GB2451682A

GB2451682A - Interference detection and avoidance in a spectral band based on stored interference profile information

Info

Publication number: GB2451682A
Application number: GB0715568A
Authority: GB
Inventors: Rafael Arcangel Cepeda Lopez; Steven Carl Jamieson Parker
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2007-08-09
Filing date: 2007-08-09
Publication date: 2009-02-11
Anticipated expiration: 2027-08-09
Also published as: GB2451682B; GB0715568D0; JP2009065653A

Abstract

Interference detection and avoidance, suitable for use on a received signal received across a spectral band and applicable in a wireless receiver, comprises monitoring for the existence of interference in one or more of a plurality of subbands of the spectral band, storing information defining an interference profile, and suppressing interference on the basis of such stored interference profile information. The claims disclose interference detection and suppression in a spectral band, based on stored interference profile information. The invention provides flexible and responsible operation within an extensive bandwidth such as UWB or cognitive radio where no established method of interference detection and avoidance is established.

Description

Wireless Communications Apparatus This invention relates to wireless communication apparatus, and is particularly concerned with interference avoidance in a wireless communication network.

The invention is contemplated for implementation in any wireless communications system that benefits from detecting interference from any third party device, or any natural source, in order to avoid causing interference to the third party device, while tolerating the impact of the interference on its own performance. For instance, the invention is contemplated for application to WLAN, WPAN, UWB and MIMO technologies, but this should not be considered as an exclusive list.

Wireless communications devices can benefit from the detection of radio interference.

Regulations have been implemented to recognise the problems that interference can cause, and there is an ongoing drive towards the development of a transmitting device that can avoid causing interference to detected third party devices. In addition, there is a need for a receiving device which, as far as possible, avoids or suppresses detected interference to improve its own performance.

At present, there is no established method for performing interference detection and avoidance (DAA). This invention could be applied to radio systems such as ultra wideband (UWB) and cognitive radio, where flexible and responsible operation within an extensive bandwidth is required.

Another problem which has been recognised in the field of the present invention is that it is difficult to detect an interferer, when the signal-to-interference noise ratio (SINR) is unknown and potentially spans a large dynamic range.

Although the field of the invention is not limited in such a way, the background art will now be exemplified in the context of ultra wide band (UWB) communication. The reader will appreciate that the concepts disclosed herein, both in terms of problems with the state of the art and in terms of solutions thereto enabled by the present disclosure, are generally applicable. For instance, they may be applied to cognitive radio systems that need to detect interference and act accordingly to give satisfactory coexistence and good performance.

This invention is particularly relevant to UWB, because spectral regulators in the EU and Japan have insisted that interference DAA must be applied in certain bands of the UWB allocated spectrum (as defined by the FCC in the US, to extend from 3.1 GHz to 10.6 GHz, with maximum power spectral densities of -41.3 dBm / M1-lz). Thus there is an inherent commercial need, which gives rise to a technical problem of implementation of the commercial need.

The FCC UWB spectrum has been partitioned into 14 bands of 528 MHz by a legacy physical (PHY) layer proposal submitted to IEEE 802.15.3a. These bands are illustrated in figure 1. This has now been backed by the WiMedia Alliance and standardised by the European Computer Manufacturers Association (ECMA) as ECMA-368. As identified in figure 1, five Band Groups' have also been defmed, with the first four Band Groups containing three bands each and the fifth Band Group containing the two highest frequency bands. Figure 2 shows the first Band Group in further detail.

In the ECMA-368 standard, support for multiple wireless personal area networks (WPANs) is provided by two mechanisms: operation in different Band Groups (frequency division); and isolation of WPANs within a common Band Group using different time frequency codes (TFCs) for each unique WPAN. The use of TFCs is implemented by dividing each packet for transmission into blocks of six OFDM symbols. Each OFDM symbol within each block of six is transmitted from a pre-assigned band from within the chosen Band Group. The bands used for each consecutive OFDM symbol are defined by the TFCs.

