GB2391747A

GB2391747A - Determining the number of access messages in a signal

Info

Publication number: GB2391747A
Application number: GB0217758A
Authority: GB
Inventors: Suresh Sharma; Christopher Smart; Vagan Shakhgidian
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2002-07-31
Filing date: 2002-07-31
Publication date: 2004-02-11
Anticipated expiration: 2022-07-31
Also published as: GB0217758D0; GB2391747B

Abstract

In a multiple access cellular communication system, a base station comprises a receiver able to receive access messages from users wishing to initiates calls. In the invention, a signal comprising at least one access message is received and a signal component processor determines a plurality of signal components 401-415 of the signal. The signal components arise from multipath propagation. Arrival times are determined for each of the components 401-415, preferably as relative time intervals relative to the beginning of a time slot or the arrival of a given component. A maximum delay interval 417, 419, relating to the time interval over which multipath propagation may spread a single access message, is determined and in response to the maximum delay interval 417, 419 and the arrival times, a determination is made of at least a minimum number of access messages in the signal. For example, fig. 4 shows that two maximum delay intervals 417, 419 are required to accommodate the arrival times of the components 401-415, indicating at least two access messages. The determination may involve further factors and component groupings. Demodulation resource appropriate for the determined number of access messages may then be allocated. Use in cellular systems, especially UMTS, to improve detection of initial access messages/preambles which collide or overlap, e.g. access messages having identical Walsh code signatures which interfere in RACH using slotted mode ALOHA protocol.

Description

239 1 747

AN APPARATUS AND METHOD FOR DETECTING MULTIPLE

ACCESS SIGNALS IN A CELLULAR COMMUNICATION SYSTEM

Field of the invention

The invention relates to an apparatus and method for detecting multiple access signals in a cellular communication system and in particular in a UMTS cellular communication system.

Background of the Invention

FIG. 1 illustrates the principle of a conventional cellular communication 15 system 100 in accordance with prior art. A geographical region is divided

into a number of cells 101, 103, 105, 107 each of which is served by base station 109, 1 11' 113, 115. The base stations are interconnected by a fixed network which can communicate data between the base stations 101, 103, 105, 107. A mobile station is served via a radio communication link by the 20 base station of the cell within which the mobile station is situated. In the example if FIG. 1, mobile station 117 is served by base station 109 over radio link 119, mobile station 121 is served by base station 111 over radio link 123 and so on.

25 As a mobile station moves, it may move from the coverage of one base station to the coverage of another, i.e. from one cell to another. For example mobile station 125 is initially served by base station 113 over radio link 127. As it moves towards base station 115 it enters a region of overlapping coverage of the two base stations 111 and 113 and within this 30 overlap region it changes to be supported by base station 115 over radio link 129. As the mobile station 125 moves further into cell 107, it

r continues to be supported by base station 115. This is known as a handover or handoff of a mobile station between cells.

A typical cellular communication system extends coverage over typically 5 an entire country and comprises hundred or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile station to a base station is known as unlink, and communication from a base station to a mobile station is known as downlink. The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition the fixed network comprises gateway functions for interconnecting to external 15 networks such as the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for 20 routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc. Currently the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile 25 communication (GSM). GSM uses a technology known as Time Division Multiple Access (TDMA) wherein user separation is achieved by dividing frequency carriers into 8 discrete time slots, which individually can be allocated to a user. A base station may be allocated a single carrier or a multiple of carriers. One carrier is used for a pilot signal which further 30 contains broadcast information. This carrier is used by mobile stations for measuring of the signal level of transmissions from different base stations, and the obtained information is used for determining a suitable serving

cell during initial access or hangovers. Further description of the GSM

TDMA communication system can be found in 'The GSM System for Mobile Communications' by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.

Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) wherein user separation is obtained by allocating to different spreading and scrambling codes to different users on the same carrier frequency. The transmissions are spread by multiplication with the allocated codes thereby causing the signal to be spread over a wide bandwidth. At the receiver, the codes are used to de-spread the received signal thereby regenerating the original signal. Each base station has a l 5 code dedicated for a pilot and broadcast signal, and as for GSM this is used for measurements of multiple cells in order to determine a serving cell. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed. Further description of CDMA and specifically of the

20 Wideband CDMA (WCDMA) mode of UMTS can be found in 'WCDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.

When a mobile station initiates a call in UMTS or GSM, it transmits an 25 initial access message to a selected base station. This access message is known as a RACH (Random Access Channel) message. In UMTS, the RACH access mechanism uses a slotted ALOHA protocol wherein the channel is divided into discrete time slots that can be used for accessing the base station. The base station broadcasts timing information that the 30 mobile station uses to synchronize to the time slots of the RACH channel.

The mobile station transmits the RACH message by choosing a RACH

time slot at random, and transmitting the RACH message in this time slot. In UMTS, an initial RACH message known as a preamble is transmitted 5 in the RACH time slot. If the preamble is successfully received at the base station, the base station transmits an acknowledgement back to the mobile station, which in return transmits a second longer RACH information message. The RACH information message includes information related to the service requested, the destination address and 10 further information required for setting up a link. If the mobile station does not receive an acknowledgement from the base station within a given time, it retransmits the preamble in a new RACH time slot with an increased power level. The increased power level increases the probability of successful reception and a successful acknowledgement. The mobile l 5 station repeats the transmission of the preamble at increasing power levels until a positive acknowledgement is received or a pre-defined retry limit is reached.

In order to increase the capacity of the RACH channel, the preamble 20 comprises a given signature in the form of a Walsh scrambling code used in the spreading process for the signal. This signature allows the base station receiver to differentiate between preambles of different mobile stations. Further, the signature acts as an identification of the mobile station transmitting the preamble, and is used in the acknowledgement 25 from the base station to identifier which preamble is acknowledged. In UMTS 16 different signatures can be used for the preamble, and upon generation of the preamble the mobile station selects one of these pre-

defined signatures at random. If the mobile station within a given time interval detects an acknowledgement on the acquisition indication channel 30 (AICH) of a preamble received in the given RACH time slot and with the selected signature, it will determine that the pre-amble has been

successfully received, and it will proceed to transmit the RACH information message.

When the base station receives the RACH information message, it 5 generates a data packet comprising the contained information and communicates it to a Radio Network Controller (RNC). The RNC is in charge of resource allocation for the air interface, and in response to the received information it proceeds to allocate communication resource to the originating mobile station. The RNC communicates the required to information back to the (or possibly another) base station, which in response proceeds to setup and configure the communication link with the mobile station.

Further description of the access mechanism for UMTS can be found in

l S Third Generation Partnership (3GPP) Technical Specification TS 25.214.

Since the mobile stations independently and pseudo-randomly selects the signature and the RACH time slot, collisions between preambles of different mobile stations occur regularly. These collisions typically lead to 20 the base station not being able to decode the received message and thus to the preamble not being acknowledged. If the base station successfully manages to decode the preamble, the acknowledgement will be received as a positive acknowledgement by both mobile stations causing them to transmit RACH information messages which will interfere with each other 25 and therefore typically cannot be decoded.

Conventionally, collisions are considered to be acceptable in communication systems such as UMTS, and slotted ALOHA collisions are typically resolved by the originating units retrying after a variable pseudo 30 random delay. However, collisions do result in a number of disadvantages including increased error rate for call setup, increased delay in call setup,

and reduction of the access capacity and thereby in the capacity of the communication system.

Accordingly, an improvement in the access mechanism for cellular 5 communication systems would be advantageous.

Summary of the Invention

10 The inventors of the current invention have realised that the conventional approach of allowing collisions to be resolved by the mobile stations repeating the access process at a later time is sub-optimal, and consequently the current invention seeks to improve the access mechanism in cellular communication systems.

Accordingly there is provided an apparatus for detecting multiple access signals in a cellular communication system having at least one base station supporting a plurality of users, the apparatus comprising: means for receiving a signal comprising at least one access message; means for 20 determining a plurality of signal components of the signal; means for determining arrival times of each of the plurality of signal components of the signal; means for determining a maximum delay interval for a single access message; and means for determining the number of access messages in the signal in response to the maximum delay interval and the 25 arrival times.

