GB2438915A

GB2438915A - Chip code selection in Direct Sequence Spread Spectrum Ultra-Wideband receivers

Info

Publication number: GB2438915A
Application number: GB0611070A
Authority: GB
Inventors: David G Bateman; Laurent S Mazet
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2006-06-06
Filing date: 2006-06-06
Publication date: 2007-12-12
Anticipated expiration: 2026-06-06
Also published as: GB0611070D0; GB2438915B

Abstract

In a Direct Sequence Spread Spectrum (DSSS) Ultra-Wideband (UWB) receiver, a correlator (103) mixes the received signal x(t) with a chip code (Ref, fig. 2), having been stored (113) and selected (111) based on a received signal level in a subband of bandwidth equal to the correlator's low-pass filter (203, fig 2). The chip codes can be either spreading codes or frequency translation codes, and the UWB system can avoid transmissions in subbands which are designated as unsuitable by selecting chip codes which attenuate such subbands. This allows detection-avoidance techniques, in which received power in a narrow band is compared to known thermal noise in the receiver, to be applied to DSSS-UWB systems having much larger bandwidth and greater noise levels than OFDM systems.

Description

A DIRECT SEQUENCE SPREAD SPECTRUM RECEIVER AND METHOD

THEREFOR

Field of the invention

The invention relates to receive signal level determination by a direct sequence spread spectrum receiver and in particular, but not exclusively, by an Ultra Wide Band receiver.

: ** Background of the Invention S... * * S...

An important area of current research is cognitive radio systems. The basic premise is that spectrum that is under-utilized by one system can be used by an alternative system, as long as this alternative system is capable of detecting :. and avoiding the presence of existing systems. This introduces a new means of allocating spectrum for unlicensed uses thereby providing a much improved utilisation of the scarce frequency use.

Also, cognitive radio techniques are equally applicable as mitigation for the interference caused by Ultra WideBand (UWB) systems on victim services. UWB signals are generated by transmitting information in very short data pulses of typically 10-100 picoseconds duration thereby resulting in a bandwidth of the transmitted signal of several Gigahertz. A high data rate may be achieved by a UWB signal and as the bandwidth is exceedingly large, the spectral power density is relatively low. Accordingly, the interference caused to and the sensitivity to narrowband interference from other signals in the frequency range is relatively low thereby allowing a UWB communication system to co-exist with other communication systems in the same frequency range.

Mitigation of interference caused by UWB systems to victim services is of particular interest as current regulations are likely to require UWB systems to be able to detect the existence of other services and to avoid causing excessive interference to these. For example, CEPT (European Conference of Postal and Telecommunications Administrations) Task Group 3 (TG3) is currently considering regulation of UWB in Europe and will with high likelihood include the need for UWB systems operating under 5GHz to "detect and avoid" victim services. The proposed approach of "detect and avoid" : ... assumes that the overlaid radio system has both the capacity to detect the presence of an existing radio system and to avoid it. Avoidance might be performed in many manners, such as prohibiting the use of the overlaid system if another 3 * * system is detected or excluding the use of individual sub-bands of the spectrum which are detected to be used by other systems.

One technique of detection and avoidance is to use a narrowband receiver and to integrate the received power over a length of time. The received power in a band is then compared to the known thermal noise level of the receiver and the presence or absence of a victim service can be discovered. Such a technique is easily applied to UWB systems based on Multi Band Orthogonal Frequency Division Multiplexing (MB-OFDM,) where the OFDM sub-carriers are spaced at 4.125MHz intervals. However, the technique cannot CMLO3O22M directly be applied to Direct Sequence Spread Spectrum (DS-SS) UWB systems because the bandwidth of the basic receiver is very high (approximately 1.5GHz). This will result in significantly more noise being detected (26dB more) than for an MB-OFDM UWB system and will result in a less detailed measurement of which specific frequency intervals are used by other systems.

A possible solution to this problem is to include an additional narrow-band receiver for measuring signal levels in smaller frequency bands. However, such an approach is cumbersome and increases complexity and cost of the UWB receiver. I... * a..

S *5S*

Hence, an improved system for monitoring receive signal levels would be advantageous and in particular a system allowing increased flexibility, reduced complexity, reduced cost and/or improved performance would be advantageous. S.... * a * S.

