GB2428921A - Spectrum sharing between radar transmissions and communications transmissions - Google Patents

Abstract

A method of spectrum sharing between radar transmissions and communications transmissions is disclosed wherein a waveform is modulated with communication data as a communication transmission and additionally serves as a radar transmission. Separate radar and communications antenna may be provided such that the radar pattern in elevation may be non-intersecting with a communication antenna pattern in elevation. Gaps may be provided in the communication transmission cycle to allow for the receipt of radar returns.

Description

1 2428921

SHARING SPECTRUM BETWEEN RADAR AND COMMUNICATIONS

APPLICATIONS

Primary surveillance radars occupy sigmficant spectrum in S-Band, around 3 GHz. On the other hand there is considerable need for radio spectrum to support wireless communications. It is therefore highly desirable to devise methods that will allow the spectrum to be re-farmed to allow the operation of primary radar and some form of cellular communications, thereby making more efficient use of the spectrum and providing greater economic benefit.

Current primary radars operate with long range. In order to achieve this they must use very large, i.e. at least lOs of kW transmit powers. This has several S.'.,.

unfortunate impacts. Firstly, the available technologies for generating such high powers are not amenable to the generation of spectra that are tightly contained. Secondly, the use of such high powers leads to the effective sterilisation of large areas for other uses.

Cellular mobile and fixed communications, on the other hand, have seen a y reverse trend in which transmit powers have fallen dramatically over the last few years S...

as the cell sizes have reduced in order to accommodate ever growing traffic capacity requirements. - : : : The present invention brings about the use of spectrum for both radar and communications through the innovation of a much tighter cellular deployment of radar monostatic stations that are located together with communications base stations and that use the same radio transmissions. Smaller coverage areas allow lower powers and better spectral properties negating the use of magnetrons or klystrons, as well as reducing the amount of spectrum needed by improving the spectral properties.

Specifically, the downlink spectrum of the communications base stations is fully used for radar transmissions. This is achieved by using some or all of the downlink communications transmission as radar transmissions. Possible antenna patterns in azimuth are shown in Figure 1.

Each sector antenna in the communications system provides independent communications to the covered sector. Each beam in the radar system is scanned across the sector to provide coverage of the applicable 120 sector. It will be appreciated that the use of three sectors is only by way of example. More or fewer sectors could equally well be considered. Indeed unsectored cells having 3600 coverage could also be implemented. Elevation patterns for a single sector are shown in Figure 2.

It will be noted that these patterns arc substantially non intersecting. The communications downtilted sector is implemented very much as is currently normal for cellular communications. The radar antenna pattern in elevation is unusual for this application. It will be noted that it does not provide coverage at the horizon. This is to minimise interference to other communications cells. However, because the radar range requirements are modest for any given radar station, it is still possible to detect radar targets down to the minimum necessary altitude.

In order to operate with modest practical transmit powers the radar operating range should be the smallest that is practical and economical. Currently, the minimum I's's.

required radar range capability is to reach an aircraft flying at maximum altitude of. : 42,000ft or about 8 miles. Sensible coverage considerations suggest that the horizontal radar range could be made a comparable figure. Then the maximum required radar range, across the diagonal, would be about 8 x 212 = 11.25 miles. The shape of the azimuth antenna pattern of Figure 2 is intended to reflect the requirement for increased S...

gain at 45 elevation where the maximum range is greatest. The round trip delay *SIe corresponding to 11.25 miles is about 120 p.s. The transmission period is based on this, *..

or the round trip delay can be computed from any other selected range. Thus, for example, transmissions can be made with 120 p.s duration. It will be appreciated that all radar returns of these transmissions except those from targets at maximum range will be at least partially overlapped with the transmission, i.e. the transmission will overlap the reception. However, conversely, there will always be a part of the radar return that is not overlapped with the transmission. Thus, if the 120 p.s transmission is followed by a 120 p.s transmission gap the available returns can be detected.

The principle, then, for each return is to correlate (or de-spread) only that portion of the return that is visible afler the end of the transmission, allowing that there will be some loss of visibility for minimum delay returns due to the transmit to receive switching time. In practice there will be some difficulties due to the very wide dynamic range of returns. The earliest returns will be much stronger than the later ones and it is likely that the processing gain due to correlation against the transmitted communications signal will be inadequate to avoid breakthrough from the short range returns to the long range measurement cells. This difficulty can be partly resolved by performing only partial correlations of the later part of the bursts so that very strong components of earlier returns are either not included or only partially included in the correlations. Typically, 60% to 80% of the portion is correlated.

However, for the earliest returns, on the one hand, the period for correlation is too short to obtain significant processing gain and on the other hand the dynamic range is very large. These requirements cannot, therefore, be satisfied by the communications transmission. For this reason it is necessary to incorporate a single pulse transmission, prefacing the coded transmission with a suitable gap for reception thereof. An example of correlation with inter-return interference is shown in Figure 3.

