerial 24 which is connected to a radio 26 where the radio frequency carrier signal used to transmit the data is detected.

The output from the radio 26 is fed to a modem 28. The output from modem 28 is fed both to a FIFO store 30 and to a synchronisation detector circuit 32 which controls a timing circuit 34. The timing circuit 34 in turn controls the time at which the received data is fed out of FIFO 30 along line 36.

In the illustrated embodiment the input data stream is shown as first passing through a forward error correction (FEC) block 40 before being passed to FIFO 6. Likewise if FEC is provided in the transmitter it is also necessary to provide a FEC block in the receiver which acts on the data stream received along line 36 from FIFO 30. The provision of forward error correction is optional and depends upon the degree of accuracy required in the output data stream. In the present example, where the input data stream is derived from a vocoder FEC is normally unnecessary as a demodulated vocoded speech signal is still readily intelligible even though a relatively large number of burst errors may have been introduced into it during transmission.Vocoders are far more tolerant of burst errors than of random errors, and the errors caused by muitipath effects tend to be of a burst type.

The FEC techniques used in blocks 40 and 42 may be of any known type and will not be discussed in detail herein. Further details of FEC techniques may be found in chapters 10, 11 and 12 of "Principles of Data Communication" by R. W. Lucky, J. Salz and E. J. Weldon Jr. published by McGraw Hill Book Company in 1968. it is also possible for the input data stream to be interleaved before passage to FIFO 6 and de interleaved in a known manner after output from FIFO 30, so as to turn the burst errors normally produced by the system into effectively. random errors, which is necessary for certain types of error correction.

The operation of the radio system described above will now be discussed in more detail with reference to Figures 3A to 3D.

Figure 3A represents a continuous input digital data stream made up of a series of "frames" derived from, for example, a vocoder (not shown).

This input data stream is divided into three notional "frames" designated Frame 1,2 and 3 each containing an equal number of vocoder frames and thus of data bits.

Figure 3B represents the output along line 16 from switch 1 2. As illustrated here each frame has been compressed by having been read out from FIFO 6 at a more rapid rate than that at which it was read in. In between each adjacent pair of frames, a short synchronisation signal generated by block 10 under the influence of the timing circuit 14 is inserted. It will be seen that the data stream has been delayed by a small time .interval during passage from line 8 to line 16.

Figure 3C represents the output of the receiver modem 28. In this example the dominant path is of such a length that the first illustrated synchronisation signal, referred to as sync 1, is delayed by a time t, with respect to its transmission time. If sync 1 is the first element of a transmission the receiver assumes an arbitrary intermediate value for the initial transmission delay t,. All subsequent transmission delays can be measured relative to this first time delay. The transmission delay t,, for sync 2 is, in this example, the same as that for sync 1. This is apparent as the beginning of sync 2 is detected exactly T seconds later than the beginning of sync 1, where T is the length of an uncompressed frame as shown in Figure 3A or, equivalently. the length of a compressed frame together with a synchronisation signal as illustrated in Figure 3B.

While the transmission delay t, remains unaltered this is indicative of the fact that the dominant path has remained the same or-just possibly -- has changed to one of exactly equal path length.

The present example shows the "dominant" path changing over a period 44 during the transmission of Frame 2. While no one path is dominant the transmitted data is corrupted. The transmission delay for the new path is now t2 and the beginning of sync 3 is not received T seconds after the beginning of sync 2 but at a slightly later time equal to (T + t2-t1) seconds. As illustrated the new "dominant" path is longer than the previous one and the time t2 is greater than t,. If the new dominant path had been shorter the time t2 would have been less than t, and sync 3 would have been received earlier than expected rather than later as in the present example.

Figure 3D illustrates the output data stream along line 36. The demodulated output from the modem 28 which is illustrated in Figure 3C is supplied to FIFO 30 and synchronisation detector 32. The sync 1 synchronisation signal is detected by detector 32; and the transmission time delay t1, or the selected intermediate delay (if sync 1 is at the beginning of the transmission), is fed to the timing circuit 34 which in turn calculates a time delay T,--t, from the end of sync 1. The timing circuit then provides a signal to FIFO 30 to cause the data stored therein to be read out at a slower rate than it was read in so that Frame 1 occupies the same amount of time as it did in the input data stream represented in Figure 3A. The time To is a constant selected to be greater than any transmission time delay.The synchronisation signal which is fed to FIFO-30 is discarded and not read out so that the output data stream is effectively identical with the input data stream of Figure 3A.

