BACKGROUND OF THE INVENTION 
     The present invention relates to a radio communication system having a random access channel for transmissions from a secondary station to a primary station, and further relates to primary and secondary stations for use in such a system and to a method of operating such a system. While the present specification describes a system with particular reference to the emerging Universal Mobile Telecommunication System (UMTS), it is to be understood that the techniques described are equally applicable to use in other mobile radio systems. In this specification the term random access packet channel refers to the logical channel on which random access packet transmissions take place, which would typically consist of a number of distinct physical channels. 
     A random access channel is a normal component of a radio communication system, enabling a Mobile Station (MS) to send short messages to a Base Station (BS). Applications include signalling to the BS when the MS is turned on, sending a packet of data to the BS when the MS may not be engaged in a call, and requesting the BS to allocate a resource for the MS to use. 
     In a system where mobile stations often have a requirement to send packets of data to the BS when not actually engaged in a call it is advantageous to provide a random access packet channel with similar characteristics to a standard random access channel but intended for the transmission of small and medium sized packets from a MS to the BS. 
     In an embodiment of a such a scheme developed for UMTS, there are a number of random access packet channels available to a MS. A request for access to a packet channel resource sent by the MS is encoded with one of 16 available signatures. Each signature corresponds to a request for a particular resource required by the MS, for example a channel having a particular bit rate. If a suitable resource is available for use, the BS allocates it to the requesting MS. 
     A problem with such a scheme is the limited amount of information that can be transferred during the access procedure, because of the limited number of signatures and channels available. This problem reduces the efficiency of resource allocation in the system. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a random access packet channel having improved resource allocation. 
     According to a first aspect of the present invention there is provided a radio communication system having a random access channel for transmissions from a secondary station to a primary station, the secondary station having means for requesting allocation of a random access channel resource by transmission of an access preamble encoded with a first signature selected from a first set of signatures and the primary station having means for receiving the access preamble, for determining the first signature and for transmitting an access acknowledgement encoded with a second signature selected from a second set of signatures indicating whether the requested resource is available, wherein the choice of first signature provides further information regarding the resource allocation request and wherein each signature of the second set corresponds to a plurality of signatures in the first set. 
     According to a second aspect of the present invention there is provided a primary station for use in a radio communication system having a random access channel for transmissions from a secondary station to the primary station, wherein means are provided for receiving an access preamble transmitted by the secondary station, which preamble is encoded with a first signature selected from a first set of signatures, for determining the first signature, for determining from the access preamble which random access channel resource the secondary station requests to be allocated and for transmitting an access acknowledgement encoded with a second signature selected from a second set of signatures indicating whether the requested resource is available, wherein the choice of first signature provides further information regarding the resource allocation request and wherein each signature of the second set corresponds to a plurality of signatures in the first set. 
     According to a third aspect of the present invention there is provided a secondary station for use in a radio communication system having a random access channel for transmissions to a primary station, wherein means are provided for requesting allocation of a random access channel resource by transmission of an access preamble encoded with a first signature selected from a first set of signatures and for receiving an access acknowledgement encoded with a second signature selected from a second set of signatures indicating whether the requested resource is available, wherein the choice of first signature provides further information regarding the resource allocation request and wherein each signature of the second set corresponds to a plurality of signatures in the first set. 
     According to a fourth aspect of the present invention there is provided a method of operating a radio communication system having a random access channel for transmissions from a secondary station to a primary station, the method comprising the secondary station requesting allocation of a random access channel resource by transmission of an access preamble encoded with a first signature selected from a first set of signatures and the primary station receiving the access preamble, determining the first signature and transmitting an access acknowledgement encoded with a second signature selected from a second set of signatures indicating whether the requested resource is available, wherein the choice of first signature provides further information regarding the resource allocation request and wherein each signature of the second set corresponds to a plurality of signatures in the first set. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a block schematic diagram of a radio communication system; 
     FIG. 2 illustrates a basic random access channel scheme; 
     FIG. 3 illustrates an enhanced random access channel scheme in which the transmission time of preamble and contention resolution preambles may be offset; and 
     FIG. 4 is a flow chart illustrating a method in accordance with the present invention for attempting to access a random access packet channel using timing offsets. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawings the same reference numerals have been used to indicate corresponding features. 
