WO2005036771A1

WO2005036771A1 - Method and apparatus for determining delay

Info

Publication number: WO2005036771A1
Application number: PCT/IB2004/052008
Authority: WO
Inventors: Brian J. Minnis
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-10-11
Filing date: 2004-10-07
Publication date: 2005-04-21
Also published as: TW200525924A; GB0323900D0

Abstract

A method and apparatus (100) for determining the relative time delay between spread spectrum signals propagated via two different paths determines the relative time delay from the slope of the difference between the phase characteristics of the signals. The method and apparatus (100) is applied in a ranging apparatus (200), in a diversity receiver (300) and in a transmitter (400) to determine the quality, and optionally to adjust the quality, of a transmitted signal.

Description

DESCRIPTION

METHOD AND APPARATUS FOR DETERMINING DELAY

The invention relates to a method of determining the relative time delay of a spread spectrum signal propagated via first and second paths and to a method of determining an indication of the quality of a spread spectrum signal. The invention also relates to an apparatus for determining the relative time delay of a spread spectrum signal propagated via first and second paths and to products comprising such an apparatus.

There are many radio applications utilising spread spectrum signals in which is it required to determining the relative time delay of a spread spectrum signal propagated via two or more paths. One example of such an application is ranging in which a spread spectrum radio signal is transmitted from a transceiver, a reflection of the transmitted signal is received back at the transceiver, and the delay between the transmitted signal and the received signal is determined in a delay measurement device in order to calculate the distance of the reflecting surface.

In this example, one path comprises the transmitter, the reflecting surface and the receiver, and another path is internal to the transceiver, comprising the coupling for the signal from the transmitter to the delay measurement device. Another example of such an application is a diversity receiver in which a spread spectrum radio signal is received via more than one antenna and the signals from each antenna are time-aligned prior to being combined and demodulated. In order to perform the time alignment the relative time delay of the signals from each antenna must first be determined. In this example, the signal follows a different path from the transmitting source to each receiving antenna. A further example of such an application is a transmitter measurement scheme in which the quality of the transmitter is assessed by generating a spread spectrum signal, passing the spread spectrum signal through the transmitter, and comparing the signal before and after passage through the transmitter in a distortion measuring device in order to assess the distortion introduced by the transmitter. In this example one path comprises the transmitter and the other path is the coupling for passing the generated signal to the distortion measuring device. Such a transmitter measurement scheme may be used, for example, to verify that the transmitter is compliant with relevant type approval specifications, or may be part of an adaptive control system which attempts to optimise the transmitter's power efficiency. One metric used for assessing transmitter distortion is the error vector magnitude (EVM). In an EVM measurement, the amplitude and phase trajectories of the signal at the transmitter output are compared with that of the baseband signal prior to passage through the transmitter, the latter acting as a reference signal. This involves taking a sample of the signal at the transmitter output, mixing it down to baseband and then passing it through an analogue- to-digital converter (ADC). The digitised signal is then treated as a complex vector whose sequence of values is supposed to follow certain allowed transitions within a prescribed constellation of points in the complex plane. On comparison with the reference signal, the differences in values of the complex vectors are the so-called error vectors and it is the ratio of the RMS value of the error vectors to the RMS value of the reference that constitutes the value of EVM. Before determining the EVM, it is necessary to time-align the signals being compared, otherwise the EVM will be erroneous. The timing accuracy required is substantially less than the sampling interval involved (i.e. 1 /sampling rate). Experiments have shown that, for a transmitter for use in the Universal Mobile Telecommunication System (UMTS), the accuracy needs to be < 1 % of the sampling interval, which is equivalent to < 0.1% of the chip period, if a reliable measure of EVM is to be obtained. In each of the application examples described above, the relative time delay of a spread spectrum signal propagated via two or more paths must be determined. One known method of determining the time delay between two spread spectrum signals is to perform correlation, sweeping one signal across the other in time whilst looking for a maximum correlation. Correlation is effective in applications where the signal bandwidth is shared with other signals such that the signal to interference ratio is poor. However correlation is an iterative, slow process. An object of the invention is to provide an alternative way of determining the relative time delay of a spread spectrum signal propagated via first and second paths.