The apparatus thus detects the number of access messages in the signal thereby resolving conflicts from simultaneously received access messages and thus improving the access procedure resulting in increased probability 30 of successful call setup, reduced average delay and increased capacity of the communication system.

According to a first feature of the invention, the apparatus further comprises means for grouping the signal components into a plurality of groups each group corresponding to one access message. This provides the advantage of each signal component being related to a specific access 5 message and therefore allows separate processing of each access message.

According to a second feature of the invention, the apparatus further comprises means for deriving an information content of an access message from the signal components comprised in a group out of the plurality of 10 groups. Hence, the grouping of signal components into access messages is used to derive an information content of the access message thereby allowing information to be communicated as part of the access message and successfully received even in the case of an access message conflict.

15 According to a third feature of the invention, each group of the plurality of groups comprises the signal components having arrival times within a time interval determined in response to the maximum delay interval and preferably the time interval is substantially equal to the maximum delay interval. This provides for a simple but reliable method of grouping signals 20 into groups corresponding to access messages.

According to a fourth feature of the invention, the apparatus further comprises means for determining a minimum time distance between arrival times of signal components of a first access message and arrival 25 times of signal components of a second access message, and the means for determining the number of access messages is operable to further determine the number of access messages in response to the minimum time distance. This provides the advantage of reducing the probability of error caused by allocating signal components to the wrong groups, and 30 thus improves the probability of a group only containing signal components from the same origin.

According to a fifth feature of the invention, the means for determining the number of access messages is operable to determine the number of access messages by grouping the signal components having time of arrival signals within a time interval substantially equal to the maximum delay 5 interval and such that each group is separated by a time of arrival interval substantially equal to the minimum time distance. This provides a simple yet very reliable method for grouping signal components into groups corresponding to individual access messages.

10 According to a sixth feature of the invention, the time interval of a group is determined so as to have the largest accumulated signal energy for signal components in the interval. In this way, the probability of the time interval mainly including signal components from one origin is increased.

15 According to a seventh feature of the invention, the apparatus further comprises means for allocating a communication resource appropriate for a number of communications corresponding to the number of access messages and preferably the communication resource is a demodulation resource. This allows the required resource to be set up for the determined 20 number of access messages such that subsequent communication from all the originating sources can be supported.

According to an eight feature of the invention, the apparatus further comprises means for determining a signature of each access message and 25 the means for determining the number of access messages is operable to determine a number of access messages in the signal having the same signature. In many communication systems, individual access messages having the same signature cannot be individually acknowledged, and therefore determining all access messages having the same signature 30 allows for the apparatus to cope with all subsequent transmissions resulting from acknowledgement of a signature.

According to a seventh feature of the invention, the apparatus further comprises means for acknowledging receipt of the received access messages. This allows for a plurality of access messages received to be acknowledged. Thus, if more than one access message has been received, 5 this is determined and all the accessing sources may be acknowledged.

According to an eighth feature of the invention, an apparatus further comprises means for negatively acknowledging receipt of the received access messages. This provides the advantage of allowing the apparatus to 10 negatively acknowledge access messages, for example if there is not sufficient available resource for the number of access messages that have been detected.

Preferably the apparatus is comprised in a base station, and preferably 15 the cellular communication system is a UMTS cellular communication system. According to a second aspect of the invention, there is provided a method for detecting multiple access signals in a cellular communication system 20 having at least one base station supporting a plurality of users, the method comprising the steps of receiving a signal comprising at least one access message; determining a plurality of signal components of the signal; determining arrival times of each of the plurality of signal components of the signal; determining a maximum delay interval for a 25 single access message; and determining the number of access messages in the signal in response to the maximum delay interval and the arrival times. 30 Brief Description of the Drawings

An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which FIG. 1 is an illustration of a cellular communication system in accordance 5 with the prior art;

FIG. 2 is an illustration of a UMTS communication system comprising a base station in accordance with a preferred embodiment of the invention; 10 FIG. 3 illustrates a flow chart of a method of detecting multiple access signals in accordance with a preferred embodiment of the invention; and FIG. 4 illustrates an example of a distribution of received signal components for two access messages.

Detailed Description of a Preferred Embodiment of the Invention

A detailed description of a preferred embodiment of the invention is in the

20 following described with specific focus on a base station of a UMTS communication system. However, it will be clear that the invention is equally applicable to other communication systems and other communication elements.