     Referring to FIG. 1, a radio communication system comprises a primary station (BS)  100  and a plurality of secondary stations (MS)  110 . The BS  100  comprises a microcontroller (μC)  102 , transceiver means (Tx/Rx)  104  connected to antenna means  106 , power control means (PC)  107  for altering the transmitted power level, and connection means  108  for connection to the PSTN or other suitable network. Each MS  110  comprises a microcontroller (μC)  112 , transceiver means (Tx/Rx)  114  connected to antenna means  116 , and power control means (PC)  118  for altering the transmitted power level. Communication from BS  100  to MS  110  takes place on a downlink channel  122 , while communication from MS  110  to BS  100  takes place on an uplink channel  124 . 
     A basic scheme for a random access packet channel operating in a frequency division duplex system is shown in FIG. 2, with the uplink channel  124  drawn above the downlink channel  122 . In an access phase, the MS  110  first transmits a preamble (P)  202  encoded with one of 16 possible signatures at a low power level in a particular access slot. A signature is a signal characterised by its scrambling code and channelisation code modulated by a specific bit sequence. A mutually orthogonal set of signatures can be obtained by defining a set of mutually orthogonal bit sequences for the modulation. Hence, a different set of signatures can be obtained by changing the scrambling code or the channelisation code (i.e. the physical channel), or by using a different mutually orthogonal set of bit sequences. In the basic system shown in FIG. 2 there is a one to one mapping between the preamble signature and the acknowledgement ( 206 ), such that receipt of an acknowledgement by the MS  110  uniquely determines which signature is being acknowledged. Although the present specification refers to sets of 16 signatures, different implementations may use sets having different numbers of signatures. 
     Each of the available signatures is mapped to a single bit rate for the packet channel. The set of available signatures, and mappings between signatures and bit rates, may be predetermined or may be regularly broadcast by the BS  100 . The MS  110  selects a signature for encoding the preamble  202  corresponding to its required bit rate. If there is more than one signature available corresponding to the required bit rate, the MS  110  selects one at random. If the BS  100  receives and decodes the preamble correctly it transmits a preamble acknowledgement (A)  206 . In the example shown in FIG. 2, after the first preamble  202  is transmitted no acknowledgement is returned in the slot  204  allocated for it (which might for example be 1 ms in length). The MS  110  therefore transmits another preamble  202  at a higher power level. This is received and decoded by the BS  100 , which transmits an acknowledgement  206  and thereby completes the access phase. 
     As well as informing the MS  110  that its preamble  202  has been received, the acknowledgement  206  may be positive, to signal that the requested resource is available, or negative, to signal that it is in use and access is denied to the MS  110 . A negative acknowledgement (NACK) may be indicated by the BS  100  inverting the phase of the signature (with respect to some reference or pilot signal). Alternatively, some of the signatures used by the BS  100  for acknowledgement may also be used as a NACK. 
     The BS  100  will only transmit one acknowledgement for each access slot, however many preambles  202  were transmitted. One basis for the selection could be to acknowledge the preamble  202  received with the highest power. The initial power level at which a MS  110  transmits the preamble  202  is typically determined by the MS  110  using open loop power control, so that a MS  110  is not at a disadvantage compared to another MS  110  nearer to the BS  100 . If more than one preamble  202  was transmitted but each preamble was encoded with a different signature then each MS  110  will know whether or not its preamble  202  was received correctly. However, it is possible that more than one MS  110  selected the same signature, and therefore believes that its preamble  202  has been received. If each of these mobile stations  110  begins to transmit its data the result will be a collision, with none of the data likely to be received correctly. 