TFCs improve the performance of systems based on the ECMA-368 standard by interleaving information bits throughout the blocks of six OFDM symbols to maximise frequency diversity. The TFCs that provide this frequency diversity are known as time-frequency interleaved (TFI) logical channels.

However, in certain circumstances, it may be preferable to operate each WPAN such that it is always allocated to the same band and therefore fixed TFCs are also provided/ These are known as fixed frequency interleaved (FF1) logical channels. FF1 logical channels are mandatory according to the standard if not all of the bands in a Band Group are available due to regulatory restrictions.

It is expected that first generation UWB devices will operate only in the first Band Group for commercial reasons; therefore, these devices will only perform interference DAA in this 1.5 GHz band. However, future devices may migrate throughout the whole 7.5 GHz of available spectrum, although there may be regionally applicable regulatory restrictions as discussed below. It is thus desirable for a DAA implementation to be capable of detecting interference, and performing avoidance and suppression, over 7.5 GHz.

Figure 1 shows how the UWB band is partitioned and regulated in the US, EU and Japan. It is apparent that in the economically attractive Band Group #1, two out of the three bands can only be used in the EU if DAA is applied. The remaining third band can only be used without DAA present until 2010, after which the whole Band Group requires DAA technology.

In Japan, the regulatory stance is even stricter and only two bands of Band Group I can be used, with one band requiring DAA and the other requiring DAA after 2008. DAA is therefore a critical technology for the future adoption and evolution of civilian UWB communications in the EU and Japan. In the US, DAA is not required, but could be adopted in the future if it is shown to be effective and can be applied cost effectively and efficiently.

The relatively coarse division of the UWB FCC spectrum into 14 bands of 528 M}Iz means that only a few narrowband interferers, distributed evenly throughout the UWB spectrum, would be required in order to render UWB devices unusable if techniques are not available to aid co-existence. It is thus desirable to provide an effective DAA solution to enable band sharing, and which manages the interference forced upon UWB receivers, while not causing interference to third parties.

An aspect of the invention provides a method of interference detection and avoidance in a spectral band comprising monitoring for the existence of interference in one or more of a plurality of subbands of said spectral band, storing information defining an interference profile, and suppressing interference on the basis of such stored interference profile information.

The method may further comprise updating said stored interference profile information on the basis of further monitoring for the existence of interference.

The method may further comprise updating an element of said stored interference profile information with the passage of time from monitoring interference leading to the creation of said element of stored interference profile information. Updating may comprise applying a decay function to the element of the stored interference profile information. The decay function may be governed by a time constant.

The interference profile information may comprise any one or more of frequency, bandwidth and received power of monitored interference.

The interference profile information may include information relating to expected behaviour of said interference with respect to time. The expected behaviour information may include probabilistic information.

Suppressing of interference may be performed on an analogue signal.

Monitoring for interference may be performed in lieu of receiving a signal for decoding.

Alternatively, monitoring may be performed alongside receiving the signal for decoding.

According to another aspect of the invention, there is provided apparatus for interference detection and avoidance suitable for use in a wireless receiver and operable to process a received signal in a spectral band comprising monitoring means operable to monitor for the existence of interference in one or more of a plurality of subbands of said spectral band, interference profile storage means operable to store information defining an interference profile, and interference suppression means operable to suppress interference on the basis of such stored interference profile information.

The monitoring means may be operable to update the stored interference profile information on the basis of information defining the existence of interference in said spectral band obtained by said monitoring means. Interference profile information management means may be provided, operable to updating an element of said stored interference profile information with the passage of time from said element of information being stored. The interference profile information management means may be operable to apply a decay function to said element of said stored interference profile information.

The interference suppressing means may be operable to receive an analogue signal and to apply interference suppression to said analogue signal.

Another aspect of the invention is in the field of interference detection and avoidance, suitable for use on a received signal received across a spectral band and applicable in a wireless receiver, and comprises monitoring for the existence of interference in one or more of a plurality of subbands of the spectral band, storing information defining an interference profile, and suppressing interference on the basis of such stored interference profile information.

