GB2338385A - Spread spectrum communication system - Google Patents

Abstract

A communications system such as a two-way paging system in which responses to downlink messages from a primary station to respective secondary stations are transmitted substantially simultaneously as spread spectrum signals, each encoded as a respective PN code sequence. The primary station distinguishes a signal from the plurality of substantially simultaneously received spread spectrum signals by multiplying the received signals with a first PN code, analysing off-line the products of multiplication by obtaining a FFT and checking the result for a peak indicative of the presence of the first PN code, and if a peak is present, reducing the amplitude of the peak, regenerating the remaining products of multiplication using an inverse FFT, multiplying the output by the first PN code to produce a regenerated received signal omitting substantially the first PN code, and repeating the sequence of operations eliminating signals with other PN code sequences present in the received signals. Optionally stronger spread spectrum signals are eliminated prior to the weaker signals.

Description

2338385 1 COMMUNICATIONS SYSTEM The present invention relates to a

communications system and particularly, but not exclusively, to a two-way communications system in which signals in one direction are transmitted substantially simultaneously as spread spectrum signals. An example of such a system is an answer back paging system.

Answer back paging systems are known from PCT Patent Specifications W096/14716, W096/16518 and W097/46033. In the systems described, paging messages are formatted according to the appropriate protocol, for example the well known CCIR Radiopaging Code No. 1, otherwise known as POCSAG, and are transmitted by a primary station as point-to-point signals to secondary stations (or selective call receivers). The down-link messages are transmitted on a time division basis. Those secondary stations wishing to transmit responses as substantially simultaneous spread spectrum uplink signals wait for an invitation signal from the primary station before transmitting.

In order not to have to apply transmitter power control to the secondary station transmitters to counter the near-far problem associated with spread spectrum signalling and thereby avoid the cost implications of increasing the complexity of the secondary station (that is the increased exchange of control signals and the increased power consumption) W096/14716 and W097/46033 disclose two methods of issuing invitation signals. In W096/14716 a sequence of invitation signals is transmitted, with successive invitation signals being at 2 an increased power level thereby increasing the coverage area from the primary station transmitter. Those secondary stations who want to reply to an invitation, transmit their responses as spread spectrum uplink signals and are then inhibited from responding for a second or more time to subsequent stronger invitations in the sequence. At the primary station, the majority if not all of the uplink signals responding to any one invitation signal will be received with comparable strengths enabling them to be processed.

W097/46033 modifies this technique by the primary station issuing an invitation and receiving responses, the primary station analyses those responses which are deemed to be intelligible, matches them to the originally is transmitted messages and transmits acknowledgements to the relevant secondary stations. The primary station then issues another invitation to those stations whose responses had not been detected requesting them to reply on the uplink. Those responses which are deemed intelligible are processed as before and acknowledgements are transmitted together with another invitation on the downlink. The cycle of operations is repeated until either a predetermined number of cycles have elapsed or it is considered that all the intelligible responses have been received. The method can be adapted to include handling requests for service as well as handling responses to messages sent on the downlink.

In the abovementioned answer back paging systems in which the responses are transmitted at a low bit rate to maximise the range of the low power transmitters in the secondary stations, the nature of the response is 3 simplified and may simply comprise a preselected PN code which it is known in advance will give a pre-identified short answer such as:

Code Sequence 1 Code Sequence 2 Code Sequence 3 Code Sequence 4 Code Sequence 5 Code Sequence 6 - secondary station in the area for the purposes of registration only received last message read message(s) answer "Yes" answer"Noll resend last message.

Each secondary station transmits the desired PN is from a block of PN codes allocated to the secondary station at the time and withdrawn afterwards to be available for re-use by another secondary station.

The primary station on receiving a batch of responses has to analyse them to recover the individual replies and then issue another invitation to those secondary stations which have not replied or from which an intelligible reply has not been received. The faster the responses can be analysed and invitations transmitted, the sooner all the intelligible responses will have been analysed and a new batch of messages transmitted, thus maximising the system capacity.

