GB2357385A

GB2357385A - Optimisation of spread spectrum signal receiver in particular direction

Info

Publication number: GB2357385A
Application number: GB9929873A
Authority: GB
Inventors: Michael Colin Wells
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1999-12-18
Filing date: 1999-12-18
Publication date: 2001-06-20
Also published as: GB9929873D0; WO2001045302A1

Abstract

Adaptive arrays have been implemented for GPS as simple analogue, power minimisation cancellers which cancel any interference which is present but do not optimise the reception of the satellite signals. Described herein is a method and apparatus (1) for optimising the reception of the wanted satellite signals using digital techniques whilst still cancelling the interference using beamsteering techniques. The apparatus (1) comprises an adaptive array (10) for receiving satellite signals and providing an output signal (20) for a GPS receiver (30). The receiver (30) processes the received signals and provides an output signal (40) indicative of the carrier to noise estimate for a particular direction. A beam direction controller unit (50) uses the output signal (40) to select a direction signal (60) which selects an array gain vector (80) from look-up table (70). The array gain vector (80) is fed to a weight computation unit (90) for providing a weight vector (100) for the adaptive array (10) in accordance with the selected direction to cancel interference.

Description

IMPROVEMENTS IN OR RELATING TO THE RECEPTION OF SPREAD SPECTRUM SIGNALS

The present invention relates to improvements in or relating to the reception of spread spectrum signals and is more particularly, although not exclusively, concerned with the reception of such signals in global positioning satellite (GPS) systems.

Adaptive arrays have traditionally been implemented for GPS systems as simple analogue, power minimisation cancellers. Such arrays cancel any interference which is present but do not optimise the reception of the satellite signals. The satellite signals are very low power so that they are not intentionally cancelled by the array but they may fall in spurious nulls and be inadvertently cancelled. More usually, some gain is presented to each satellite but not as much as if beams were formed in the direction of each satellite. The gain also varies as the adaptive array weights vary thereby modulating the GPS signals.

It is therefore an object of the present invention to provide improved reception of GPS signals whilst cancelling any interference also present with the signals.

In accordance with one aspect of the present invention, there is provided a method of improving the reception of spread spectrum signals, the method comprising the steps ofi- a) receiving signals; b) optimising the received signals from particular directions; c) processing the optimised signals to generate carrier to noise estimates; and d) using the carrier to noise estimates to select an optimum direction which maximises the carrier to noise estimates.

Advantageously, step b) comprises applying weighted values to the received signals to remove interference. It is preferred that the weighted values are determined in accordance with characteristics for a particular direction of receiving apparatus used in step a). The characteristics may comprise gain vectors for the receiving apparatus.

In accordance with another aspect of the present invention, there is provided apparatus for improving the reception of spread spectrum signals comprising:- receiving means for receiving signals from a particular direction; first processing means for optimising the received signals; second processing means for processing the optimised signals to generate carrier to noise estimates; and third processing means for using the carrier to noise estimates to select an optimum direction which maximises the carrier to noise estimates.

Preferably, the receiving means comprises an antenna array.

Advantageously, the first processing means comprises removal means for removing the interference from the received signals.

In a preferred embodiment of the present invention, the spread spectrum signals comprise GPS signals and the second processing means comprises a GPS receiver.

The third processing means may comprise selecting means for selecting a direction from the carrier to noise estimates and for producing weighting values for the receiving means. The selecting means may comprise a beam controller unit and a look-up table in which gain vectors are stored in relation to particular directions. Additionally, the selecting means further comprises a weight computation unit for computing a weight vector for applying to the receiving means to cancel interference and form a beam in the particular direction.

Using digital techniques, it is possible to optimise the reception of the wanted signals by forming beams on each satellite while cancelling any interference. This has the advantage of improved navigation accuracy, increased resilience to interference and decreased time to first fix.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings, in which:Figure 1 illustrates a block diagram of beam direction controller apparatus in accordance with the present invention; Figure 2 is a block diagram of an adaptive array shown in Figure 1; Figure 3 is a block diagram of one embodiment of a GPS processor shown in Figure 1; and Figure 4 is a block diagram of another embodiment of a GPS processor.

The present invention will be described with reference to GPS systems, but it will readily be appreciated that the invention can be used in any system which receives spread spectrum signals.