Another aspect of the invention provides an interference detection and avoidance (DAA) architecture that enables interference profiling during transceiver idle periods.

Another aspect of the invention provides an interference detection and suppression unit (IDSU) that is equipped to detect interferers and can configure front-end filters to suppress incoming interference.

Another aspect of the invention provides an interference controller unit (ICU) interoperable with the IDSU to detect interferers and to configure front-end filters.

Another aspect of the invention provides an interference profiling unit ([PU) that is interoperable with the ICU, a Fast Fourier Transform (FF1) and a baseband processor, for gathering and processing interference statistics that will enable the prediction of future interference. This unit may instruct and configure a transmitter to apply frequency notching, or migration to other bands or Band Groups. It may also instruct the IDSU to apply an appropriate receive filter to suppress incoming interference.

Another aspect of the invention provides an interference memory unit (IMU) for storing information about interferers, including one or more of: updateable a priori information about other radio systems, their transmission properties and their immunity and vulnerability to interference; and statistics of detected interferers and related interference information.

Another aspect of the invention provides a DAA architecture that can monitor several bands concurrently for interference and retune the receiver to enable a more accurate analysis of the interference.

Another aspect of the invention provides a method of interference detection and avoidance that involves the use of a threshold function governed by an expected number of interference events, that will determine whether it is acceptable to transmit on any chosen frequency without causing interference to third party devices.

The threshold function may be weighted to account for the importance and vulnerability of third party devices. For example, frequencies used for safety critical devices may have different weights assigned compared. with those.used for frequencies dedicated to entertainment systems. . Another aspect of the invention provides a method of interference detection and avoidance that involves the use of an interference event mean rate function that predicts the risk of causing interference.

Another aspect of the invention provides a method of interference detection and avoidance that involves the use of an interference event mean rate function that decays over time using a time constant. The time constant may be programmable.

In a similar maimer to that discussed above, the memory time constant may be weighted to account for the importance and vulnerability of third party devices.

Another aspect of the invention provides a method of interference detection and avoidance that involves the use of a scaled rate function and memory time constant that determines whether selective front-end interference suppression filtering is applied, and on what frequencies, to quantif' a perceived risk of interference to a receiver.

It will be appreciated by the skilled person that, in some circumstances, impinging interference is only detrimental to the receiver, and will not cause regulatory infringement, so the probability thresholds and memory time constants adopted may be more lax than used by the transmitter when deciding to apply frequency notching.

Aspects of the invention provide an architecture, and corresponding method, that detects and characterises interference for configuring a transceiver to avoid causing interference to third parties, while remaining robust to the reception of intended packets.

Other aspects of the invention provide, in general purpose analogue signal processing apparatus, a digital controller suitably configured to perform processor executable steps to enable performance of one of the methods so described, or to cause the analogue signal processing apparatus to become configured as apparatus so described.

The processor executable steps may be provided as a factory setting, or may be introduced thereafter. The processor executable steps may be in the form of a computer program product, which may be embodied on a storage medium (such as an optical or magnetic device), a programmable medium (such as a flash memory or a PROM of whatever type) or as a computer receivable signal.

Further aspects, advantages and features which may be provided in relation to the invention may become evident from the following description of specific embodiments thereof, with reference to the accompanying drawings, in which: Figure 1 is a spectral allocation diagram for the WiMedia standard; Figure 2 is a further spectral allocation diagram for a band group of the diagram illustrated in figure 1; Figure 3 is a schematic diagram of a receiver in accordance with a first specific embodiment of the invention; Figure 4 is a schematic diagram of an interference detection and suppression unit in the receiver of figure 3; Figure 5 is a schematic diagram of a receiver in accordance with a second specific embodiment of the invention; Figure 6 is a schematic diagram of an interference suppression element in the receiver of figure 5; and Figure 7 is a schematic diagram of an interference detection element in the receiver of figure 5.