Summary of the Invention

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided a direct sequence spread spectrum receiver comprising: a correlator for correlating a received signal with a chip code, the correlator comprising: a mixing means for mixing the received signal and a first chip code to generate a mixed signal; a low pass filtering means for CMLO3O22M generating a correlator output signal, the low pass filter having a first bandwidth smaller than a bandwidth of the received signal; means for storing a plurality of chip codes, at least one of the plurality of chip codes corresponding to a direct sequence spreading chip code and at least one of the plurality of chip codes corresponding to a frequency translation chip code; means for selecting the first chip code as a chip code corresponding to a spreading code or a chip code corresponding to a frequency translation chip code in response to a mode of operation of the correlator; and means for generating an indication of a receive signal level in a first frequency subband having a bandwidth of the first bandwidth in response to the correlator output signal. *I,.

The invention may allow an improved receive signal level detection in direct sequence spread spectrum receivers. In particular, the invention may increase sensitivity and/or I dq*ê I.... allow a lower frequency granularity of measurements. The complexity and/or cost may be reduced.

The bandwidth of the received signal may in some scenarios be considered equivalent to a bandwidth of a receive front end coupled to the correlator.

The mixing means and the low pass filtering means may be implemented by separate operations or may be implemented by a single combined function. Specifically, the mixing means may be a multiplier followed by a low pass filter or may be integrally implemented as a matched filter.

CMLO3O22M According to an optional feature of the invention, the means for storing comprises a plurality of frequency translation chip codes with different translation frequencies, each frequency translation chip code corresponding to a different frequency subband of the frequency bandwidth of the received signal.

The invention may allow a low complexity way of making a direct sequence spread spectrum receiver provide functionality equivalent to a series of narrowband receivers thereby improving e.g. detection performance when detecting a presence of other systems. * .. * . .

According to an optional feature of the invention, the * ** receiver is arranged to iteratively select different chip codes of the plurality of frequency translation chip codes to generate receive signal level indications for a plurality of different frequency subbands. **1*

I d S

* 20 The receiver may specifically scan a given frequency bandwidth, such as the bandwidth of the received signal and/or a bandwidth of a receive front-end, by sequentially measuring each narrow band interval of the given frequency bandwidth.

According to an optional feature of the invention, the correlator output signal is a sampled signal with a first sample frequency and the plurality of frequency translation chip codes have translation frequencies offset by a frequency offset corresponding to the first sample frequency.

CMLO3O22M The first bandwidth may correspond to the first sample frequency. The feature may allow an efficient monitoring of a given bandwidth by sequentially measuring subbands having a bandwidth corresponding to the first bandwidth.

According to an optional feature of the invention, the receiver is arranged to generate three different frequency subbands for the received signal.

This may allow reduced complexity and/or improved performance. Specifically, three frequency subbands may be generated by a correlator of length four and the frequency translation chip codes may specifically comprise values only I'.

ii. selected from the set consisting of 1, -1, j and -j. Is.

According to an optional feature of the invention, the received signal is a complex base band signal and the frequency translation chip code comprises values I...

corresponding to quantised values of a complex exponential function having a frequency corresponding to the translation frequency of the frequency translation chip code.

This may allow reduced complexity and/or improved performance.

According to an optional feature of the invention, the received signal is an analog signal, the mixer is an analog mixer and the low pass filter is an analog filter and the correlator furthermore comprises sampling means for sampling the correlator output signal with a first sample frequency.

CMLO3O22M The analog signal is a continuous time signal which may or may not be quantised. The sampling means may be an Analog-to-Digital Converter. The feature may allow implementation benefits in many embodiments.

According to an optional feature of the invention, the received signal is a sampled signal and the correlator furthermore comprises down-sampling means for down-sampling the correlator output signal to a first sample frequency lower than a sample frequency of the received signal.

The sampled signal is a discrete time signal which may or may not be quantised. The feature may allow implementation S...

benefits in many embodiments.

According to an optional feature of the invention, the direct sequence spread spectrum receiver further comprises S.....

* * Discrete Fourier Transform, DFT, means for performing a DFT on the correlator output signal to generate receive signal level indications for subbands having frequency bandwidths smaller than the first bandwidth.

This may allow improved performance and may in particular allow increased sensitivity and/or increased frequency resolution while allowing low complexity implementation.

According to an optional feature of the invention, the selection means is arranged to select the first chip code as a chip code corresponding to a spreading chip code when the correlator is in a spread spectrum receive mode of operation and to select the first chip code as a chip code corresponding to a frequency translation chip code when the CMLO3O22M correlator is in a receive signal level measurement mode of operation.