25 FIG. 2 is an illustration of a UMTS communication system 200 comprising a base station in accordance with a preferred embodiment of the invention.

FIG. 2 illustrates two communication units 201, 203 communicating with a base station 205, in UMTS known as a Node B. over air interface links 30 207, 209. The communication units 201, 203 may typically be mobile stations, communication terminals, wireless devices, user equipment (UE), remote terminals, subscriber units etc. In the preferred embodiment, the

( communication units 201, 203 communicate with the base station 205 in accordance with the protocols and requirements as specified in the Technical Specifications of the UMTS communication system.

5 Specifically, the communication units 201, 203 initiate a call by transmitting an access message to the base station 205 on the BACH access channel using a slotted ALOHA protocol i.e. wherein the access channel is separated into separate time slots. In particular, the communication units 201, 203 initially transmit a preamble comprising an t O identifying signature and upon receiving acknowledgement of this signature from the base station 205 the communication units 201, 203 transmit an information message, as previously described.

The base station 205 comprises an antenna 211 connected to a duplexer l5 213, which is further connected to a transmitter 215 and a receiver 217.

The antenna 211 receives and transmits radio signals over the communication links 201, 203. The duplexes 213 isolates between the transmitter 215 and receiver 217 such that the same antenna can simultaneously be used for both receiving and transmitting.

The transmitter 215 is operable to transmit signals to the communication units 201, 203 in accordance with the UMTS specifications. The

transmissions from the base station 205 to the communication units 201, 203 comprise traffic data, control data and broadcast data and are 25 communicated on logical channels, including traffic channels, broadcast channels, control channels and access channels.

Similarly, the receiver 217 is operable to receive signals transmitted from the communication units 201, 203 in accordance with the UMTS 30 specifications. The transmissions from the communication units 201, 203

comprise both traffic data and control data (including access and configuration data), and the uplink communication therefore also

comprises logical channels including traffic channels, control channels and access channels.

Specifically, the receiver 217 is able to receive a signal comprising at least 5 one access message. In the preferred embodiment, the receiver will thus receive radio signals within each time slot of the access channel. As each of the plurality of communication units may transmit an access message in a time slot of the access channel, the radio signal may comprise none, one or more access messages from communication units.

The receiver 215 is connected to a signal component processor 219 for determining a plurality of signal components of the signal. As is well known for cellular communication systems, the radio propagation typically causes radio signals to travel between the communication units and the l 5 base station through a plurality of paths of different length. Since the propagation delay depends on the distance travelled, a received signal from one communication unit comprises a plurality of signal components where each of the signal components correspond to one path of the multi path propagation environment. Thus, the received signal comprises a 20 number of signal components depending on the propagation environment and on the number of communication units transmitting in the given time slot. The signal from a given communication unit is thus received at the base station as a number of multi path signal components over a given time interval that depends on the path length variation of the multi paths.

25 A statistical measure of the time interval over which signal components from one communication unit is received is known as the delay spread.

The signal component processor 219 is connected to an arrival time processor 221 for determining arrival times of each of the plurality of 30 signal components of the signal. Thus for each of the signal components that have been identified by the signal component processor 219, the arrival time processor 221 determines what the arrival time of that signal

component is. The arrival times are in the preferred embodiment determined as relative time intervals and thus reflect the relative delay between signal components. Any suitable reference for the arrival time may be used including e.g. the beginning of a time slot or the arrival time 5 of one of the signal components.

The arrival time processor 221 is connected to an access signal processor 223. The access signal processor 223 is further connected to a delay interval processor 225 for determining a maximum delay interval for a 10 single access message. The maximum delay interval is a measure related to the time interval over which signal components from a single communication unit may be spread. In a simple embodiment, the maximum delay interval is predetermined based on knowledge of typical propagation environments for cellular communication systems. In this is embodiment, the delay interval processor 225 may simply consist in a memory or even as a predefined variable in a software or firmware program executed in the access signal processor 225.