Before commencing description of the specific embodiments so illustrated, by way of background discussion, it should be noted that it is a preferred objective of implementation of an interference DAA solution in a communications system, that any third party interference should be reliably detected. Once such interference has been detected, the system must be capable of making transmissions in a Band Group shared by the narrowband frequency, with certainty that it will not cause interference. Radio systems that only operate in listen mode', and whose presence is not advertised, cannot be detected. It is likely that these systems (e.g. radio astronomy) would have to declare their presence to regulators in advance, so that static and permanent frequency notching can be enforced.

It has been found impossible to predict with complete certainty that a UWB transmission will not cause interference to a third party device. However, it has been found possible to make the probability of causing interference diminishingly small as long as a system is educated in advance about the characteristics of third party systems, that it uses conservative default settings, that it has had time to profile the traffic of its radio environment, and that it has received information about interferers from other devices on the same network. By diminishing this probability, the problem of interference can be made effectively insignificant.

It is considered insufficient simply to apply interference detection immediately prior to a transmission and then, once the all clear' has been given, continue with a transmission regardless of what might happen in the future. For instance, interference can arise through third party activity commencing during transmission. An additional mechanism can be provided at the transmitter, to detect interference which commences during a transmission, so that the transmission can be terminated immediately.

However, this is still unsatisfactory, as the third party transmission will already have suffered some interference. Furthermore, it is difficult to detect weak interference while the local transmission is effectively obscuring interference from being detected.

A conservative approach to interference DAA is to log interference events regularly over a long period and then to avoid completely the use of these frequencies for future transmissions. However, this is potentially inefficient, as third party devices may meanwhile have been removed from the area; in such circumstances, transmissions could be made without fear of causing interference, but interference avoidance would have been incorporated into the spectrum needlessly. If detected interference frequencies are not re-used at some point in the future then the device will slowly lose performance as ever more bands become eliminated due to the potential presence of narrow-band interferers.

Having regard for the above, the embodiments of the invention to be described below each operate on the principle of learning about the interference environment in which a device is operating, and to determine an accurate mean interference event rate function r, (f) based on repeated observation. An interference event density function p1 (f, t) is defined, which determines the number of observations of interference in an infinitesimal duration öt, in the limit as öt -* 0.

Hence, an integration of p, (f, r) over a specified duration determines the expected number of interference observations that will be made in that period. Consequently, the mean rate of interference events r, (f) is given by equation 1, if it is assumed that the interval [t, t + TJ is statistically representative: r,(f)=-Jp,(f,t).dt (I) In practice, r, (f) would be determined by repeated short observations, under the assumption that the ensemble average is of a stationary process. The expected number of interference events N, (f) in a transmission time T is then approximated by: N,(f)r1(f).T (2) If the activity of a device is known, the number of interference events occurring in a defined period may therefore be determined from equation 2. For example, one event per hour may be deemed acceptable when using a frequency occupied by a WLAN, whereas one event per year may be deemed too high on a frequency used for a radar system.

Traffic is likely to evolve over time, potentially in a predictable manner, and therefore a table of r, (f) can be defined, such as ones for diurnal or nocturnal operation, or for acknowledged busy periods. Deterministic behaviour over a short time period may also arise if the interference is identified as belonging to a known radio system. This information could also be included in the interference model to improve prediction.

A threshold for the number of expected interference events, N,hh (f,t), is assigned to defme what is considered an acceptable risk of causing interference to a third party.

This threshold is calculated by defming how often an interference event' is allowed to happen (for example, once a year) based on a worst-case' estimate of how busy the transmitting UWB device is likely to be (from the average duration of transmissions and their regularity). Therefore the threshold is determined by taking into account the nature of the implementation, as mentioned above. The threshold is, in this embodiment, a function of frequency and possibly time: the use of discrete time (for example diurnal and nocturnal in the coarsest sense), would define a table of thresholds that are in essence oniy frequency dependent in a shorter timescale. Frequencies that are used for safety critical systems (e.g. radar) would be given a very low threshold, whereas other non-critical systems that have recovery mechanisms', such as error correction and retransmission capability, would be assigned higher thresholds.