The direct sequence spread spectrum receiver may specifically be a Ultra WideBand direct sequence spread spectrum receiver. The invention may allow improved performance and/or reduced complexity of UWB receivers and systems and may in particular allow efficient and low complexity functionality for detecting the presence of other systems within the bandwidth of the UWB system.

According to another aspect of the invention, there is provided a method of monitoring receive signal levels by a direct sequence spread spectrum receiver comprising a correlator for correlating a received signal with a chip code, the method comprising: the correlator mixing the received signal by a first chip code to generate a mixed signal; the correlator generating a correlator output signal by low pass filtering the mixed signal, the low pass filter having a first bandwidth lower than a bandwidth of the received signal; storing a plurality of chip codes, wherein at least one of the plurality of chip codes correspond to a direct sequence spreading code and at least one chip code corresponds to a frequency translation chip code; selecting the first chip code as a chip code corresponding to a spreading code or a frequency translation chip code in response to a mode of operation of the correlator; and generating an indication of a receive signal level in a first frequency band having a bandwidth of the first bandwidth in response to an output level of the low pass filter.

CMLO3O22M These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which FIG. 1 illustrates a UWB DS-SS receiver in accordance with some embodiments of the invention; * * .. I... * S 5.55

FIG. 2 illustrates a correlator for a UWB DS-SS receiver in accordance with some embodiments of the invention; and

S S..

FIG. 3 illustrates a method of monitoring receive signal S.....

levels by a direct sequence spread spectrum receiver.

Detailed Description of Some Embodiments of the Invention The following description focuses on embodiments of the invention applicable to a direct sequence spread spectrum (DS-SS) receiver for a Unique Wide Band (UWB) transceiver.

However, it will be appreciated that the invention is not limited to this application but may be applied to many other DS-SS receivers.

FIG. 1 illustrates a UWB DS-SS receiver in accordance with some embodiments of the invention.

CMLO3O22M The receiver comprises a radio frequency (RF) front end 101 which amplifies, filters and down-converts the received signal as will be well known to a person skilled in the art.

In the example, the RF front end 101 generates an analog (continuous time) complex base band signal.

The RF front-end 101 is coupled to a correlator 103. FIG. 2 illustrates the correlator 103 in more detail. The correlator 103 comprises a mixer 201 which mixes the incoming signal with a selected chip code. The mixer 201 is specifically a multiplier which multiplies the received signal and the chip code. The output of the mixer 201 is coupled to a low pass filter 203 which specifically can be an integrator. Thus, the correlator 103 in the DS-SS UWB :. 15 receiver can be represented by a multiplication of a known :. template signal corresponding to the chip code followed by a low-pass filter 203 acting as an integrator. The filtering furthermore has the effect of reducing the data rate of the received signal from the DS-SS chip rate to the symbol rate of the received signal. In the example of FIG. 1 the multiplication is a complex multiplication as the received signal is a complex base band signal. The chip code signal may also be a complex signal but may also be a real signal corresponding to e.g. a binary chip code signal.

It will be appreciated that the mixing and filtering may be performed sequentially as in the example of FIG. 2 or may e.g. be performed by a single integrated operation.

Specifically, the correlator may comprise a matched filter.

The matched filter may in a digital implementation specifically perform the operation of: CMLO3O22M y = C,1

I I

where x,-is the input signal to the correlator and c are the chip code values. In the example, the multiplication may be considered to provide the mixing functionality and the summation may be considered to provide the low pass filtering. It will be appreciated that a digital correlator with a multiplier being followed by a summer (over K values) will result in the same relation between the input signal and the output signal of the correlator.

As will be well known to the person skilled in the art, the receiver can select a specific received spread spectrum signal by selecting the chip code that corresponds to the specific chip code used when transmitting this signal. S..

The correlator 103 is coupled to a receive processor 105.

* * The receive processor 105 receives the low pass filtered :. correlated signal from the correlator 103 and extracts the transmitted data. The receive processor 105 may specifically comprise functionality for data detection and error correction as will be known to the person skilled in the art.

In addition to functionality for receiving a specific DS-SS UWB signal, the receiver of FIG. 1 furthermore comprises functionality for supporting cognitive radio services.

Specifically, the receiver comprises functionality for monitoring received signal levels of frequency subbands within the bandwidth of the RF front end 101. The monitoring of these signal levels is used to detect the presence of other radio communication system within the subbands.