Figure 1 illustrates a beam direction controller arrangement 1 which implements beamsteering in GPS systems using carrier to noise ratio (CNR) estimates. The arrangement comprises an adaptive array 10 connected to a GPS receiver 30. The adaptive array 10 receives signals from satellites and provides an output signal 20 which is input to the receiver 30. The receiver processes the signal 20 and provides an output signal 40 which is passed to a beam direction controller unit 50. In the beam direction controller unit 50, the signal 40 is used to provide a signal 60 indicative of a particular direction which produces the best carrier to noise (CNR) estimates. Signal is fed to a look-up table 70 to select array gain vectors for the adaptive array 10 which correspond to a particular direction. Output 80 from look- up table 70 is fed to a weight computational unit 90 which calculates a weight vector 100 which must be applied to the signals received by the adaptive array 10 to produce output signal 20.

As shown in Figure 2, the adaptive array 10 comprises a plurality of antennas 1101,., 110,, for receiving GPS signals. Although only two antennas 110 1, 11 Q, are shown in Figure 2, any suitable number of antennas can be used. In typical adaptive arrays, between four and seven antennas are used. Signals 120,,..., 120, received at respective ones of the antennas 110 1, .. 1 1 lonare passed to respective conversion units 13 01,..., 13 0,, where they are converted to a lower frequency and digitised. Each signal 120,,., 120, comprises a composite signal corresponding to all signals received together with any interference present. The conversion units 130,,..., 130, provides output digital signals 1401 140nwhich are passed to respective combining units 150,,..., 150,, which combine each signal 140,., 140n with a weighting coefficient W,..., W,, as will be described in more detail later. Output signals 160,..., 160,, from the combining units 1501,., 150n are summed together in a summator 170 which provides an output signal 20 from the array 10 which corresponds to the GPS signal with no interference.

Output signal 20 forms the input signal to GPS receiver 30 as described above.

GPS receiver 30, as shown in more detail in Figure 3, comprises a first combining unit 3 10 which combines a signal 3 20 with signal 20 to effect frequency offset removal therefrom to produce a frequency corrected signal 330. The frequency corrected signal 330 is input to a second combining unit 340 where it is combined with a despreading signal 350 to provide a despread signal 360. The despreading signal 350 corresponds to a particular satellite from which signals are received at the adaptive array 10. Despread signal 360 is input to a correlator 370 which provides an output signal 380 which is passed to a processor unit 390.

In practice, signals are received from at least four satellites using parallel correlators as shown in Figure 4. In Figure 4, four parallel correlators 370,, 3702,3703,3704are shown. Additional correlators will also be present (not shown) for maintaining the synchronisation of the despreading signals. For each correlator 3701, 370, 3703,3704, a respective signal 201, 202,203,204received from an associated adaptive array (not shown) is passed to respective combining units 310,, 3102,3103,3104where they are combined with respective signals 320,, 3202,3203,3204 to correct frequency offset for each signal and then the frequency offset corrected signals 330,, 3302,3303,3304are passed to respective combining units 3401, 3402,3403, 340, where they are despread with respective signals 3 501, 3 5025 3503,3504. Despread signals 360,, 3602,3603,3604are then passed to respective correlators 3701, 3702,3703,3704,, outputs 380, 3802,3803,3804 being correlation estimates which provide inputs for a GPS processor 390.

GPS processor 390 processes the correlator outputs 380, 3802,38035 3804 to recover satellite data messages and to compute the position of the GPS receiver 30 (Figure 1). The processor 390 also uses the correlator outputs 380,, 3802,3803,3804 to estimate the carrier to noise ratio (CNR) of each satellite signal, the CNR estimates being output as indicated by reference numeral 40. In accordance with the present invention, the CNR estimates are used to implement beamsteering.

Referring back to Figure 1, output 40 is passed to a beam direction controller 50 where the CNR estimates are used to estimate the satellite directions. The required directions are output as signals 60 and passed to a look-up table (LUT) 70 which stores antenna gain vectors for each direction, the antenna gain vectors being specific to the particular antenna array 10 being utilised. The LUT 70 specifies how the relative gains of the antennas 11011... I "Onwithin the adaptive array 10 vary with direction. The LUT 70 looks up the appropriate array gain vector for the direction indicated by signal 60 and passes that array gain vector 80 to weight computation unit 90.