Some interference events may be rare, or a device may not have sufficient resources or time to amass sufficient statistics to build up an accurate rate function that can be tracked over time. The present embodiment therefore defines a memory time constant' that is used in the absence of any updated information. A long memory time constant would be used for a frequency known to be used by safety critical services, whereas a shorter value would be used for less important services. The new time dependent rate function would therefore be of the form: r(t,f) = r0(f)exp[-r(f)t] (3) where r0 (f) is the mean rate of a transmission causing interference directly based on the latest best estimate; r is the decay constant assigned to the rate function for a particular frequency and t is the time elapsed since the determination of r0 (f).

If no value of r (f) is available then an empirically deduced default value can be used that errs on the side of caution. A transmission is prohibited, and notching must be implemented, if the following condition is met: r (f)exp[-r(f)t] T �= Nthh (f,t) (4) Although the described approach assumes that probabilities calculated from past events may be applied to predicting future events, in practice, this provides a sufficiently good performance as to meet expectations, particularly if the following procedures are followed: * Maximum a priori information is used when learning about any interference sources. For example, a detected source of interference might be suspected to be an IEEE8O2. 11 a wireless local area network (WLAN). This hypothesis can be tested by listening' at appropriate times based on the known packet structure.

Once a source has been identified, and its traffic statistics logged, then prediction will be more dependable and past detections can be more accurately extrapolated to future probabilities.

* Interference traffic evolves over time and therefore r0 (f) and r(f) must be constantly updated to reflect current traffic and improve the reliability of future prediction.

* The thresholds used are conservative estimates, especially on safety critical frequencies.

A specific embodiment of a receiver will now be described in further detail, with reference to Figure 3 of the drawings. This receiver is of a generally conventional structure, as will be appreciated by the reader, but includes also features specific to the present example of the invention as will be described in due course.

Interference detection may be performed using a receiver 10 as illustrated in figure 3 which constitutes a first specific embodiment of the invention. This architecture is based around a UWB embodiment, but the concept is generally applicable to other radio systems. Only the receive branch is shown as the invention as exemplified by the presently described specific embodiment generally concerns the operation of the receive branch. However, the reader will appreciate that the corresponding transmit branch should preferably be capable of applying frequency notching based on information passed to the baseband processing block that performs transmit as well as receive operations. This is to avoid the situation wherein the transmitter transmits on the frequencies where the interference was detected. This is achieved by feeding back information to the transmitter regarding detected interference and is done so as not to transmit data in those sub-carriers causing interference to the narrowband user.

The receive chain is typical of a traditional UWB receiver, except that it has been modified to equip it for DAA operation. The receiver 10, in more detail, comprises an antenna 20 whose function is controlled by an RX/TX switch 22. The setting of the RX/TX switch 22 is controlled by a baseband processing functions unit 50, which is representative of data processing elements of the receiver 10. In the receive mode, signals pass from RX/TX switch 22 to a balun 24 which, in the usual manner provides balancing and compatibility between the antenna and the signal processing parts of the receiver.

The receiver 10 according to the first specific embodiment is operable in one of two modes: an interference detection mode, and a signal reception mode.

Interference detection is active in the period when the transceiver is normally idle, and neither transmitting nor receiving packets. In practice, the device would alternate between a sleep state and an interference detection state to minimise power consumption. The sleep state would dominate the duty cycle, to help conserve power, once a receiver has amassed sufficient statistics.

The radio frequency (RF) signal is detected and amplified by a variable gain low noise amplifier (LNA) 26. The LNA 26 output passes into a tuneable band-pass filter 28 to limit the signal to the Band Group of interest. This has been demonstrated to be feasible in "A dual-antenna phase-array ultra-wideband CMOS transceiver" (I. Sever et al, IEEE Comm. Mag., August 2006), which uses a 0.18 pm CMOS implementation. The resulting RF signal, which has a bandwidth of approximately 1.5GHz and thus covers the Band Group of interest, is subsequently down-converted by a band selection mixer that is driven by a synthesizer capable of selecting the required band. Band selection is appropriate as products from the original 1.5 0Hz bandwidth signal persist at the output. This mixer stage is currently implemented in UWB chipsets for implementing TFCs.