The diagram shows a radar transmission consisting of a single short pulse followed by a longer transmission burst. The radar returns follow the profile shown, with the earliest returns stronger than the later ones. Iii reality the range of signal levels is much greater but this would be difficult to illustrate. Beneath the radar return profile we see the responses to the radar transmission corresponding to each of the six returns shown. The height of each one corresponds to the amplitude and the shift to the right, :. . to the delay. The signal receivable at the radar site would be the sum of these.

Each of the returns is annotated with the method of detection. For the first three * the receiver simply detects the initial pulse. For the last three detection is by means of partial correlation of the end of the pulse. i.e. that some of the pulse that is after the end of the transmission. For example, examine the correlation (shaded dark grey) at the end of return 4. It will be noted that the selected period for correlation is overlapped with returns 3, 5 and 6, but not with returns I and 2. It thus has less interference than if it started immediately after the end of the transmit pulse. The interference from returns 5 and 6 should not be problematic because they are weaker and are suppressed by the processing gain involved in the correlation process. Thus the period of correlation for return 4 is a compromise between the number of earlier returns included and the processing gain through maximising the period of correlation. Similarly, return 5 only experiences interference from return 4 and return 6 only experiences interference from return 5. As the period of return reduces, the period of correlation increases. The pulse structure is shown in Figure 4.

The signal in the radar and communications transmission is identical. The only difference is that the communications signal is radiated through the sector antenna whilst the radar signal is radiated through the elevated beam.

Normally FM radar operates allowing reception of returns during the transmission of the extended duration pulse. However, typically this relies on the following considerations. The pulses are typically constant envelope which makes cancellation of the leakage signal much more straightforward and often separate antennas are used for transmit and receive, providing increased attenuation for the leakage path. If phased array antennas are used this is not very attractive.

It is not essential for the gap in transmission to be quite as long as the period of transmission. However, to the extent that it is shorter, there is a loss of sensitivity and therefore, of range. It is also possible to make the radar transmission pulse shorter than the communications burst whilst using the same modulation for the radar pulse that is being used for the corresponding part of the communications burst. However, if this is done there is a discontinuity in phase and amplitude in the signal as received at the mobile station, arising at the time when the radar signal is activated. It is possible to mitigate this effect by incorporating separate channel estimation reference data in the two parts of the burst. This reference data could take the form, for example, of ambles' :. . (pre, post or mid) or periodically transmitted pilot symbols. The communications receiver in the mobile station can then capture the first, radar free, part of the burst and:* the second, radar present, part of the burst, either separately or as part of a single * : ::: capture process, in the form of digitised complex baseband. Independent channel estimates can then be formed from the separate reference data in the two parts of the burst and used separately to demodulate the two parts. There is no point in making the radar burst component any longer than the transmission gap.

One possible example of this operation is shown in Figure 5. In this case the communications transmission is roughly twice as long as the radar transmission.

However, both parts contain training data to allow independent channel estimation.

In general the combined radar and communications transmissions followed by transmission gaps need not appear on a regular basis. They can be incorporated according to some form of pseudo random pattern such as that shown in Figure 6.

This approach allows ambiguity resolution over target doppler shifts whilst also, in general, reducing the overhead associated with the radar operation. In multiple doppler filters the returns corresponding to the same delay from each of several radar transmissions can be summed after multiplying each received complex sample by the phasor having a phase shift denved from doppler shift of interest and the (possibly non uniform) transmission delay associated with each sample.

Alternatively, a variety of fixed transmission rates can be cycled through as illustrated in Figure 7. A further alternative is to toggle between two or more transmission durations in a regular pattern as illustrated in Figure 8.

A coded signal may be transmitted over a period of time longer than the resolution of the radar. At the end of the transmission, the transmitter is switched off and receive begins. A correlation of a delayed transmitted signal against a possible radar return arriving with that delay for a period beginning after the beginning of the receive period and ending by the end of the delayed transmitted signal is performed.

The correlation can begin after the beginning of the receive period, delayed by a suitable fraction of the maximum possible correlation period for that radar return delay such that a substantial optimum is established between maximising processing gain on one hand and minimising possible interference from stronger earlier returns on the other.

A pulse can be transmitted prior to the coded signal and radar returns are accumulated over the period between the transmission of this pulse and the beginning * :* of the coded signal. The gap in time between the transmission of the pulse and the * : : transmission of the beginning of the coded signal is chosen such that when detection of the earliest delay that cannot be detected from the pulse is attempted as describe above, then a suitable delay in beginning the correlation is adequate to achieve acceptable processing gain to discriminate between this return and earlier arriving returns.

The coded transmission may consist of a carrier wave modulated with physical layer data according to a standard method of modulation. The physical layer data in a burst can be scrambled according to a pseudo random function with substantially random-like properties. The scrambling function may generate substantially statistically independent random sequences over a significant number of transmission bursts. The scrambling functions in different radars can be set to generate random sequences that are substantially statistically independent at any given time between two or more radars A pulse transmission system is proposed suitable for contemporaneous radar and communications operation in which transmission bursts containing physical layer data are followed by transmission gaps suitable for reception of radar returns. Some physical layer transmission bursts are followed by transmission gaps and others are not.