In weight computation unit 90, the array gain vector 80 is used to calculate weight vector 100 which cancels any interference which is present while forming a beam in the required direction. Weight vector 100 comprises elements W,,..., W,, which are applied to respective ones of the combining units 1501,..., 150,, in the adaptive array 10 to cancel interference. The weight vector 100 can be calculated by several well known techniques, for example, as described in "Introduction to Adaptive Arrays" by

R.A. Monzingo and T.W. Miller, Wiley Interscience 1980, ISBN 0-471-05744-4.

It will readily be appreciated that the beam direction controller 1 works either in an acquisition mode or in a tracking mode.

In the acquisition mode, beams are formed in a number of directions which cover the sky. The direction with the highest CNR estimate gives the direction of the satellite. The beams may be formed sequentially or in parallel if there are enough networks available, each network comprising a weight computation unit and correlators as discussed above.

Once the initial direction of a satellite has been found, then the beam direction controller I operates in a tracking mode for that particular satellite.

This may be achieved in one of two ways:- a) by dithering the beam in direction and observing the effect of the dithering on the CNR estimate obtained. If a particular dither offset improves the CNR estimate, then that becomes the new estimate of satellite direction. Interpolation may be employed to refine the satellite direction estimate.

b) by implementing a monopulse approach if at least two networks are available per satellite. In this case, a 'sum' beam is formed using one network and a 'difference' beam is formed using another network. The ratio of the CNR estimates in the 'sum' beam and the 'difference' beam then indicates the satellite direction. This technique ensures that the maximum gain is continually steered on the satellite.

The beam direction controller 1 may operate independently on each satellite. Alternatively, almanac information from the GPS receiver may be exploited. In this case, it is only necessary to acquire the direction of a minimum of two satellites by searching the sky.

Although the present invention has been described with reference to the use of CNR estimates, it will readily be appreciated that the beam direction controller could also use the correlator outputs directly.

In order to form a beam, the weight computation unit needs knowledge of the array gain vectors. These may initially be calculated from knowledge of the antenna array construction, but the presence of a platform or housing on which the antenna array is mounted may cause the actual values to differ from the calculated values.

A calibration can be implemented using the beam direction controller as shown in Figure 1 and satellite almanac information. If the GPS receiver directs several correlators at the same satellite in synchronisation and the weight computation unit directs the individual antenna signals to different correlators, then the correlator values give the relative array gain values. The complete table of array gain vectors can be constructed as the satellites traverse the sky.

Claims

CLAIMS:

1. A method of improving the reception of spread spectrum signals, the method comprising the steps of. - a) receiving signals; b) optimising the received signals from particular directions; c) processing the optimised signals to generate carrier to noise estimates; and d) using the carrier to noise estimates to select an optimum direction which maximises the carrier to noise estimates.

2. A method according to claim 1, wherein step b) comprises applying weighted values to the received signals to remove interference.

3. A method according to claim 2, wherein the weighted values are determined in accordance with characteristics for a particular direction of receiving apparatus used in step a).

4. A method according to claim 3, wherein the characteristics comprise gain vectors for the receiving apparatus.

5. A method according to any one of the preceding claims, wherein the spread spectrum signals comprise GPS signals.

6. Apparatus for improving the reception of spread spectrum signals comprising:- receiving means for receiving signals; first processing means for optimising the received signals from particular directions; second processing means for processing the optimised signals to generate carrier to noise estimates; and third processing means for using the carrier to noise estimates to select an optimum direction which maximises the carrier to noise estimates.

7. Apparatus according to claim 6, wherein the receiving means comprises an antenna array.

8. Apparatus according to claim 6 or 7, wherein the first processing means comprises removal means for removing the interference from the received signals.

9. Apparatus according to any one of claims 6 to 8, wherein the spread spectrum signals comprise GPS signals and the second processing means comprises a GPS receiver.

10. Apparatus according to any one of the preceding claims, wherein the third processing means comprises selecting means for selecting a direction from the carrier to noise estimates and for producing weighting values for the receiving means.

11. Apparatus according to claim 10, wherein the selecting means comprises a beam controller unit and a lookup table in which gain vectors are stored in relation to particular directions.

12. Apparatus according to claim 11, wherein the selecting means further comprises a weight computation unit for computing a weight vector for applying to the receiving means to cancel interference and form a beam in the particular direction.

13. A method of improving the reception of spread spectrum signals substantially as hereinbefore described with reference to the accompanying drawings.

14. Apparatus for improving the reception of spread spectrum signals substantially as hereinbefore described with reference to the accompanying drawings.