The I and Q elements of the selected Band Group of interest are passed from the mixer to an interference detection and suppression unit (IDSU) 32. When performing interference detection, this unit, and indeed the mixer 30, is under the control of an interference controller unit (ICU) 34. The ICU 34 operates on the basis of information received from the IDSU 32 concerning detected interference in the Band Group of interest. On the basis of processing carried out at this stage, interference suppressed signals can then be passed to an ADC 36 and then a resultant digital signal passed to a Fast Fourier Transform. The resultant frequency domain data can then be passed to the baseband processing functions 50, and then is also passed to an interference profiling unit IPU 40 which cooperates with an interference memory unit (ITvlU) 42. The ICU 34, IPU 40 and the IMU 42 will be described in due course with regard to their function.

The IDSU 32 will now be described in further detail with reference to Figure 4. The IDSU illustrated therein comprises firstly a block of variable gain amplifiers 44, one per each of the I and Q elements of the Band Group of interest. Then, the amplified signals are each passed to respective programmable filters 46 for interference suppression in use. Then, the signals are passed to respective power detectors 48.

When performing interference detection rather than transmitting or receiving packets, the IDSU 32, ICU 34, IPU 40 and IMU 42 are all active, and the programmable filter 46 is configured as an all-pass filter. The ou tput from the power detector 48 is sensed and digitised by an ADC in the ICU 34. A threshold is set for the received power, above which the presence of an interferer in the active Band Group is signalled. When in interference detection mode, the receiver 10 patrols all of the supported Band Groups; the time spent scanning each band is in proportion to the anticipated usage of the respective band, relative to other bands, by third party devices. An initial default setting of the receiver 10 causes the receiver 10 to scan all of the UWB bands equally often, with prioritised scanning used once a priori information or statistical behaviour becomes available.

In an alternative embodiment, the programmable filter 46 cou1d simply be switched out during interference detection.

If the ICU 34 detects interference then the receiver must quickly characterise the interference before it disappears.

To achieve this, the first step is to determine the band(s) in the Band Group in which the interference resides. In the described first embodiment, this is accomplished by sequentially retuning the band selector down-converter mixer 30 to each of the bands in turn. The programmable filter 46 is reconfigured as a band-pass filter, tuned to pass a band 528M1-Iz wide (and thus corresponding to the width of a band group). Nominally, of course, this would suggest that only signals within such a bandwidth are passed by the filter, although some tolerance on this will be accepted.

If the power detector 48 yields a signal that exceeds the power detection threshold indicative of interference, then the measured power level is used to set the gain of the VGAs so that the signal can be sampled within the dynamic range of the ADC in the main receive chain. In practice, this balances the signal distortion that is caused by saturation and quantisation. Custom power measurement chips are available that have a logarithmic response, which would enable interference power characterisation over a large dynamic range. For example, the Analog Devices AD83 18 offers a bandwidth from 1 MHz to 8 GHz and a dynamic range of 58 dB. In practice, the gain would be set to err on the side of too little gain, thereby to avoid saturation.

Subsequently, the IPU 40 samples the output of the FFT 38 and analyses the spectral content of the signal. For a UWB system based on the ECMA-368 standard, the period of the FFT 38 is only 242.42 ns, which would enable rapid characterisation of most sources of interference. For example, interference from a system conforming to IEEE 802.11 a uses symbols that persist for 3 200-4000 ns (depending on the channel). The centre frequency, bandwidth, power and time of occurrence of the interferer is logged in the IMU 42 and used to calculate the function r0 (f) (and reset r(f)) for controlling transmit frequency notching using either Active Interference Cancellation (AIC) as described in "Active interference cancellation technique for MB-OFDM cognitive radio," (H. Yamaguchi, Proc. of the 34" Microwave Conference, Vol. 2 Oct 2004, pp 1105-1 108). In addition, interference suppression can then be administered if needed, by configuring the front-end filter of the receiver. Interference suppression will, in the context of this embodiment, be described in due course.