     To reduce the chances of this happening, a contention resolution phase may follow the transmission of an acknowledgement  206  which indicated that the requested resource was available. Each MS  110  which transmitted a preamble  202  encoded with a signature corresponding to that acknowledged by the BS  100  now transmits a further contention resolution preamble (CP)  208 . This preamble  208  is encoded with a signature randomly selected from another set of 16 possible signatures. This set may be different from the set used for the access preamble  202  (either by changing the set of modulating bit sequences, the scrambling code or the channelisation code), or alternatively the set of signatures may be shared between access and contention resolution phases. The BS  100  then issues a contention resolution acknowledgement (CA)  210  corresponding to the selected preamble  208 , for example that received with the highest power, which acknowledgement  210  enables the MS  110  to transmit its data. Hence, if more than one MS  110  selected the same access preamble  202  the chance of the same contention resolution preamble  208  also being selected is small. A channel assignment message may optionally be transmitted at substantially the same time as the contention resolution acknowledgement  210 , or even as part of the acknowledgement  210 , as disclosed in our co-pending International patent application PCT/EP00/06988 (our reference PHGB 000003). 
     After this contention resolution phase the BS  100  begins transmission of a Physical Control CHannel (PCCH)  212 , which includes power control information to instruct the MS  110  to adjust its transmission power as necessary, and the MS  110  transmits one or more data packets (PKT)  214  on the allocated packet channel, which is normally on a different physical channel to those used for the preamble transmissions. The PCCH  212  may begin simultaneously with the transmission of the data  214 , or may precede it sufficiently for closed loop power control to be established before the data transmission. 
     A particular problem with the basic scheme described above is that the efficiency of resource allocation is limited by the number of choices available for the access preamble  202 . In an embodiment of the scheme for UMTS, for example, there are 16 signatures and 12 random access sub-channels available, giving a total of 196 degrees of freedom. 
     This problem is solved in a system made in accordance with the present invention by defining a larger set of signatures, which may have low cross correlations rather than strict orthogonality. Such a set of signatures could be defined by modifying the bit sequences used to define the signatures or by using different scrambling codes, or by a combination of both techniques. In order to avoid the need to allocate a larger number of downlink channelisation codes, the same acknowledgement ( 206 ) in FIG. 2 may be transmitted in response to any one of a defined set of preamble signatures. This may increase the probability of collisions, but typically this would be an acceptable drawback compared the other advantages of the system. 
     The availability of many more degrees of freedom enables significantly more efficient resource allocation. For example, in the case of channel assignment it enables there to be more bit rates. 
     A system made in accordance with the present invention can make use of the much greater number of degrees of freedom in a number of ways. Possible assignments for each combination of signature, channel and (non-zero) timing offset include the following (either singly or in combination): 
     as a request for a particular bit rate; 
     as a request for a particular access class (with associated priority); 
     as a request for additional resources (e.g. a downlink shared channel); 
     for indicating the priority of the message. 
     for indicating the length (or minimum or maximum length) of the packet to be sent; 
     for use by a specific MS  110 ; and 
     for use by MSs  110  using one of a set of common higher layer messages. 
     Some of these assignments could reduce the quantity of higher layer information required to be included in the data packet  214 . For example, if a particular MS  110  is assigned the exclusive use of a subset of the possible uplink signals, its identity is automatically known by the BS  100  and need not be included in the data packet  214 . Such a reduction in overhead on packet transmission could be particularly useful for low data rate applications. The same principle could be applied to other commonly-used items of higher layer information. Other assignments could improve system responsiveness. For example, indicating the minimum length of a packet to be sent enables the BS  100  to send an ARQ (Automatic Repeat reQuest) to the MS  110  if it does not receive at least the indicated minimum length. Indicating the maximum length of a packet may enable the BS  100  to improve its planned resource allocation. 
