GB2306057A - An adaptive system for multi-signal discrimination - Google Patents

Abstract

An interference reduction scheme and method is disclosed for use in telemetry and private mobile radio trunking. Adaptive systems as are known require knowledge of the signal characteristics of both wanted and unwanted transmissions. Timing information has been required of the wanted signal so that a known period of non-transmission enables interference to be determined and thereby adaptively cancelled. Further the interference must be transmitting during this period of wanted signal silence, which is not always the case. The present invention provides a continuously sampling scheme where the time constant of the adaption processor P is such that interference is nulled and the wanted signal is not, owing to their different durations.

Description

ADAPTIVE SYSTEM FOR MULTISIGNAL DISCRIMINATION The present invention relates to systems for discriminating among multiple radio signals to recover information and more specifically to an adaptive system for recovering a signal from among several signal sources in a common channel.

The separation of independent sources from received data is a well known and difficult problem in signal processing. Generally, the signal sources as well as their exact frequency charateristics are unknown.

Typically, in any country radio telecommunication operators are allocated several bands of usage. In the UK, for example, the UHF2 band (450-470 MHz) is currently assigned to Private Mobile Radio (PMR), which is typically split into channels of several kHz.

Frequency reuse is obtained by dividing the country into cells for Telemetry and Private Mobile Radio Trunking use. This can give rise to inter-cell interference when propagation distances are increased either by geographical features e.g. valleys or atmospheric effects ('lift conditions'). Adjustment of cell sizes and shapes are controlled so interference can be avoided except under extreme conditions. Much more difficult to avoid is interference from transmitters based in other countries which are outside of the particular national jurisdiction.

A PMR system used for scanning telemetry typically comprises a single base station surrounded by several outstations. The base station is equipped with an omni-directional antenna and the outstations with directional Yagi antennass.

Scanning telemetry is used to remotely monitor and control a number of outstations. Interference effects are not as noticeable on the link from the base station to the outstations since situations where the wanted link has the same line of sight as the interference can be avoided. For cases not in line of sight, the gain of the Yagi antennas used by the outstations give better signal to interference levels. Since the scanner must receive signals from outstations in different directions, an omni-directional antenna is currently used.

This leaves the link from the outstation to the basestation vulnerable to interfering signals operating on the same band.

One technique to overcome interference is that of antenna nulling the aim is to reduce the gain of the receiving antenna in the direction of the interference source, while at the same time retaining good coverage in the direction of the outstations. There are two solutions that can be employed: The first solution entails the use of a fixed directional antenna to obtain a sample of the interference, which could then be combined in anti-phase with the response from a second omni-directional antenna. This would desensitise the main antenna system to interference, but installation requires knowledge of the direction of the interference and precise phase matching of the two signals.

This can only be easily achieved in the presence of the interfering signal. Since the interference can be intermittent, depending on the atmospheric conditions, installation of the system is difficult.

The second solution entails the use of an adaptive cancelling scheme. As is known, adaptive antenna schemes employ phased antenna arrays which can steer nulls in the coverage pattern of the antenna whereby unwanted signals are not received. This technique requires a method of discrimination such that only unwanted signals fall into nulls in the reception pattern. The unwanted signal is thereby said to be pulled our Presently, known adaptive base station antenna arrangements for telemetry stations operate by sampling interference in a time slot or window in the scanning sequence when it is known that there is no wanted signal present. Typically, this may be a period of 25ms each minute.This system is not particularly efficient since any pulsed interference will only be detected whilst this is pulsing and noncontinuous interference will only be detected during its on period of duty cycle. This type of sampling to determine background interference, however, is employed because of its convenience and simplicity in operation.

In the UK, for example, mobile users in Scandinavian countries using Nordic mobile telephone systems can cause substantial interference, especially under certain atmospheric conditions and localities of usage. A similar sort of problem can however be present in any country. Nordic interference has a duty cycle and duration of call corresponding to a typical mobile radio system, the length of the call will be a minute or so and the duty cycle and frequency re-use will typically be high. No knowledge of time and arrival of unwanted signal is given and the presently used interference discriminator can miss some bursts of non-continuous and pulse like interference, such as Nordic interference.