According to a first aspect of the invention there is provided a method of determining the relative time delay of a spread spectrum signal propagated via first and second paths, comprising: determining the phase versus frequency characteristics of the signal after propagation via the first and second paths; from the phase versus frequency characteristics determining the phase difference as a function of frequency for at least a portion of the bandwidth of the signal; fitting a straight line function to the phase difference as a function of frequency; and determining the relative time delay from the slope of the straight line function. According to a second aspect of the invention there is provided a method of determining an indication of the quality of a spread spectrum signal, comprising: generating a spread spectrum signal at baseband in a transmitter; routing the signal to a delay measurement apparatus via a first path comprising up-conversion in the transmitter and a down conversion stage, and via a second path at baseband; in the delay measurement apparatus, determining according to the method of the first aspect of the invention the relative time delay of the signal propagated via the first and second paths; equalising the delay of the signal propagated via the first and second paths by applying a differential delay equal to the relative time delay; determining after equalising the delay an indication of the quality of the signal propagated via the first path relative to the signal propagated via the second path. According to a third aspect of the invention there is provided an apparatus for determining the relative time delay of a spread spectrum signal propagated via first and second paths, comprising: phase determining means for determining the phase versus frequency characteristics of the signal propagated via the first and second paths; phase differencing means for determining from the phase versus frequency characteristics the phase difference as a function of frequency for at least a portion of the bandwidth of the signal; fitting means for fitting a straight line function to the phase difference as a function of frequency; and delay determining means for determining the relative time delay from the slope of the straight line function. According to a fourth aspect of the invention there is provided a radio transceiver for use in ranging, comprising: generation means for generating a spread spectrum signal at baseband; transmitter means for transmitting the signal; receiver means for receiving the transmitted signal from a reflecting surface; apparatus according to the third aspect of the invention coupled to the receiver means and to the generation means for determining the relative time delay of the signal propagated via first and second paths, wherein the first path comprises the transmitter means, receiver means and the reflecting surface, and the second path comprises the coupling to the generation means; and distance determining means adapted to determine from the relative time delay the distance of the reflecting surface from the transmitter means and receiver means. According to a fifth aspect of the invention there is provided a receiver for receiving a spread spectrum signal, comprising: first receiver means for receiving the spread spectrum signal propagated via a first path comprising a first antenna; second receiver means for receiving the spread spectrum signal propagated via a second path comprising a second antenna; apparatus according to the third aspect of the invention coupled to the first and second receiver means for determining the relative time delay of the signal propagated via the first and second paths; delay equalisation means coupled to the apparatus according to the third aspect of the invention and to the first and second receiver means for equalising the delay of the signal received via the first and second paths by applying a differential delay equal to the relative time delay; combining means coupled to the delay equalisation means for combining after delay equalisation the signal received via the first and second paths; and demodulation means coupled to the combining means for demodulating the signal resulting from the combining. According to an sixth aspect of the invention there is provided a transmitter for transmitting a spread spectrum signal, comprising generation means for generating a spread spectrum signal at baseband; up-conversion means for up-converting the baseband spread spectrum signal for transmission; down-conversion means for down-converting to baseband the up-converted baseband spread spectrum signal; apparatus according to the third aspect of the invention coupled to the down-conversion means and to the generation means for determining the relative time delay of the signal propagated via first and second paths, wherein the first path comprises the up- conversion means and the down-conversion means, and the second path comprises the coupling to the generation means; delay equalisation means coupled to the apparatus according to the third aspect of the invention and to the first and second paths for equalising the delay of the signal propagated via the first and second paths by applying a differential delaying equal to the relative time delay; and processing means adapted to determine after the delay equalisation an indication of the quality of the signal propagated via the first path relative to the signal propagated via the second path. In one embodiment of the invention any phase shift introduced by one or more devices, such as a filter, in at least one of the first and second paths may be compensated for prior to fitting the straight line function. In one embodiment of the invention the indication of quality is an indication of Error Vector Magnitude (EVM). In one embodiment of the invention the delay between the signal propagated via the first and second paths may be equalised in more than one step by making an initial coarse measurement and adjustment followed by a fine measurement and adjustment. By providing an alternative to using correlation which does not require an iterative process, the invention provides a fast way of determining the relative time delay of a spread spectrum signal propagated via first and second paths. The invention is most effective in situations where the signal paths are not subjected to high levels of interference, for example in an in-building propagation environment. The invention can be effective even in a rich multipath environment by virtue of the large bandwidth of the spread spectrum signal.