CMLO3O22M In the receiver of FIG. 1, the measuring of the received signal levels is based on the output from the correlator 103. Specifically, the correlator 103 is coupled to a monitoring processor 107 which is arranged to determine the received signal level in one or more subbands based on the output from the correlator 103.

Thus, in the receiver of FIG. 1 the correlator 103 is used in different modes of operation. In a first mode of operation, the correlator 103 is used to de-spread a wanted : ** signal such that this can be received. In a second mode of S...

operation, the correlator 103 is used to generate signals that are indicative of the received signal level in one or more subbands.

The receiver of FIG. 1 specifically comprises a controller S....

* * 109 which is coupled to the monitoring processor 107 and which controls whether the receiver is operating in the first mode of operation or in the second mode of operation.

The controller 109 is furthermore coupled to a chip code selector 111 which is also coupled to the correlator 103.

The chip code selector 111 is also coupled to a chip code store 113 wherein the different chip codes are stored.

In the receiver, the controller 109 controls the operation of the correlator 103 by selecting between different types of chip codes. Specifically, the chip code store 113 comprises all the chip codes that may be used to de-spread a wanted signal. However, in addition, the chip code store 113 comprices one or more frequency translation chip codes which CMLO3O22M do not result in a de-spreading of a wanted signal but rather results in a frequency translation of the incoming signal.

When the controller 109 controls the receiver to perform normal spread spectrum receiving, it controls the chip code selector 111 to select the appropriate dispreading chip code from the chip code store 113 for the specific wanted signal.

The chip code selector 111 then feeds this to the correlator 103. Accordingly, the correlator 103 de-spreads the signal and feeds this to the receive processor 105. * .*

When the controller 109 controls the receiver to perform receive level measurement operations, it informs the monitoring processor 107 that such an operation is ongoing.

In addition, it controls the chip code selector 111 to select a frequency translation chip code from the chip code store 113 and to feed this to the correlator 103. S. *S * S * * S

Accordingly, the received complex base band signal is multiplied by the frequency translation chip code resulting in a frequency translation. The output of the multiplication process is low pass filtered in the low pass filter 203 with a first bandwidth. Accordingly, the output of the correlator 103 corresponds to a frequency translated and low pass filtered version of the received signal. Thus, the output of the correlator 103 is an indication of the signal level in the subband with a bandwidth of the low pass filter and at a frequency corresponding to the frequency translation value of the selected frequency translation chip code.

CMLO3O22M Furthermore, by selecting between different frequency translation chip codes corresponding to different frequency translations, the signal levels in different subbands can be determined by the monitoring processor 107.

Thus, the receiver of FIG. 1 allows chip codes to be used that result in a frequency shift in the received signal x(t) rather than a correlation or despreading. The low pass filter will filter the frequency shifted received signal x(t)such that only a single sub-band of the received signal is selected. This will have the effect of reducing the bandwidth of the receiver and thus increasing its S...

sensitivity for use in a cognitive radio system. *5I*

In the receiver of FIG. 1, the chip store code comprises a set of frequency translation codes which have translation frequencies offset from the surrounding translation *s...S :. frequencies by a fixed amount. Specifically, a number of * 1 frequency translation chip codes are stored that can allow the receiver to scan the entire frequency bandwidth of the RF front end 101 in a sequence of subbands corresponding to the bandwidth of the low pass filter 203. Furthermore, the offset between the different translation frequencies are selected correspond to the bandwidth of the low pass filter 203.

In some embodiments, the correlator output is a discrete time sampled signal with a given first sample frequency/rate. Specifically, the correlator 103 can be implemented as analog lot correlator allowing processing at high frequencies and high bandwidths without necessitating an Analog-to-Digital conversion at high frequencies (which CMLO3O22M tends to be complex and have high power consumption).

However, the output of the low pass filter 203 has a substantially reduced bandwidth and may accordingly be sampled in an Analog-to-Digital converter with much lower sample rate.

In a sampled solution, the bandwidth of the low pass filter 203 is typically selected to correspond to the sample rate.

Specifically, in order to meet the Nyquist criterion, the bandwidth of the low pass filter 203 is typically selected to be around half the sample rate of the correlator output.

In such an example, the frequency translation chip codes are S...

5.' selected such that they are offset by a value corresponding to half the output sample frequency. *5 * ...