The problems of the known systems can thus be summarised as follows: Since the background interference is sampled during a particular period when outstations do not transmit, timing information is required to synchronise the adaptation with the scanning sequence. A timing interface is thus required with the existing system if the adaptive antenna is fitted. Also, if the interference is pulsed then a pulse must be coincident in time with the sample time if it is to be cancelled.

A particular example of a problem that can arise from the use of this technique is that in the transmission of data by utilities concerning e.g. gas leaks and the like, it is imperative that a transmitted signal is received. Under certain atmospheric conditions such communications can be rendered ineffective for several days at a time.

The present invention seeks to overcome the aforementioned problems.

In accordance with the invention, there is provided a method of discriminating between first and second groups of signals received by a base station including an adaptive antenna array operable to form nulls in the area of coverage, wherein the first group of signals have a duration of transmission less than the second group of signals; the method comprising the step of: forming nulls in the area of coverage of the antenna array responsive to the received signals; wherein the repetition period of the first group of signals is greater than an adaptation time constant and the adaptation time constant is greater than the duration of the first group of signals, whereby nulls are formed against the second group of signals.

In accordance with another aspect of the invention, there is provided a radio communications system operable to discriminate between signals in a multisignal environment wherein a first group of signals have a duration of transmission greater than a second group of signals; the radio system including a base station and a number of outstations, the base station comprising an adaptive antenna operable to form nulls in its area of coverage to discriminate between the groups of signals; wherein all signals are sampled and control means within the base station are operable to adapt the area of coverage to null out all received signals, wherein the repetition period of the first group of signals is greater than an adaptation time constant and the adaptation time constant is greater than the duration of the first group of signals, whereby nulls cannot be formed at a rate to cancel the first group of signals.

By using a system that operates with a deliberately long time constant of adaptation, a null will not be formed instantaneously.

However, since the system operates in a continuous fashion, new sources of interference will be catered for as and when they arrive, while significant nulls are formed for signals of a duration greater than the duty cycle of the wanted signals.

In the adaptive antenna approach, an array of several antennas is employed. In a preferred embodiment a total of four antennas are deployed at the base site. In a receive mode, the outputs of the antennas are shifted in phase and magnitude (within a weighting network) before being combined into a single signal fed to the receiver. The effect of this combination is to alter the sensitivity of the antenna array with azimuth direction.

The interference reduction device can use omni-directional antennas for all four elements in the array. This configuration enables interference sources from any direction in azimuth to be cancelled.

Such a systems is useful when knowledge of the bearing of the interferer is either unknown or known to vary in direction. It can be shown by computer simulation that an asymmetrical antenna array used is less likely to form spurious nulls than a symmetrical array.

An arrangement using one or more directional antennas as auxilliary elements is also possible, provided the sector from which the interference is arriving is known a priori.

In order that a greater understanding of the invention may be obtained, reference shall now be made to the Figures as shown in the accompanying drawing sheets, wherein: Figure 1 shows a scanning telemetry system.

Figure 2 shows a base station schematic for a telemetry system in accordance with one embodiment of the invention.

Figure 3 shows a system configuration in accordance with one embodiment of the invention.

Figure 4 shows the intemal architecture of the interference reduction device in accordance with one embodiment Figure 5 shows an omnidirectional response and an adapted response for an antenna arrangement made in accordance with the invention Figure 6 is a graphical representation of a short wanted pulse masked by a long interference pulse.

Figure 7 shows the effect of an adaptive antenna operating in accordance with the invention on the interference shown in Figure 6.

Referring now to Figure 3, there is shown an embodiment of the invention wherein a base station comprises a radio 21 which transmits on a main antenna M and receives signals via the main antenna and several auxiliary antennas A1-A3. Each antenna communicates with the radio via a diplexer D,D1-D3: in receive mode all diplexers are routed through an interference reduction device (IRD). The unused transmit inputs of the auxiliary diplexers are terminated. The auxiliary antennas in the array cause a small amount of shadowing of the main antenna, however the transmit pattern remains essentially omni-directional. Since the IRD adapts against interference while the base station transmits, filtering to avoid limiting the auxiliary channels is required. This filtering is performed by the diplexers. All four channels are mixed down to an intermediate frequency.