An object of the present invention is to be able to process spread spectrum signals effectively.

According to a first aspect of the invention there is provided a method of distinguishing a signal from a plurality of simultaneously received spread spectrum 4 signals, each encoded with a different code, the method comprises:

i) multiplying the received signals with a first code, is ii) analysing the products of multiplication by transforming the products and checking the result for an indication of a signal which had been encoded with the first code, if an indication is present, iii) reducing the amplitude of the indication in the transformed output, iv) regenerating the remaining products of multiplication using an inverse of the transform, v) multiplying the output by the first code to produce a regenerated received signal omitting substantially the first code, and vi) repeating the sequence of operations eliminating signals with other codes, present in the received signals.

According to a second aspect of the invention there is provided a two way communications system comprising:

primary station, and plurality of secondary stations, the primary station having means for transmitting a plurality of point-to- point downlink signals to preselected ones of the secondary stations, said secondary stations having means for transmitting uplink signals as substantially simultaneous spread spectrum signals encoded using a respective code, wherein the primary station has is means for transmitting an invitation signal inviting the preselected ones of the secondary stations to transmit their uplink signals, and means for recovering a selected uplink signal from the substantially simultaneously received signals, said recovering means comprising multiplying means for multiplying the received signals with a first code, means for analysing the products of multiplication by transforming said products and checking the result for an indication of a signal having the first code, means responsive to an indication being present for reducing the amplitude of the indication in the transformed output, means for regenerating the remaining products of multiplication using an inverse of the transform, means for multiplying the output by the first code to produce a regenerated received signal omitting substantially the first code, and means repeating the sequence of operations eliminating signals with other codes, present in the received signals. According to a third aspect of the present invention a primary station for use in a two way communications system includes the primary station and a plurality of secondary stations, said secondary stations having means for transmitting uplink signals as substantially 6 simultaneously received spread spectrum signals encoded using a respective code, the primary station comprises: means for transmitting a plurality of point-to-point downlink signals to preselected ones of the secondary stations, means for transmitting an invitation signal inviting the preselected ones of the secondary stations to transmit their uplink signals, means for recovering a respective uplink signal from the substantially simultaneously received signals, said recovery means comprises multiplying means for multiplying the received signals with a first code, means for analysing the products of multiplication by transforming said products and checking the result for an indication of a signal which has been encoded with the first code, means responsive to an indication being present for reducing the amplitude of the indication in the transformed output, means for regenerating the remaining products of multiplication using an inverse of the transform, means for multiplying the output by the first code to produce a regenerated received signal omitting substantially the first code, and means repeating the sequence of operations eliminating signals with other codes, present in the received signals. Any suitable transform may be used for transforming the signal, for example a Fast Fourier Transform, and the 7 inverse of the transform is used for regenerating the signal.

The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein:

Figure 1 is a block schematic diagram of a two-way paging system; Figure 2 is a block schematic diagram of a primary station comprising a system controller and a base station transceiver; Figure 3 is a block schematic diagram of a secondary station; Figure 4 is a timing diagram showing the transmission of invitation signals, reception of spread spectrum responses and the analysis of the responses; Figures SA and 5B illustrate, respectively, a string of messages transmitted by a primary station and an invitation message including acknowledgements to successfully recovered responses; Figure 6 is a flow chart showing a sequence of operations involved in transmitting messages, receiving responses and acknowledging responses; Figure 7 is a block schematic diagram of stages in a primary station for recovering individual responses from a batch of simultaneously received spread spectrum responses; and Figure 8 is a flow chart of the process carried out by the stages in Figure 7.

In the drawings the same reference numerals have been used to indicate corresponding features.