If the spectrum is found to be broadband then it may be either a UWB transmission or a broadband interferer (or an aggregation of interferers). The baseband processing functions 50 will then attempt to decode the transmission to determine whether it is the intended recipient.

A second embodiment of the invention will now be described with reference to Figure 5 and 6. As before, an RX/TX switch 122 receives a signal from an antenna 120, and passes it through a balun 124 to a variable gain amplifier 126 and then to a band pass filter 128. The outcome of this is to limit the received signal to the Band Group of interest.

The receiver 100 of the second embodiment is again under the control of an interference controller unit (ICU) 134 which communicates with an interference profiling unit (IPU) 140, which in turn refers to an interference memory unit (IMU) 142.

A cycling switch 129 passes the band limited signal to either of a first mixer 130 or a bank of second mixers 131. The first mixer 130 communicates with an interference suppress ion element 146 of an interference detection and suppression unit, and the bank of second mixers 131 pass information to interference detection elements 148 of the same IDSU 132. The latter interference detection elements 148 output analogue information to the ICU 134 which then converts the same to digital information to be processed.

The interference suppression element 146 is under the control of the ICU 134, in response to the information collected from the interference detection elements 148. The interference suppressed signals output from the interference suppression element 146 are passed to the ADC 136 and the resultant digital signal then undergoes Fast Fourier Transform 138 to bring it into the frequency domain, so the IPU 140 can identif' the position of the interference in the frequency domain, and then inform filters in the interference suppression element 146 as to the portion of the spectrum to remove.

These frequency domain signals can then be passed to baseband processing functions in the conventional manner.

The interference suppression element 146 is illustrated in further detail in Figure 6.

This consists of three stages. A first stage comprises a variable amplifier 160 per I and Q component to be processed. Further, the signal thereby amplified is passed to a respective programmable filter 162, which configured on the basis of information received from the ICU 134, and intended for interference suppression. It will be noted that, in contrast to the first embodiment, there is no need for this programmable filter to be disabled at any time, as this element does not perform interference detection. Then, the signals are passed through a further bank of VGAs 164, before presentation to the IDC 136.

The interference detection element 148 is of similar form. In this case, the signalled input thereto are connected to VGAs 170, and then these modified signals are passed to low pass filters 172. the resultant signal is then passed to a bank of power detectors 174 which can then monitor for detection of interference of the Band under investigation.

Any outputs of these power detectors can then be passed to the ICU 134 for processing.

The parallel architecture of the second embodiment operates slightly differently from the first embodiment. For receiving packets, the switch 129 is configured to pass the received signal into the normal UWB receive chain signified by the interference suppression element 146. However, for the interference monitoring mode, the switch 129 is configured to pass the signal into both the receive chain and the filter bank (band selection mixers 131) that enables concurrent power detection in each of the bands in the Band Group.

Once it has been ascertained which band(s) have interference in them then the switch 129 is set to pass the signal into just the UWB receive chain (if the signal does not already reside in the band currently being monitored by the receive chain). The FFT is subsequently used to characterise the spectrum of the interference. The gain of the VGA of the interference suppression element 146 is set, as with the sequential system, to ensure that the ADC samples are accurate. If more than one band contains interference then it is necessary to retune the receiver sequentially in a manner similar to that used for the sequential architecture. In many situations, interference will only affect one band and therefore this parallel architecture enables more rapid characterisation of interference than the sequential architecture, to mitigate the risk that the interference will vanish before it has been fully characterised. In addition, it can detect intended UWB transmissions earlier because it does not need to interrogate the bands sequentially.

The interference profile information is also vitally important for the receive chain when it is receiving packets. If a signal is received and the SINR is poor (e.g. <6 dB) then the VGA alone will be insufficient to set up the receiver for the ADC. If a strong interferer is present then the transmitted information signal will be poorly resolved if the VGA is set up according to the total power of the received signal. If the gain is increased to reduce quantisation of the intended signal then the aggregate signal will saturate leading to inter-carrier interference (IC!) in the information signal. However, if the interferer has been characterised then a programmable analogue filter can be applied to suppress the interference prior to the ADC to increase the SINR.