The controller 109 can in such an example scan the entire bandwidth of the receiver by sequentially selecting one of * * the frequency translation chip codes and for each selected frequency translation chip code measure the output signal level of the correlator 103. This signal level is a direct indication of the signal level received in the frequency subband with a bandwidth equivalent to the bandwidth of the low pass filter 203 and centred around the translation frequency of the selected frequency translation chip code.

By stepping through the stored frequency translation chip codes an accurate measurement of received signal levels in frequency subbands can be obtained.

The frequency translation chip codes can specifically be chip codes that correspond to quantised values of a complex exponential function having a frequency corresponding to the CMLO3O22M translation frequency of the frequency translation chip code.

As a simple example of a frequency translation chip code is the sequence of chip values corresponding to 1, j, -1,-j,1, j, -1,-j,l... sequence, which has a translation frequency of a quarter of the chip rate and which will result in three subbands being generated.

It will be appreciated that in an analog implementation, the application of the frequency translation chip code as a square wave signal will also result in higher harmonics of I..

the signal being mixed with the incoming signal. However, I...

such mixing products will be of higher frequencies and will be sufficiently attenuated by the following low pass filtering of the low pass filter 203.

* In some embodiments, the correlator 103 can be implemented *:* entirely in the digital domain. Thus, the received signal from the RF front end 101 can be a signal sampled at the chip rate and the frequency translation chip codes can be provided as digital values. The multiplication of these two digital signals can then be low pass filtered in e.g. a digital FIR (Finite Impulse Response) filter. If the FIR filter is implemented as an integrator (non-weighteed summation), the overall operation of the correlator 103 corresponds to the processing of a subband for a Discrete Fourier Transform (DFT). Thus, the receiver of FIG. 1 provides an elegant way of sequentially determining signal levels in different subbands corresponding to DFT subbands.

Furthermore, the output of the integrator can be down--sampled to the symbol rate.

CMLO3O22M Furthermore, it will be appreciated that the overall operation of the correlator can correspond to a sequence of band filters. As a specific example, which is equivalent to applying a DFT, if N is the ratio between the sample frequency of the correlator output signal and the chip frequency, N-i band filters can be defined by the chip code coefficients 2ur*n*k h,,(k)=e N N-i N-I where nE[-.. } * In order to facilitate implementation, there is typically only very few bits available to represent the chip code coefficients. In other words, the chip code coefficients are typically heavily quantised and the quantisation error may introduce significant aliasing to the system.

For example, simulations have shown that for a correlator with 8 delays (i.e. 8 samples being summed in the integrator and N=4) the use of 2+1 bits (2 bit for data, 1 bit for sign) allows a detection of more than 93% of the energy of 4 narrow bands. For a 24 delay correlator with 4+1 bits, 12 narrow bands are selectable with only less then 4% of aliasing. However, for a 4 delay correlator, no degradation is introduced when detecting energy in two narrow bands even if only 1+1 bits (1 bit for data, 1 bit for sign) are used to store the filter coefficients. This, is due to the coefficients reverting to the previously described sequences CMLO3O22M of 1, j, -1,-j,1, j, -1,-j,l.... which can be quantised by two bits without introducing any quantisation error.

The aliasing introduced by quantisation errors of the chip code coefficients in some embodiments can result in an increased risk of false detection, as a signal that is not in the band of increase is being detected as if it is. It should be noted that out-of-band aliases as seen in the 8 delay case might be ignored as they fall in a zone that will normally be filtered by the systems receive filter. For the case of the in-band aliases, it is possible to resolve this * problem by comparing the energy detection in all of the I;,) bands together. As the aliases fall in known positions and with known power levels, resolving the values of the actual * 15 power levels in each of the bands comes down to solving the linear equation representing the mixing with the aliases. * is...

* Also, in a system that implements M-ary modulation, multiple 0 bits are used in the receive correlator in any case, and so in these cases the cost of the additional bits in the correlator are already necessitated by the M-ary modulation.

The described technique introduces a means of reducing the UWB receivers bandwidth in the correlator and increasing its sensitivity as a receiver in a cognitive radio system. It allows a lower cost solution for the UWB cognitive radio, using the maximum number of components from the existing system, and at a sensitivity that is determined by the implementer.