Sync 3 is received after a longer transmission time delay t2 because of the dominant path change during the transmission of Frame 2.

Therefore the time delay T0-t2 introduced by timing circuit 34 before Frame 3 is read out of FIFO 30 is less than previously, so as to compensate for the jump in the data stream that would otherwise have been present.

The period 46 in Frame 2 following the dominant path change, displaced by the time delay T,--t, introduced after sync 2, is in error.

The first portion of this burst of errors 46 corresponds to the corrupted data 44 which was caused by there being no dominant path. The remaining errors in period 46 are due to the jump in the data stream which is corrected when sync 3 has been received so that Frame 3 is read out in its correct relation to the previous two frames.

Each frame read out from FIFO 30 has an identical length although a frame such as Frame 2 which is being transmitted when the dominant path changes to a longer path has a longer duration than the others in the demodulated output from modem 28 as represented in Figure 3C.

If the synchronisation signal is for any reason not detected, the timing circuit 34 continues to apply the same time delay which was established by the previously received synchronisation signal during the reading out of the following "frame" from the FIFO 30. Thus the assumption is made that there has been no "dominant" path change if the synchronisation is not received.

If the FEC blocks 40, 42 are included in the transmitter and receiver respectively, the error period 46 may be at least partially corrected, particularly if interleaving of the input data stream was also employed.

Although the FIFOs 6, 30, synchronisation generator 10 and detector 32 and timing circuits 14, 34 have been illustrated as independent blocks they may be dedicated digital circuitry or alternatively their functions may be performed by suitably programmed microprocessor circuits at the transmitter and receiver respectively.

As the above described technique is employed in a frequency-hopping radio system, it is preferable if a synchronisation signal is transmitted immediately after the carrier frequency has been changed to a new frequency.

A dominant path change is more likely to occur in coincidence with the frequency change than at other times as the carrier signal frequency represents an important factor in determining which path is dominant. This keeps errors to a minimum if the dominant path has changed as a result of the change to a new carrier frequency because a new delay for FIFO 30 will be immediately calculated from the new transmission delay so that the next frame will be in registration with the previous frames.

Although the transmitter output has been described as if it were continuous, in frequency hopping radio systems it is normally necessary to provide a period of "dead" time during which the transmitter and receiver are changing frequency and no data is transmitted. In the present system this "dead" time would occur between the end of one frame and the transmission of the following synchronisation signal.

It will be appreciated that although a frequency-hopping radio system for transmitting data derived from a vocoder has been described above, it is also possible to employ the above described technique for data derived from other sources and/or with radio systems other than frequency-hopping systems.

Claims (8)

1. A radio system comprising a radio transmitter for transmitting digital signals and having means for periodically inserting synchronisation information into a data stream, a radio receiver comprising means for detecting said synchronisation information, and buffer means for receiving and storing the data received and reading out said data with a time delay dependent on a time lapse between an expected time of arrival of the synchronisation information preceding that data and its actual time of arrival.

2. A radio system as claimed in claim 1, in which the data is read out at a lower rate from the buffer than the rate at which it is received so that the output data stream is substantially continuous.

3. A radio system as claimed in claim 1 or 2, in which the transmitter and receiver further comprise means for encoding and decoding the data respectively to provide error correction.

4. A radio system as claimed in any one of the preceding claims, in which the transmitter and receiver are part of a frequency hopping radio system.

5. A radio system as claimed in any one of the preceding claims, in which the data stream is derived from a vocoder.

6. A method of transmitting a data stream, comprising the steps of dividing the data stream into frames, inserting a synchronisation signal between each pair of adjacent frames, transmitting and receiving the data and synchronisation signals, detecting the receipt of each synchronisation signal, storing the frame of data received after each synchronisation signal, comparing the time of receipt of the synchronisation signal relative to the time of receipt of the previous synchronisation signal so as to detect any change in the transmission time due to a change in dominant transmission path, and reading out each stored frame of data after a time delay which is dependent on and compensates for any said detected change in transmission time.

7. A radio system substantially as herein described with reference to the accompanying drawings.

8. A method of transmitting data substantially as herein described with reference to the accompanying drawings.