It will be appreciated by the reader that it may be undesirable to apply an analogue interference rejection filter in the receive arm if no interference is actually present, as this would appear as a deep fade to the baseband processor (50, 150). It is therefore important to know whether interference is likely to occur during the reception of a signal. If an estimate of the time-of-arrival of the receive signal is available then the IDSU, ICU and IPU can monitor the medium for interference leading up to the anticipated reception to gauge whether it is prudent to apply front-end filtering and the form in which it should take. In addition, the IPU will have profiled the medium for interference between transmissions and will have an estimate of the probability of encountering interference on specific frequencies during the reception period from r0(f).

If interference probability thresholds are exceeded then a dynamically controllable analogue filter is configured to suppress the interference prior to the ADC (termed programmable filter in figures 4 and 6). The probability thresholds used would be scaled versions of those used for deciding whether to apply notching of the transmitted signal. In reception, it is less critical if the wrong decision is made and therefore a higher probability threshold can be used that balances the performance lost due to applying a front-end filter when it is not required and not applying one when interference is present.

If the interference is too severe then the system will elect to move bands. In practice, this would be explicitly signalled to the receiver by the transmitter, or requested by the receiver to the transmitter in the event of a hidden node by explicit feedback.

UWB devices have severe restrictions placed upon them in the EU and Japan if they do not employ DAA; and there are plans to make the rules even stricter. Described embodiments are intended to enable UWB devices to operate in the economically attractive DAA-only' bands. Furthermore, UWB devices using this technology would be less prone to receiver front-end overload that would otherwise have a massive impact on link performance, when considering that UWB transmissions are so weak.

Claims

CLAIMS: I. A method of interference detection and avoidance in a spectral band comprising monitoring for the existence of interference in one or more of a plurality of subbands of said spectral band, storing information defining an interference profile, and suppressing interference on the basis of such stored interference profile information.
2. A method in accordance with claim 1 and further comprising updating said stored interference profile information on the basis of further monitoring for the existence of interference.
3. A method in accordance with claim 1 or claim 2 and further comprising updating an element of said stored interference profile information with the passage of time from monitoring interference leading to the creation of said element of stored interference profile information.
4. A method in accordance with claim 3 wherein said updating comprises applying a decay function to said element of said stored interference profile information.
5. A method in accordance with claim 4 wherein said decay function is governed by a time constant.
6. A method in accordance with any one of the preceding claims wherein said interference profile information comprises frequency, bandwidth and received power of monitored interference.
7. A method in accordance with claim 6 wherein said interference profile information includes information relating to expected behaviour of said interference with respect to time.
8. A method in accordance with claim 7 wherein said expected behaviour information includes probabilistic information.
9. A method in accordance with any one of the preceding claims wherein said suppressing is performed on an analogue signal.
10. A method in accordance with any one of the preceding claims wherein the monitoring is performed in lieu of receiving a signal for decoding.
11. A method in accordance with any one of claims 1 to 9 wherein the monitoring is performed together with receiving the signal for decoding.
12. Apparatus for interference detection and avoidance suitable for use in a wireless receiver and operable to process a received signal in a spectral band comprising monitoring means operable to monitor for the existence of interference in one or more of a plurality of subbands of said spectral band, interference profile storage means operable to store information defining an interference profile, and interference suppression means operable to suppress interference on the basis of such stored interference profile information.
13. Apparatus in accordance with claim 12 wherein said monitoring means is operable to update said stored interference profile information on the basis of information defining the existence of interference in said spectral band obtained by said monitoring means.
14. Apparatus in accordance with claim 12 or claim 13 and comprising interference profile information management means operable to updating an element of said stored interference profile information with the passage of time from said element of information being stored.
15. Apparatus in accordance with claim 14 wherein said interference profile information management means is operable to apply a decay function to said element of said stored interference profile information.
16. Apparatus in accordance with any one of claims 12 to 15 wherein the interference suppressing means is operable to receive an analogue signal and to apply interference suppression to said analogue signal.