8 is Referring to Figure 1 the two-way paging system comprises a primary station PS consisting of a paging system controller 10 which is connected to at least one base station transceiver 12, if necessary by land lines or other suitable links. In the event of there being more than one base station transceiver they may be geographically spaced apart and may operate in a quasi-synchronous mode.

Selective call receivers or secondary stations SS1, SS2 are provided, each of which comprises a transceiver which is able to receive transmissions from the transceiver 12 and is able to transmit a limited number of types of messages, including acknowledgements, at significantly lower power than the output power of the transceiver 12, for example 30dB lower. The messages are transmitted as spread spectrum signals typically at an information rate of one thousandth of that transmitted by the transceiver 12 and a spreading sequence length of the order of 101, for example 8191 chips per bit.

Figure 2 shows an arrangement of the system controller 10 connected to the transceiver 12. The system controller 10 comprises an input 18 for data messages to be relayed via the transceiver 12. The messages are held in a store 20 from where they are relayed to a formatting stage 22 which appends Receiver Identity Code word (RIC) to the message and divides the message into a plurality of concatenated code words of a predetermined length, each code word including error detection/correction bits and optionally an even parity bit. The RICs are held in a store 24. A processor 26 is provided which controls the operation of the system controller in accordance with a 9 program which is stored in a memory 28. Also connected to the processor 26 are a clock/time stage 30, an invitation signal generator 32 and a store 34 for storing details of the code sequences which may be used by the secondary stations in transmitting their responses to the invitation messages. Once the data messages in the store 20 have been formatted in the stage 22 the processor 26 causes them to be transmitted by the base station transceiver 12. The formatting of the data messages may conform to any known message format such as CCIR Radiopaging Code No. 1 (otherwise known as POCSAG), ERMES or to any other signal format which is known or yet to be devised. once the messages have been transmitted, the processor arranges to transmit the invitation-torespond signals generated in the stage 32.

The processor 26, following the transmission of an invitation signal, switches the transceiver 12 to receive and is ready to accept signals received by the transceiver 12, the outbound propagation path to the or each secondary station being substantially the same as that of the inbound propagation path. In order to identify each of the responses which is sent as a spread spectrum signal, each of the code sequences is mixed sequentially with the received signals which are held in a buffer and when a correlation is obtained then the response is noted and further code sequences are mixed with the received signal in order to recover any other responses which may be present. The recovery of the responses will be described later in greater detail with reference to Figures 7 and 8.

Figure 3 is a block schematic diagram of a secondary station SS having the capability of transmitting responses to invitation signals as spread spectrum signals. The secondary station SS comprises an antenna 36 which is coupled to a receiver stage 38. An output of the receiver stage 38 is coupled to an input of a decoder 40. A microcontroller 42 is coupled to the output of the decoder 40 and controls the operation of the secondary station in accordance with a program held in a Read Only Memory (ROM) 44. The microcontroller 42 has inputs/outputs, as appropriate, coupled to annunciating means 46 which may be audio, visual and/or tactile, a keypad 48, data output means, for example an LCD driver 50 which is coupled to an LCD panel 52, and a Random is Access Memory (RAM) 56 for storing any messages which have been received and decoded.

In operation the receiver stage 38 is energised in response to the particular battery economising protocol followed by the secondary station SS. Optionally the decoder 40 and the microcontroller 42 may ',sleep,' when not required, the microcontroller 42 being woken by an internal timer (not shown) or an interrupt signal and in so doing waking up other stages of the secondary station, as appropriate. When a RIC is received, it is demodulated, decoded, error corrected and checked to see if it is one assigned to the secondary station or an invitation to send a message to the primary station. Assuming it is the RIC assigned to the secondary station then depending on the programming of the microcontroller 42, the annunciating means 46 may be activated to inform the user that a call has been received. However a user, 11 by actuating a key or keys of the keypad 48 can inhibit one or more of the output devices of the annunciating means. If a short message at the same data rate as the address code word is concatenated with the paging call, then once it has been decoded and error checked/corrected, the microcontroller 42 causes the decoded message to be stored in the RAM 56. By actuating a key or keys of the keypad 48, a user can instruct the microcontroller 42 to read-out the message from the RAM 56 to the LCD driver 50 which will cause the message to be displayed on the screen 52. The operation described so far is typical for many alphanumeric message pagers conforming to the POCSAG standard.