The invention will now be described, by way of example only , with reference to the accompanying drawings wherein; Figure 1 is a flow chart of a method for determining the relative time delay of a spread spectrum signal propagated via first and second paths; Figure 2 is the frequency spectrum of a reference spread spectrum signal; Figure 3 is the phase characteristic of a reference spread spectrum signal; Figure 4 is the phase difference between two spread spectrum signals delayed with respect to each other. Figure 5 is the frequency spectrum of a spread spectrum signal after passing through a transmitter; Figure 6 is the phase characteristic of two spread spectrum signals delayed with respect to each other and with noise added to the delayed signal; Figure 7 is the phase difference between two spread spectrum signals delayed with respect to each other and with noise added to the delayed signal; Figure 8 is a block schematic diagram of an apparatus for determining delay in accordance with the present invention; Figure 9 is a block schematic diagram of an apparatus for ranging; Figure 10 is a block schematic diagram of a receiver for receiving a spread spectrum signal; Figure 11 is a flow chart of a method of determining an indication of the quality of a spread spectrum signal; and Figure 12 is a block schematic diagram of a transmitter for transmitting a spread spectrum signal. In the drawings the same reference numerals have been used to indicate corresponding features in different Figures. Referring to the flow chart of Figure 1 , at step 10 the phase versus frequency characteristic θy (f) of a spread spectrum signal propagated by a first path is determined, and at step 12 the phase versus frequency characteristic Q₂φ of the spread spectrum signal propagated by a second path is determined, /is the frequency in Megahertz. At step 14 the phase difference as a function of frequency Aθ(β is determined as Aθ(β= θ φ- θ_ 0. At step 16 a mathematical function of the form y=mx+c representing a straight line is fitted to the function Aθφ, and at step 18 the relative time delay t_ between the signals transmitted by the first and second paths is determined from the slope m of the straight line as

The mathematical basis for the above method is explained as follows.

The phase shift Qφ of a single frequency signal travelling along a single path is proportional to the frequency and the delay introduced by the path i.e.

where / is the frequency of the signal and t is the delay. Therefore, for a spread spectrum signal comprising a range of closely-spaced spectral components, the slope of the phase difference versus frequency characteristic is proportional to the delay, i.e. — — = In . Therefore, the time delay can be df determined from the slope of the phase difference versus frequency characteristic, i.e. t = — . — — . An advantage of basing the time delay