All physical layer transmission bursts may be transmitted from one or more antennas specilcally dedicated to communications functions. If the physical layer transmission bursts are followed by gaps they may be radiated from one or more antennas or antenna systems specifically dedicated to radar functions.

The physical layer transmission bursts that are not followed by gaps can be disabled from being radiated any antennas or antenna systems specifically dedicated to radar function. The duration of communication only transmissions, not associated with the radar function, may be set to be non-constant, for example varying according to a pseudo random pattern. The duration of communication only transmissions, not associated with the radar function, can be varied between a restricted set of allowed values and a substantially constant, but variable across the members of the set, number: of transmissions is made using each allowed value from that set in turn.

A system is proposed in which a radar transmitter / receiver and communications transmitter are collocated and in which at least one of the operational ** frequencies used for communications is also used by the radar transmitter I receiver.

Separate antennas or antenna systems can be used for the radar transmitter / receiver * : : :* and for the communications transmitter. The elevation patterns for the antennas or antenna systems used for the radar transmitter I receiver may be arranged to be substantially nonoverlapping with the elevation patterns for the antenna or antenna systems used for the communications transmitter.

A radar transmitter / receiver and communications transmitter are collocated and at least one of the operational frequencies used for communications is also used by the radar transmitter / receiver. The communications transmission bursts are arranged to be transmitted according to a pattern in time that has a repeating frame and in which the radar antenna pattern or patterns is/are scanned in time wherein the radar antenna pattern scanning and the repeating transmission frame are synchronised together.

The repeating frame may constitute a time division multiple access (TDMA) frame such that signals transmitted at different times within the frame are intended for reception at different destinations. The radar antenna pattern scanning can be applied in azimuth to provide detection of targets from a range of directions and may be achieved by means of mechanically rotating a directional antenna, or by electronic means In the case of electronic means these can consist of the adjustments of the complex coupling weights applied to the outputs of a range of antenna element outputs forming a phased array antenna.

A system can have a radar transmitter / receiver and communications transmitter are collocated and at least one of the operational frequencies used for communications is also used by the radar transmitter / receiver, with the communications transmission bursts arranged to be transmitted according to a pattern in time that has a repeating frame. The radar antenna pattern or patterns is/are scanned in time wherein the radar antenna pattern scanning and the repeating transmission frame are syncbronised together. The pattern in time constitutes a TDMA frame and a frequency re-use pattern of synchronised communications base station surround the combined radar transmitter / receiver and communications transmitter. Means may be provided to minimise interference from the radar transmission to receivers of the communications transmissions in other cells using the same frequency by altering as appropriate the assignments of TDMA time slots to users in those other cells in such a.. :* way as to avoid such a receiver being required to receive its wanted signal at the same time as it is experiencing substantial interference from the radar pulse, due either to that pulse or a side lobe of it being directed towards that receiver at that time. The receiver.

can measure interference in all time slots and reports back to its base station, ranking information that allows the base station to propose a preferable time slot. Part of a signal in radar is visible to a communications terminal which recognises it as being a radar signal, obtains a pattern of interference and sends this back to the base station, so it can modify transmission time to avoid the interference.

Multiple simultaneous beams can be used, with all the antenna beams produced from one antenna. The beams are either staring beams, or scanning beams. The system is optimised for simultaneous communications and radar use, in both monostatic and multistatic radar configurations.

Claims (12)

1. A method of spectrum sharing between radar transmissions and communication transmissions, the method comprising providing a waveform modulated with communication data as a communication transmission; wherein the modulated waveform also serves as a radar transmission.

2. A method according to claim I, wherein separate radar and communications antennas are provided.

3. A method according to claim 2, wherein a radar pattern in elevation is substantially non-intersecting with a communication antenna pattern in elevation.

4. A method according to any preceding claim, wherein gaps in the 1 5 communication transmission cycle are provided to permit receipt of radar returns. ..

5. A method according to claim 4, wherein only radar returns visible in the gaps are correlated. p * . p * *.

6. A method according to claim 4 or claim 5, wherein the transmission gap is of equal length to the transmission. S... * S * S

7. A method according to any preceding claim, wherein communication transmissions are partially at radar free periods and partially concurrent with radar transmissions.

8. A method according to any preceding claim, wherein communication transmissions follow a pseudo random pattern.

9. A method according to any preceding claim, wherein radar and communication transceivers are co-located.

10. A method according to any preceding claim, wherein radar beam scanning and communication transmission bursts are synchronised.

11. A method according to claim 10, wherein the communication transmission bursts are TDMA transmissions adapted such that TDMA receive time slots are allocated in periods when no radar interference is being received.

12. A method according to claim 11, wherein the TDMA receive time slots are allocated based on measurements at the receiver reported to a base station of the receiver. *S * * S S * II * * S.. Si S * * * * I* 5.' * elf a a ** .