Referring now to Figure 4, the signals from all the antennas are amplified and then mixed to a certain frequency using a local oscillator as a source. The outputs from the mixers MX -MX3 are then fed to an antenna control unit (ACU). The ACU includes combining means , weighting elements W1 -W3, a power meter PM, a processor P and an output. Prior to being input to the combiner, the signals from the auxilliary antennas are weighted by weighting elements.

The summed output from the combining means is split by splitter S into two parts. The first is up-converted to the receive RF frequency before being output to the radio. The second is fed to a logarithmic amplitude strip, the voltage envelope output from which is used as a feedback error signal in the adaptive control algorithm. The algorithm used is typically a perturbation gradient descent technique which can be implemented by computer. The perturbation algorithm is based on the standard technique of adaptive algorithms as is known.

The sensitivity of the logarithmic amplitude strip is adjusted such that thermal noise is just seen at the bottom of its range. This is set so that, in the absence of interference, the weight set will tend to decay to zero since any non-zero values of weighting increase the noise of the output. A zero weight set in the absence of interference is important to maintain a consistent quiescent antenna response.

The weight solution of each element in turn is perturbed up and down and the weight set is moved towards the solution which offered the lower power at the beamformer output. The IRD is able to adapt fully against an interferer within only 25ms, but in this application, the time constant is increased such that it is longer than the duration of the wanted signal.

A study was performed to investigate the antenna pattern of the protected system and to make a comparison of bit error rates for protected and unprotected systems. The omni-directional response of the antenna was plotted by setting the weights to zero, and rotating the antenna array while monitoring the receive power at the output of the IRD. The antenna response is shown as 'Omni' in Figure 5. The IRD was then allowed to adapt against this reference signal as though it were an interference source. The weight set was then frozen and the antenna array rotated while monitoring the power of the reference signal. The response is shown as 'Adapted' in Figure 5.

Figures 6 and 7 show amplitude -v- time graphs of a short duration ( < is) wanted signal and a long duration (30s) interfering signal before and after applying the technique as taught herein. In Figure 7, in the first period of cancellation, the interference has been substantially cancelled out after the first four seconds; during the second period, the cancellation is more pronounced for the first few seconds. At the beginning of each cycle of adaptation against the longer duration signal, a period of re-adaptation is required due to decay of the previous null.

Since the adaptive arrangement is responsive to signals having a constant phase front i.e. the sources of signals are static, thus for a mobile radio user, continuous transmission will not be affected by the adaptative canceller provided that the mobile radio user moves such that the signals do not provide a constant phase front at the base station.

The present invention provides an adaptive antenna system using low cost technology and benefits from a robust but hitherto non utilised wanted signal discriminant.

Claims (2)

Claims

1 A method of discriminating between first and second groups of signals received by a base station including an adaptive antenna array operable to form nulls in the area of coverage, wherein the first group of signals have a duration of transmission less than the second group of signals; the method comprising the step of: forming nulls in the area of coverage of the antenna array responsive to the received signals; wherein the repetition period of the first group of signals is greater than an adaptation time constant and the adaptation time constant is greater than the duration of the first group of signals, whereby nulls are formed against the second group of signals.

2 A method according to claim 1 wherein the first group of signals have a duration of transmission at least an order of magnitude less than the second group of signals 3 A radio communications system operable to discriminate between signals in a multisignal environment wherein a first group of signals have a duration of transmission greater than a second group of signals; the radio system including a base station and a number of outstations, the base station comprising an adaptive antenna operable to form nulls in its area of coverage to discriminate between the groups of signals; wherein all signals are sampled and control means within the base station are operable to adapt the area of coverage to null out all received signals1 wherein the repetition period of the first group of signals is greater than an adaptation time constant and the adaptation time constant is greater than the duration of the first group of signals, whereby nulls cannot be formed at a rate to cancel the first group of signals.