measurement on a range of frequencies, rather than a single frequency, is that the measurement error is averaged resulting in improved accuracy. The phase difference versus frequency characteristic can be determined by subtracting from the phase versus frequency characteristic of the delayed signal the initial phase versus frequency characteristic of the signal. Therefore, for a signal propagated via two paths, because the initial phase characteristic will be the same for both paths, the relative time delay can be determined from the difference in the phase versus frequency characteristics i.e. t_d = — . ^J — JJ Compared with known, iterative methods of 2π df correlating signals to find their relative time delay, the above method is non- iterative so enables a fast solution. Further refinements of the basic method for a practical scenario will now be described. Figure 2 shows an example of the baseband spectrum of a spread spectrum signal. The modulation bandwidth is approximately 4 MHz, the chip rate is 3.84 MHz and an over-sampling ratio of 8 was used to generate the signal. Hence the sampling rate is 30.72 MHz and the sampling interval is 32.6 ns. The phase versus frequency characteristic of this signal is shown in Figure 3 and can be seen to be noise like in character. If this signal is propagated along a path each frequency experiences a different phase shift as described above, but the phase difference introduced by the propagation path varies linearly with frequency, as described above and illustrated in Figure 4. In a practical implementation noise and distortion is likely to be introduced to the spread spectrum signal. For example, digital-to analogue converters can introduce quantisation noise, filters can introduce phase shift, and amplifiers can introduce noise and intermodulation distortion. Figure 5 illustrates an example of the baseband spectrum of a noisy spread spectrum signal; the noise level outside of the central bandwidth of 4 MHz has risen by approximately 40 dB. The result of the noise is that the phase characteristic is changed. Figure 6 illustrates the phase characteristics of the spread spectrum initially, labelled the "reference signal", and after propagation via a path which introduces noise, labelled the "delayed signal". Figure 7 illustrates the difference between these phase characteristics. Despite the seemingly random nature of the phase characteristics, the difference between them still exhibits a linear relationship in the 4 MHz region of the central frequencies; the noise has primarily disturbed the sidebands. The consequence of this is that the determination of time delay should be based only on the central portion of the phase versus frequency characteristic. The bandwidth of the portion used can be selected based on knowledge of modulation bandwidth and of the type and extent of noise and distortion introduced along the signal paths. Even dispersion can be tolerated provided the dispersion introduces a roughly periodic perturbation of the straight line phase characteristic of the signal; the straight line function can then be fitted through the perturbations. Devices, such as filters, used in transmission or reception apparatus, may introduce a phase shift, resulting in a phase difference versus frequency characteristic that is not a straight line. However, the effect of such devices on the signal may be compensated for by determining and removing the phase characteristic of such devices from the phase versus frequency characteristics or the phase difference versus frequency characteristic to leave a straight line characteristic to which a straight line function can be fitted. Referring to Figure 8, there is shown a block schematic diagram of an apparatus 100 for determining the relative time delay of a signal propagated via first and second paths. The apparatus 100 comprises first and second inputs 110, 120 for supplying first and second signals to respectively first and second phase determining means 130, 140 for determining the phase versus frequency characteristic of the first and second signals. There is an output from each of the first and second phase determining means 130, 140 for supplying the phase versus frequency characteristic to a phase difference determining means 150 for determining the difference between the phase versus frequency characteristics of the first and second signals. There is an output of the phase difference determining means 150 for supplying the difference between the phase versus frequency characteristics of the first and second signals to a fitting means 160 for fitting a straight line function y=mx+c to the difference. There is an output of the fitting means 160 for supplying the gradient m of the fitted straight line function to a delay determining means 170 for determining the relative delay t_d of the first and second signals as t_d=m/2π. There is an output 180 for delivering the determined delay t_d. The blocks 130 to 170 described within the apparatus 100 may be implemented in one or more processors. The first and second phase determining means 130, 140 may operate by digitising the signals supplied to the first and second inputs 110 120 and determining the spectra of these signals by using either a Discrete Fourier Transform (DFT) or a Fast Fourier Transform (FFT), depending on the number of samples and the computational