     FIG. 3 illustrates how the set of possible signatures can be extended by introducing different time offsets. The time of transmission of the access preamble  202  may be offset by T off  with respect to the boundary  302  of the access slot (which is itself defined relative to timing signals transmitted by the BS  100 ). The access preamble  202  and contention resolution preamble  208  both comprise 4096 chips, while the length of the access slot is 5120 chips. By allowing timing offsets T off  of multiples of 256 chips, up to 19 different non-zero values of T off  are possible without introducing ambiguity about which slot contained the preamble  202 ,  208 . When T off =0 the behaviour of the system is identical to that of a system without the possibility of timing offsets. 
     The timing offset T off  preferably advances the transmission time of the preambles  202 ,  208 , since any delay in its transmission might mean that the BS  100  is unable to detect the preamble in time to transmit an access acknowledgement  206  in an appropriate time slot. 
     The timing offset for the contention resolution preamble  208  is preferably the same as that for the access preamble  202 , so that the BS  100  can readily identify the two preambles  202 ,  208  as coming from the same MS  110 . 
     It is preferable for there to be no timing offsets in the downlink channels  122 , so that the acknowledgement  206  of an access preamble  202  is the same irrespective of the timing offset used. This enables collisions between offset and non-offset transmissions to be resolved during the contention resolution phase. The use of timing offsets has the further advantage of increasing the number of different signals available for use during the contention resolution phase, thereby reducing the probability of unresolved collisions. 
     As a variation on the above scheme, the preamble signature could act purely as an identifier for the access attempt, while the information relating to the resource allocation request could be carried by the timing offset and sub-channel selected by the MS  110 . 
     A flow chart summarising a method in accordance with the present invention, including the use of time offsets, for a MS  110  attempting to access a packet channel resource is shown in FIG.  4 . The method starts, at step  402 , when the MS  110  has data for transmission on the random access packet channel. The MS  110  firstly, at step  404 , selects a signature corresponding to the required resource, for example the required bit rate. Next the MS  110 , at step  406 , determines an appropriate timing offset T off , for example to indicate its identity or the urgency of the data for transmission. Having selected these parameters, the MS  110 , at step  408 , attempts to access the packet channel by transmitting one or more access preambles  202  and a contention resolution preamble  208 , as described above. 
     In alternative embodiments, not all the features shown in FIG. 3 may be needed. For example, inner loop uplink power control may not be essential, so the PCCH  212  may not be present. The collision resolution part may not be essential, in which case the contention resolution preamble  208  and acknowledgement  210  messages may not be present. The consequences of these changes would be simpler implementation, but at the cost of poorer Eb/No performance and increased collision probability of message parts. In some cases, all the required signalling information may be conveyed by the choice of access sub-channel and signature, in which case the message part would not be needed and could be omitted. As an alternative to selecting the bit rate by reference to the preamble signature, it may be determined by the MS  110  and indicated by signalling in the message part  214 . 
     As well as its application in a FDD system as described above, the present invention could be applied in other types of communication system. For example, it could be used in a Time Division Multiple Access (TDMA) system provided that the uplink transmissions take place in different time slots to the downlink transmissions. 
     The embodiments described above relate to packet transmission. However, the same principles can equally well be applied to a system in which circuits are set up for data transmission. 
     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 radio communication systems and 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 combinations of features during the prosecution of the present application or of any further application derived therefrom. 
     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.
A radio communication system has a random access channel for the transmission of data ( 214) from a secondary station to a primary station. Such a channel is intended for use by secondary stations having data ( 214) to transmit to a primary station while not actually engaged in a call. By enabling access requests ( 202) to be transmitted with a greater range of possible signatures, a much greater number of degrees of freedom is available to a secondary station requesting access to a random access channel. This enables significantly improved efficiency of resource allocation by increasing the amount of information transmitted to the primary station by the access request ( 202).