A narrowband system operating within the bandwidth of the UWB receiver is likely to represent only a relatively small CMLO3O22M part of the total received signal power in the entire bandwidth. However, the received signal level due to noise is typically relatively constant across the frequency bandwidth and the noise power is therefore proportional to the measurement bandwidth. Accordingly, by measuring in subbands with lower bandwidth the noise power will be reduced in each subband whereas the power received from a narrow band system will be constant in thecorresponding subband. This allows for a much improved sensitivity in detecting the presence of narrow band systems as it is sufficient to just detect this in one subband with a substantially lower noise power. Thus, the described el.., approach allows a more accurate detection of narrowband S...

systems thereby providing improvements for cognitive radio systems which can use such detection to avoid the narrow band systems.

I

S.....

In some embodiments it may be advantageous to measure signal levels in subbands that are smaller than the bandwidth corresponding to the low pass filter 203. In such embodiments, the monitoring processor 107 can comprise a DFT (such as an FFT) which can be applied at the output of the correlator 103. Thus, a number of DFT subbands can be produced for each subband generated by the correlator 103.

Such an approach may allow facilitated implementation and can specifically allow reduced complexity as a complex DFT func.ion is only applied to signals with a reduced sample rate.

Thus, the described UWB receiver comprises functionality which is suitable for detecting the presence of other CMLO3O22M systems within the bandwidth of the receiver. This allows an improved cognitive radio approach. Specifically, the controller 109 is used to measure received signal levels in the different subbands to detect that a given subband is already used by another system, such as a narrowband system, and therefore should not be used by the UWB system. In a simple embodiment, the controller 109 can simply compare the received signal level in a given subband to a predetermined threshold and designate the subband as unsuitable for use if the threshold is exceeded.

If the given subband is designated as unsuitable, the UWB S...

system can adapt such that this subband is not used for *

...DTD: transmissions (or that these transmissions have reduced power in the designated subbands). These transmissions may for example be transmissions from a UWB transmitter which is part of the same UWB transceiver as the UWS receiver. Thus, I.....

the controller 109 can control an integrated UWB transmitter * not to use designated subband(s). The transmitter may for example select a transmitter chip code which is frequency selective and which has a strong attenuation in the designated subband.

In some embodiments, the transmissions may alternatively or additionally be transmissions of a transmitter which is remotely located from the UWB receiver. For example, the transmissions may be from a UWS transmitter that currently transmits signals to the UWB receiver. In such a case, a feedback path may be established between the UWB receiver and the UWB transmitter and this feedback path may be used to provide the UWB transmitter with information of the subbands that should be avoided. The remote UWB transmitter CMLO3O22M may then proceed to select frequency selective chip codes that attenuate the designated subbands.

FIG. 3 illustrates a method of monitoring receive signal levels by a direct sequence spread spectrum receiver comprising a correlator for correlating a received signal with a chip code. The receiver furthermore comprises a chip code store which stores a plurality of chip codes, wherein at least one of the plurality of chip codes corresponds to a direct sequence spreading code and at least one chip code corresponds to a frequency translation chip code. I... * IS. * * * ***

The method initiates in step 301 wherein a first chip code is selected as a chip code corresponding to a spreading code or to a frequency translation chip code in response to a mode of operation of the correlator.

Is.... S * S. s*

* Step 301 is followed by step 303 wherein the correlator mixes the received signal and the selected chip code to generate a mixed signal.

Step 303 is followed by step 305 wherein the correlator generates a correlator output signal by low pass filtering the mixed signal. The low pass filter has a first bandwidth lower than a bandwidth of the received signal.

Step 305 is followed by step 307 wherein an indication of a receive signal level in a first frequency band having a bandwidth of the first bandwidth is generated in response to an output level of the low pass filter.

CMLO3O22M It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors.

However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or sea.

physical structure or organization. S...

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least a.....

:. partly as computer software running on one or more data * processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may CMLO3O22M appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in a...

different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim S....

categories as appropriate. Furthermore, the order of features in the claims does not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order.

CMLO3U22M

Claims

CLAIMS

1. A direct sequence spread spectrum receiver comprising: a correlator for correlating a received signal with a chip code, the correlator comprising: mixing means for mixing the received signal and a first chip code to generate a mixed signal; a low pass filtering means for generating a correlator output signal, the low pass filter having a first bandwidth smaller than a bandwidth of the received signal; I..

means for storing a plurality of chip codes, at least S...

one of the plurality of chip codes corresponding to a direct sequence spreading chip code and at least one of the plurality of chip codes corresponding to a frequency translation chip code; *8**S* :. means for selecting the first chip code as a chip code * corresponding to a spreading code or a chip code corresponding to a frequency translation chip code in response to a mode of operation of the correlator; and means for generating an indication of a receive signal level in a first frequency subband having a bandwidth of the first bandwidth in response to the correlator output signal.