resources available. The duration of the signals processed by the apparatus 100 is a trade off. The longer the duration of the signals, the more reliable the time delay calculation will be, but the higher the computational load. Also, the duration must be large enough for the expected time delay to cause no more than a few degrees of phase shift from one frequency bin to the next in the DFT or FFT. The phase difference determining means 150 may operate by dividing the complex spectra supplied by the first and second phase determining means 130, 140 and then converting the result into polar coordinates. The alternative approach of converting to polar coordinates prior to performing an FFT or DFT and then subtracting the phase versus frequency characteristics would be prone to errors, particularly if the time delay is large. Before fitting the straight line function in the fitting means 160, the phase difference characteristics may be unwrapped so that phase values exceeding ±π are not expressed modulo π. The compensation referred to above for compensating for the effect of devices in the transmission or reception apparatus may be performed, for example, by the first and second phase determining means 134, 140, the phase difference determining means 150, or in a separate compensation means. One application of the method and apparatus for determining relative time delay in accordance with the invention, as described above with reference to Figure 1 and Figure 8 is for determining range. Referring to Figure 9, there is shown a block schematic diagram of a ranging apparatus 200. There is a generator 210 for generating a spread spectrum signal. There is a transmitter 220 coupled to transmit the generated spread spectrum signal via an antenna 240. The transmitter 220 is coupled to the antenna 240 by a coupling means 230, which may be for example a circulator. The transmitted signal will be propagated and reflected from a reflector 290 located a distance d from the ranging apparatus and received back at the antenna 240. A receiver 250 is coupled via the coupling means 230 to the antenna 240 for receiving the reflected signal. Apparatus 100 as described above with reference to Figure 8 for determining the relative time delay of a signal propagated along first and second paths is coupled to receive on its first input 110 the reflected signal from the receiver 250, and on its second input 120 the generated signal directly from the generator 210. The first path comprises the transmitter 220, the route to and from the reflector 290, the receiver 250, and the second path comprises the direct coupling from the generator 210 to the apparatus 100. The calculation of delay can take into account any delays occurring in equipment, which can themselves be determined by calibration of the equipment. The output of the apparatus 100 is coupled to a range determining means 260 for calculating the distance of the reflector 290 from the ranging apparatus 200 as d = t_dc/2 where t_d is the relative time delay and c is the speed of light. As an example, using a spread spectrum signal having a signal bandwidth of 4MHz, a im range is equivalent to a phase shift of approximately 4.8°. This degree of resolution is well within the scope of the delay calculation method and current processing equipment, and enables an accuracy of approximately +1 m over a range in the order of 1 m to 100m. Another application of the method and apparatus for determining relative time delay in accordance with the invention, as described above with reference to Figure 1 and Figure 8, is for receiving a spread spectrum signal received via more than one antenna. Referring to Figure 10 there is illustrated a receiver 300 for receiving a spread spectrum signal. There are first and second antennas 310, 320 for receiving the signal propagated via first and second paths. Coupled to the first and second antennas 310, 320 there are respectively first and second receiver front ends 330, 340 for amplifying, filtering and down-converting the received signals to baseband. An output from each of the first and second receiver front ends 330, 340 is coupled respectively to the first and second inputs 110, 120 of apparatus 100, as described above, for determining the relative time delay, of the signals received at each antenna 310, 320. The output 180 of the apparatus 100 is coupled to supply an indication of the relative delay t_d to an input of a delay equalisation stage 350. The output from each of the first and second receiver front ends 330, 340 is also coupled respectively to further inputs of the delay equalisation stage 350. The delay equalisation stage 350 operates using the supplied value of the relative delay t_d to equalise the delays experienced by the signals received at each antenna 310, 320. First and second outputs of the delay equalisation stage 350 couple the delay equalised signals to a combining stage 360 for combining the signals. An output of the combining stage 360 is coupled to a demodulator 370 for demodulating the combined signals. The demodulated signal is supplied on an output 380. Another application of the method and apparatus for determining relative time delay in accordance with the invention, as described above with reference to Figure 1 and Figure 8, is a method for determining an indication of the quality of a spread spectrum