The illustrated secondary station SS includes a low is power transmitter 58 whereby acknowledgements and/or short messages can be relayed to the or any in-range base station transceiver. The actual acknowledgement or message is generated by the microcontroller 42 and will be transmitted as a spread spectrum signal. One or more near orthogonal pseudo-random code sequences may be stored or generated in a stage 60. The microcontroller 42 controls the reading out of a code sequence from the stage 60 which is coupled to a transmitter 58. The code sequence may be one of a set of near orthogonal sequences or a time shifted version of such a sequence. The chosen sequence may represent the identity of the secondary station and/or the number of a message received and/or coded reply as shown below.

12 Code Sequence 1 Code Sequence 2 Code Sequence 3 Code Sequence 4 Code Sequence 5 Code Sequence 6 secondary station in the area for the purposes of registration only. Received last message. Read message (s). Answer "Yes" Answer "No" Resend last message.

As an alternative to allocating sets of predetermined code sequences to secondary stations allocated to respective frames, the paging system controller and the secondary stations may each store the same block of code sequences, say 1000 code sequences. When a data message is to be transmitted to an addressed secondary station the system controller anticipates that one of say 20 possible answers may be possible and the overall transmission of the data messages includes an indication that twenty of the 1000 possible code sequences have been allocated dynamically to the secondary station for use in transmitting its answer, each code sequence representing one of twenty possible answers. Once a response to an invitation signal has been received and relayed to the system controller it is compared with each of the twenty dynamically allocated code sequences and the code sequence which achieves the best correlation is deemed to give the answer to the message. Once the answer has been determined the allocation of the twenty code sequences for an answer from that secondary station is withdrawn, either explicitly or implicitly.

13 Figure 4 is a timing diagram illustrating an example of a sequence of invitation signals INV1 to INV3 alternating with response signals RES1 to RES3 and code search routines SCH. In Figure 4 messages (not shown) already have been transmitted on the downlink. A first invitation INV1 is transmitted on the downlink. The secondary stations which have detected a message addressed to them respond to the invitation signal INV1 by transmitting a code sequence within a defined time slot RESI. A search routine SCH is initiated following expiry of the time slot. In the search routine, codes stored in the store 34 (Figure 2) are successively compared with the response code sequences and one by one the responses to particular ones of the messages are identified. However, due to the near/far problem only the strongest of the response signals are detected and these are eliminated from the next search by acknowledgement signals being transmitted on a downlink to inform those pagers which have been successful not to respond to the subsequent invitation signals INV2 and INV3 in the sequence.

It is anticipated that in a practical system the majority of the secondary stations SS (Figure 1) will be some distance from the antenna of the transceiver 12 which means that the response signals will have a low power at the antenna. Accordingly, although the durations of the time slots RES1, RES2 and RES3 may be equal, as shown, it is preferable that variable slot lengths be allocated according to the anticipated number of responses, for example a low number of relatively high powered responses and a high number of relatively low 14 powered responses. Short slots are allocated initially so that the few, strong powers contending against low noise and interference can be eliminated efficiently. Longer slots are then allocated to accommodate the weak received powers contending against significant levels of noise and interference.