2. The direct sequence spread spectrum receiver of claim 1 wherein the means for storing comprises a plurality of frequency translation chip codes with different translation frequencies, each frequency translation chip code corresponding to a different frequency subband of the frequency bandwidth of the received signal.

CMLO3O22M 3. The direct sequence spread spectrum receiver of claim 2 wherein the receiver is arranged to iteratively select different chip codes of the plurality of frequency translation chip codes to generate receive signal level indications for a plurality of different frequency subbands.

4. The direct sequence spread spectrum receiver of claim 3 wherein the correlator output signal is a sampled signal with a first sample frequency and the plurality of frequency translation chip codes have translation frequencies offset by a frequency offset corresponding to the first sample frequency. S.. * S.. * S * a..

5. The direct sequence spread spectrum receiver of claim 4 wherein the receiver is arranged to generate three different frequency subbands for the received signal.

S

S.....

:. 6. The direct sequence spread spectrum receiver of any * S previous claims wherein the received signal is a complex base band signal and the frequency translation chip code comprises values corresponding to quantised values of a complex exponential function having a frequency corresponding to the translation frequency of the frequency translation chip code.

7. The direct sequence spread spectrum receiver of any previous claims further comprising means for detecting that the first frequency subband is unsuitable for transmissions by a transmitter associated with the receiver in response to the indication of the receive signal level.

CMLO3O22M 8. The direct sequence spread spectrum receiver of any previous claim wherein the received signal is an analog signal, the mixer is an analog mixer and the low pass filter is an analog filter and the correlator furthermore comprises sampling means for sampling the correlator output signal with a first sample frequency.

9. The direct sequence spread spectrum receiver of any of the previous claims 1 to 7 wherein the received signal is a sampled signal and the correlator furthermore comprises down-sampling means for down- sampling the correlator output signal to a first sample frequency lower than a sample I...

ii.. frequency of the received signal. * **.

10. The direct sequence spread spectrum receiver of claim 8 or 9 wherein the first bandwidth corresponds to the first sample frequency.

S..... * S *. *5

* 11. The direct sequence spread spectrum receiver of any of the claims 8 to 10 further comprising Discrete Fourier Transform, DFT, means for performing a DFT on the correlator output signal to generate receive signal level indications for subbands having frequency bandwidths smaller than the first bandwidth.

12. The direct sequence spread spectrum receiver of claim 11 further comprising means for detecting that one or more DFT subbands is unsuitable for transmissions by a transmitter associated with the receiver in response to the receive signal level indication for the one or more of the DFT subbands.

CMLO3O22M 13. The direct sequence spread spectrum receiver of any previous claim wherein the selection means is arranged to select the first chip code as a chip code corresponding to a spreading chip code when the correlator is in a spread spectrum receive mode of operation and to select the first chip code as a chip code corresponding to a frequency translation chip code when the correlator is in a receive signal level measurement mode of operation.

14. The direct sequence spread spectrum receiver of any previous claim wherein mixer means corresponds to a ::::. multiplier and the low pass filter means corresponds to an integrator. S... I.

15. The direct sequence spread spectrum receiver of any I..

* previous claim wherein the correlator comprises a matched filter corresponding to the mixing means and the low pass filter means. * . I * *

16. The direct sequence spread spectrum receiver of any previous claim wherein the direct sequence spread spectrum receiver is an Ultra WideBand, UWB, receiver.

17. A method of monitoring receive signal levels by a direct sequence spread spectrum receiver comprising a correlator for correlating a received signal with a chip code, the method comprising: the correlator mixing the received signal and a first chip code to generate a mixed signal; the correlator generating a correlator output signal by low pass filtering the mixed signal, the low pass filtering CMLO3O22M having a first bandwidth smaller than a bandwidth of the received signal; storing a plurality of chip codes, at least one of the plurality of chip codes corresponding to a direct sequence spreading chip code and at least one of the plurality of chip codes corresponding to a frequency translation chip code; selecting the first chip code as a chip code corresponding to a spreading code or a chip code corresponding to a frequency translation chip code in response to a mode of operation of the correlator; and generating an indication of a receive signal level in a first frequency subband having a bandwidth of the first I...

bandwidth in response to the correlator output signal. S.. a..

S

* S... * S * ** S a

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