signal, for example when assessing the quality of a transmitter, and when adjusting the transmitter to improve signal quality. Referring to Figure 11 there is illustrated a method of determining the quality of a spread spectrum signal. The method starts at step 40 where a spread spectrum signal is generated. At step 41 the spread spectrum signal is up-converted and amplified in the transmitter. At step 42 the output of the transmitter is sampled and the sampled signal is down-converted. At step 43 the relative time delay between the generated spread spectrum signal and the down-converted signal is determined. At step 44 the time delay of these two signals is equalised. At step 45 the EVM of the down -converted and delayed signal is determined by comparing this signal with the generated signal. The EVM provides an indication of the quality of the transmitter. Optionally at step 46 the transmitter may be adapted to improve the EVM. Optionally the equalisation of relative time delay at step 44 may be preceded by a coarse time delay estimation and adjustment, for example equalising the relative time delay to the nearest digital time sample. Such a two stage approach may be advisable if the time offset of the two signals is, or might be, very large. A very large time delay would correspond to a very large phase slope in the time delay determination and this can be prone to errors if the delay is too large, for example phase steps of greater than π can become ambiguous. The coarse adjustment can be performed by sweeping one signal across the other using steps of a whole sampling interval and looking for the minimum value of EVM. It cannot produce the wrong answer if sufficiently long signals are processed. Referring to Figure 12 there is block schematic diagram of a transmitter 400 equipped to determine the quality of a transmitted spread spectrum signal. There is a generator 410 for generating a spread spectrum signal. An output of the generator 410 is coupled to a front end stage 420 for up-conversion and amplification of the generated signal. An output of the front end stage 420 is coupled to an antenna 430. The output of the front end stage 420 is also coupled to a down-conversion stage 440 for down converting the signal provided by the front end stage 420. The output of the generator 410, and an output of the down-conversion stage 440, are coupled to respectively the first and second inputs of an apparatus 100, as described above, for determining the relative time delay of the signals supplied to the inputs 110, 120. The output 180 of the apparatus 100 is coupled to supply an indication of the relative delay t_d to an input of a delay equalisation stage 350, which is as described above. The output of the generator 410, and the output of the down-conversion stage 440, are also coupled to respectively the further inputs of the delay equalisation stage 350. The delay equalisation stage 350 operates using the supplied value of the relative delay t to equalise the delays experienced by the signal at the output of the generator 410 and the signal at the output of the down conversion stage 440. It may additionally operate to remove any amplitude or phase offset between the two signals by complex scaling of the signals. The first and second outputs of the delay equalisation stage 350 are coupled to a quality assessment stage 460 which operates to compare the delay-equalised signals and generate an indication of quality of the down-converted signal on an output 450. Optionally an output 450 of the quality assessment stage 460 is coupled to a control means 470 which is coupled to the generator 410 and to the front end stage 420. The control means 470 operates to adjust one or more parameters of the generator 410 and the front end stage 420 in order to improve the quality of the signal at the output of the front end stage 420. Such adjustments may comprise, for example, predistorting the signal generated by the generator 410, or adjusting imbalance in quadrature mixers used for up-conversion in the front end stage 420. The transmitter 400 may be the transmitter portion of a transceiver. In this case the down-conversion stage 440 could be part of the receiver portion of the transceiver, and the delay equalisation stage 350 may be implemented using filters, such as digital root raised cosine filters, provided in the receiver for channel selection and sampling rate reduction (i.e. decimation). In an alternative embodiment, as described above in relation to the method of determining the quality of a spread spectrum signal, two stages of time delay estimation and adjustment may be incorporated; the apparatus 100 may be preceded by, or include, a coarse time delay estimation and adjustment stage for reducing the differential delay, for example for adjusting time delay to the nearest time sample, prior to a fine time delay estimation by the apparatus 100 and fine adjustment by the delay equalisation stage 350. In the present specification and claims the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Further, the word "comprising" does not exclude the presence of other elements or steps than those listed. From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the art of wireless technology and spread spectrum communication and which may be used instead of or in addition to features already described herein.