If desired, the secondary stations SS may have power control on their transmitters in order to vary the strength of their response signals and in so doing reduce the number of invitation/response cycles. Also the population of pagers may be subdivided.

one method of issuing an invitation message whilst simultaneously informing those pagers whose responses have been analysed successfully is to send the messages is M1 to M14 in an ordered sequence as shown in Figure SA and in the invitation signal, Figure 5B, providing a field FD with a corresponding number of time slots on a 1 to 1 basis, thus slot 3 corresponds to message M3. When the first invitation signal INV1 is transmitted, say all the bits in the field FD are zero indicating that no responses have been received. However, after the first round of analysis, acknowledgements are transmitted to say the pagers to which the messages Mi, M3, M4, M9, Mil and M12 were addressed by changing the bits in slots 1, 3, 4, 9, 11 and 12 of the f ield FD f rom 110 11 to IT 111. Further bits are changed as more of the messages are acknowledged.

The number of cycles in which invitations are transmitted may be fixed. However, if it is determined that the number of successfully decoded responses exceeds is a statistically determined threshold value, then further iterations are stopped.

Figure 6 is a flow chart showing the sequence of operations involved in transmitting messages, receiving responses and acknowledging responses. Block 70 represents start. Block 71 relates to the transmission of a sequence of messages and this is followed in block 72 by the transmission of an invitation signal. Block 73 relates to the reception of the spread spectrum responses which are then analysed and matched with their respective messages in block 74. Block 75 relates to the transmission of the acknowledgements. In block 76 a check is made to see if the number of successful responses exceeds a threshold value indicating that as many as possible responses have been received. If the answer is No(N) the flow chart proceeds to block 77 in which a check is made to see if the predetermined maximum number of invitations has been exceeded. If the answer is No(N) the flow chart reverts to the block 72. A Yes(Y) answer from the blocks 76 and 77 causes the flow chart to revert to the block 71 and the cycle is repeated.

In accordance with the present invention a progressive elimination process is used to recover the responses identified by the PN codes in the spread spectrum signals transmitted by the secondary stations. Referring to Figure 7, the spread spectrum signals are received by the receiver section of the transceiver 12 which provides quadrature related frequency down-converted I and Q signals which are applied to an Analogue to Digital Converter (ADC) 80 which provides digitised versions of the raw data which are stored in a 16 raw sample store 82. The raw data R is read-out and supplied to a despreading stage comprising a multiplier 84 having a second input connected to the PN code store 34 (Figure 2). The code store supplies a predetermined PN Code Si which is multiplied with the raw data R to produce a despread signal Ni and products W2 + W3 + W4... Wh, where Wh = S1 o Sh and h = 2, 3, 4.. n. After despreading, the signal is cut into segments of the size of a Fast Fourier Transform stage (FFT)86 and the FFT is calculated on each segment, and its magnitude is averaged over the number of segments. The FFTs are stored in bins of a store 88. The largest peak in the spectrum is compared to a threshold to detect the presence of a given sequence. If the sequence S1 is present, indicated by the is largest peak exceeding the threshold, the bin with the largest peak and its neighbouring bins on each side or the neighbouring bins in the case of the maximum energy being shared between 2 bins are attenuated to zero or excised in stage 90. An output 91 is sent to the processor 26 which recovers the response, for example "Yes", associated with that PN code sequence and relays it to the originator of the message. The F7T signal is applied to an inverse FFT stage (FFT) and the output (W2 + W3 + W4.. Wn) is applied to another multiplier 94 in which it is spread again with the detected sequence S1. The output from the multiplier 94 comprises the original signal R from which the component corresponding to the PN code sequence Si has been removed, that is R - S1 = S2 + S3 + S4... Sn.

This signal is applied to the multiplier 84 to be multiplied by another PN code sequence. say S2.

17 In the event of a PN Code sequence read-out of the PN code store 34 not being present in the raw data, there will not be a peak in the output of the FFT stage 86. As a consequence the operation reverts to another PN code sequence being read-out of the store 34 and being applied to the multiplier 84. The sequence of operations carried out in the stages 84 to 94 is repeated until all the PN code sequences have been detected or the remaining signal is deemed to be unintelligible.