Claims

1. A method of determining the relative time delay of a spread spectrum signal propagated via first and second paths, comprising: determining the phase versus frequency characteristics of the signal after propagation via the first and second paths; from the phase versus frequency characteristics determining the phase difference as a function of frequency for at least a portion of the bandwidth of the signal; fitting a straight line function to the phase difference as a function of frequency; and determining the relative time delay from the slope of the straight line function.

2. A method as claimed in claim 1 , further comprising compensating for phase shift introduced by a device in at least one of the first and second paths prior to fitting the straight line function.

3. A method of determining an indication of the quality of a spread spectrum signal, comprising: generating a spread spectrum signal at baseband in a transmitter; routing the signal to a delay measurement apparatus via a first path comprising up-conversion in the transmitter and a down conversion stage, and via a second path at baseband; in the delay measurement apparatus, determining according to the method of claim 1 or 2 the relative time delay of the signal propagated via the first and second paths; equalising the delay of the signal propagated via the first and second paths by applying a differential delay equal to the relative time delay; determining after equalising the delay an indication of the quality of the signal propagated via the first path relative to the signal propagated via the second path.

4. A method as claimed in claim 3, wherein determining the indication of quality comprises determining an indication of Error Vector Magnitude.

5. A method as claimed in claim 3 or 4, comprising reducing the differential delay between the signal propagated via the first and second paths prior to equalising the delay.

6. Apparatus (100) for determining the relative time delay of a spread spectrum signal propagated via first and second paths, comprising: phase determining means (130, 140) for determining the phase versus frequency characteristics of the signal propagated via the first and second paths; phase differencing means (150) for determining from the phase versus frequency characteristics the phase difference as a function of frequency for at least a portion of the bandwidth of the signal; fitting means (160) for fitting a straight line function to the phase difference as a function of frequency; and delay determining means (170) for determining the relative time delay from the slope of the straight line function.

7. Apparatus as claimed in claim 6, further comprising compensation means for compensating for phase shift introduced by a device in at least one of the first and second paths prior to determining the straight line function.

8. An apparatus (200) for use in ranging, comprising: generation means (210) for generating a spread spectrum signal at baseband; transmitter means (220) for transmitting the signal; receiver means (250) for receiving the transmitted signal from a reflecting surface (290); apparatus (100) according to claim 6 or 7 coupled to the receiver means (250) and to the generation means (210) for determining the relative time delay of the signal propagated via first and second paths, wherein the first path comprises the transmitter means (220), receiver means (250) and the reflecting surface (290) , and the second path comprises the coupling to the generation means; and distance determining means (260) adapted to determine from the relative time delay the range of the reflecting surface from the transmitter means (220) and receiver means (250).

9. A receiver (300) for receiving a spread spectrum signal, comprising: first receiver means (330) for receiving the spread spectrum signal propagated via a first path comprising a first antenna (310); second receiver means (340) for receiving the spread spectrum signal propagated via a second path comprising a second antenna (320); apparatus (100) according to claim 6 or 7 coupled to the first and second receiver means (330, 340) for determining the relative time delay of the signal propagated via the first and second paths; delay equalisation means (350) coupled to the apparatus (100) according to claim 6 or 7 and to the first and second receiver means (330, 340) for equalising the delay of the signal received via the first and second paths by applying a differential delay equal to the relative time delay; combining means (360) coupled to the delay equalisation means (350) for combining after delay equalisation the signal received via the first and second paths; and demodulation means (370) coupled to the combining means (360) for demodulating the signal resulting from the combining.

10. A transmitter (400) for transmitting a spread spectrum signal, comprising generation means (410) for generating a spread spectrum signal at baseband; up-conversion means (420) for up-converting the baseband spread spectrum signal for transmission; down-conversion means (440) for down-converting to baseband the up- converted baseband spread spectrum signal; apparatus (100) according to claim 6 or 7 coupled to the down-conversion means (440) and to the generation means (410) for determining the relative time delay of the signal propagated via first and second paths, wherein the first path comprises the up-conversion means (420) and the down-conversion means (440), and the second path comprises the coupling to the generation means; delay equalisation means (350) coupled to the apparatus (100) according to claim 6 or 7 and to the first and second paths for equalising the delay of the signal propagated via the first and second paths by applying a differential delaying equal to the relative time delay; and processing means (460) adapted to determine after the delay equalisation an indication of the quality of the signal propagated via the first path relative to the signal propagated via the second path.

11. A transmitter (400) as claimed in claim 10, wherein the processing means (460) is adapted to determine the indication of quality as an indication of Error Vector Magnitude.

12. A transmitter (400) as claimed in claim 10 or 11 , further comprising delay adjustment means (350) for reducing the differential delay between the signal propagated via the first and second paths prior to the delay equalisation.

13. A transmitter (400) as claimed in claim 10, 11 or 12, comprising adaptation means (470) responsive to the indication of quality for adapting a parameter of the transmitter (400).

14. A transceiver comprising the transmitter (400) as claimed in any one of claims 10 to 13.