If it is estimated that insufficient responses have been received then as indicated by Figure 6, another invitation signal is transmitted. When analysing the subsequent response signals, the PN code sequences which have already been identified are not reused.

is Figure 8 is a flow-chart of the process for analysing raw data off-line. Block 100 relates to receiving and demodulating responses transmitted by the secondary stations. Block 102 relates to the raw data, or data recycled from a previous iteration, being despread using a PN code sequence read-out from the code store 34 (Figure 2). Block 104 relates to transforming the despread data using a FFT and in block 106 storing the FFT output. Block 108 relates to checking if there is a peak in the spectrum which exceeds the threshold. If the answer is No (N) the flow chart reverts to the block 102, but if the answer is Yes (Y) then in block 110 the bin and/or the neighbouring bins having the peak is/are attenuated or reduced to zero and an output is provided which is used by the primary station to provide the response associated with the particular PN code sequence. In block 112 the remaining spectrum undergoes an inverse 18 transform to that in the block 106, that is FFT-1. Block 114 denotes despreading the transformed signal by multiplying the signal by the PN code sequence used to despread the raw or recycled data. Block 116 relates to checking if the spread signal is considered intelligible. If the answer is Yes(Y) the flow chart reverts to the block 102, but if the answer is No (N) then in block 118 the primary station transmits either another invitation signal or a new block of messages.

Refinements can be made to the analysing process described with reference to Figure 8 to take into account that stronger signals may make weaker signals difficult to detect.

In a first refinement the process comprises two or is more iterative cycles, where a cycle is defined as a plurality of operations on the spread spectrum signal as received or remaining after a search made by sequentially cycling the PN codes which have not been detected in a previous cycle. In so doing the stronger spread spectrum responses are detected and eliminated. In the next cycle the less strong responses are detected and so on.

As an illustration of this refinement assume that there are five sequences S1 to SS present in a spread spectrum signal to be analysed. In a first cycle, sequences S1 and S3 are found and their associated bins are excised. However sequences S2, S4 and SS are not detected. In the next iterative cycle S2 is found and its bins are excised, leaving S4 and S5 undetected. In the third iterative cycle SS is found and its bins are excised, leaving S4 undetected. In a fourth and final iterative cycle, S4 is found and its bins are excised.

19 In a second refinement which is a variant of the first refinement, the sequences S1 to SS are searched for in a decreasing order of magnitude beginning with the strongest first. During the first search all sequences are searched for and the resulting peaks of undetected signals are sorted in decreasing order of magnitude thereby determining the order of search for the next cycle(s).

The first and second refinements ensure that the same piece of received signal is fully exploited.

Although the illustrated and described embodiment of the present invention uses FFT and FFT-1 transforms, other suitable transforms may be used such as the Walsh-Hadamard transform.

Although the present invention has been described with reference to a digital paging system the present invention may also, for example, be used in a cellular or cordless telephone system having provision for two- way paging or two way data transfer systems such as remote meter reading applications and is also applicable to CDMA (Code Division Multiple Access) communications systems.

Further the signals transmitted on the uplink may also comprise requestsfor services, such as registration, and accordingly the present invention is equally applicable to processing such requests for services in the same way as a response with the exception that there will be no match with an outgoing message.

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 design, manufacture and use of message transmission systems or component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or is combinations of such features during the prosecution of the present application or of any further application derived therefrom.

21

Claims (16)

1. A method of distinguishing a signal from a plurality of simultaneously received spread spectrum signals, each encoded with a different code, the method comprising the steps of:

i) multiplying the received signals with a first code, ii) analysing the products of multiplication by transforming the products and checking the result for an indication of a signal which had been encoded with the first code, if an indication is present, iii) reducing the amplitude of the indication in the transformed output, iv) regenerating the remaining products of multiplication using an inverse of the transform, v) multiplying the output by the first code to produce a regenerated received signal omitting substantially the first code, and vi) repeating the sequence of operations eliminating signals with other codes, present in the received signals.

2. The method of distinguishing a signal as claimed in claim 1, wherein stronger spread spectrum signals are eliminated before weaker signals.

3. The method of distinguishing a signal as claimed in claim 1 or claim 2, wherein the spread spectrum signals are 22 arranged in decreasing order of magnitude and the strongest spread spectrum signals are eliminated first.

4. The method of distinguishing a signal as claimed in claim 3, wherein during the first pass through said steps, peaks of undetected signals are sorted in decreasing order of magnitude.

5. The method of distinguishing a signal as claimed in any one of the preceding claims, wherein the products of multiplication are transformed using a Fast Fourier Transform (FFT) and the remaining products of multiplication are retransformed using an inverse Fast Fourier Transform (FFT-1).

is

6. A two way communications system comprising: a primary station, and a plurality of secondary stations, the primary station having means for transmitting a plurality of point-to-point downlink signals to preselected ones of the secondary stations, said secondary stations having means for transmitting uplink signals as substantially simultaneous spread spectrum signals encoded using a respective code, wherein the primary station has means for transmitting an invitation signal inviting the preselected ones of the secondary stations to transmit their uplink signals, and means for recovering a selected uplink signal from the substantially simultaneously received signals, 23 said recovering means comprising multiplying means for multiplying the received signals with a first code, means for analysing the products of multiplication by transforming said products and checking the result for an indication of a signal having the first code, means responsive to an indication being present for reducing the amplitude of the indication in the transformed output, means for regenerating the remaining products of multiplication using an inverse of the transform, means for multiplying the output by the first code to produce a regenerated received signal omitting substantially the first code, and means repeating the sequence of operations eliminating signals with other codes, present in the received signals.

is

7. The communications system as claimed in claim 6, wherein the recovery means comprises means for eliminating spread spectrum signals in at least two cycles.

8. The communications system as claimed in claim 6 or claim 7, further comprising means for determining the magnitudes of undetected spread spectrum signals and for eliminating the respective spread spectrum signals in a decreasing order of magnitudes.

24

9. The communications system as claimed in any one of claims 6 to 8, wherein the transform is a Fast Fourier Transform (FFT) and the inverse transform is an inverse Fast Fourier Transform (FFT-1).

10. A primary station for use in a two way communications system including the primary station and a plurality of secondary stations, said secondary stations having means for transmitting uplink signals as substantially simultaneously received spread spectrum signals encoded using a respective code, the primary station comprising:

means for transmitting a plurality of point-to-point downlink signals to preselected ones of the secondary stations, is means for transmitting an invitation signal inviting the preselected ones of the secondary stations to transmit their uplink signals, means for recovering a respective uplink signal from the substantially simultaneously received signals, said recovery means comprising multiplying means for multiplying the received signals with a first code, means for analysing the products of multiplication by transforming said products and checking the result for an indication of a signal which has been encoded with the first code, means responsive to an indication being present for reducing the amplitude of the indication in the transformed output, means for regenerating the remaining products of multiplication using an inverse of the transform, means for multiplying the output by the first code to produce a regenerated received signal omitting substantially the first code, and means repeating the sequence of operations eliminating signals with other codes, present in the received signals.

11. A primary station as claimed in claim 10, wherein the recovery means comprises means for eliminating spread spectrum signals in at least two cycles.

is

12. A primary station as claimed in claim 10 or claim 11, further comprising means for determining the magnitudes of undetected spread spectrum signals and for eliminating the respective spread spectrum signals in a decreasing order of magnitudes.

13. A primary station as claimed in any one of claims 10 to 12, wherein the transform is a Fast Fourier Transform (FFT) and the inverse transform is an inverse Fast Fourier Transform (FFT- 1).

14. A method of distinguishing a signal from a plurality of substantially simultaneously received spread spectrum signals, substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.

15. A two way communications system constructed and arranged to operate substantially as hereinbefore described 26 with reference to and as illustrated by the accompanying drawings.

16. A primary station constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.