The access signal processor 223 is operable to determine the number of 20 access messages in the signal in response to the maximum delay interval and the arrival times the signal components of the signal. In a simple embodiment, the maximum delay interval is a predetermined value which corresponds to the maximum time over which signal components from one communication unit can be spread. Thus if signal components are received 25 having arrival times distributed over a larger time interval, they must have originated from more than one communication unit. In a very simple embodiment, the access signal processor simply determines the number of time intervals of a duration substantially equal to the maximum delay interval, which are required for all signal components to belong to one 30 time interval. The number of access messages is then given as the number of time intervals required. In more complex embodiments, more advanced

algorithms can be used to group signal components into groups where each group corresponds to one access message from one communication unit.

The access signal processor is connected to a receiver processor 227 which S performs all further functions required for the communication with the communication unit to be supported. Thus the receiver processor 227 performs functions including interfacing with the communication network infrastructure, and in particular it is connected to a Radio Network Controller (RNC) 229. It further performs the functions required for lO receiving and decoding information content of received signals, and specifically it comprises functionality required to (typically together with the receiver 217) determine the signature of the received access messages.

In addition, the receiver processor 227 contains functionality for controlling and configuring the receiver 217 and other circuitry of the base lS station 205 to enable it to perform the functions required for operation in accordance with the specifications for the communication system.

The receiver processor 227 thus completes the reception of the access messages, and in the preferred embodiment it uses the determination of 20 the access signal processor 223 to receive and decode all the access signals received in the given time slot of the access channel. Thus the receiver processor 227 combines all signal components determined to belong to a group corresponding to a single access message into one access message signal. The combined signal(s) are then demodulated and decoded as is 25 well known in the art.

The receiver processor 227 is further connected to the transmitter 215. If one or more access signals are detected by the receiver processor 227, this connection is used to control the transmitter 215 to transmit an 30 acknowledgement signal to the communication units 201, 203.

( 15 FIG. 3 illustrates a flow chart 300 of a method of detecting multiple access signals in accordance with a preferred embodiment of the invention. The method will be described with reference to the communication system 200 and base station 205 of FIG. 2.

In step 301, the receiver 217 receives a signal comprising at least one access message. In this step, the antenna signal is received for a time interval wherein one or more access messages may be received. In the preferred embodiment the antenna signal for a full time slot of the slotted 10 ALOHA access channel is filtered, down-converted, amplified, sampled and de-spread (apart from the signature codes) to provide a digital receive signal, which can be further processed. Thus in the preferred embodiment the subsequent steps are all performed by operations on a digital representation of the received signal. As such, the steps are preferably l 5 performed by a firmware program in a single suitable Digital Signal Processor (DSP) and thus the processor elements 219, 221, 223, 225 and 227 are all implemented as firmware in the same DSP.

In the preferred embodiment of a UMTS communication system, the 20 receiver 217 receives and digitises the antenna signal for a time interval equal to the duration of a time slot. As the communication units of a UMTS system transmits access messages pseudo randomly, the received antenna signal may comprise none, one or more preamble RACH messages. As the communication units further select a signature Walsh 25 code pseudo randomly, the digitised receive signal may comprise a plurality of preamble messages having different signature codes, or in some situations a plurality of preamble messages may comprise the same Walsh code. The digitised receive signal is in parallel de-spread by each of the 16 different signature codes to generate a de-spread signal. The 30 following steps describe the operation performed on each of the generated de-spread signals. As the Walsh codes have strong cross-correlation properties any signal component from access messages having a different

signature than the one used for the de-spreading is relatively insignificant, and is ignored in the following description. It will be

apparent to the person skilled in the art that for any embodiment not employing signature codes, the following steps can for example be 5 performed on the received digitized signal.

In step 303, the signal component processor 219 determines a plurality of signal components of the signal. It is well known from receivers operating in multi-path environments to determine a number of signal components 10corresponding to the received multi path components. Specifically, in the preferred embodiment, the base station 205 comprises a RAKE receiver, which as part of its operation performs the function of determining a number of signal components and tracking these. Hence, in the preferred embodiment, step 303 is part of the RAKE receiver process but is further l S used to determine a number of access messages in the received signal as described below. Specifically, the signal components can be determined from a correlation between a local replica of a known part of the received signal with different time offsets. In the preferred embodiment of the invention, the preamble does not contain any variable information apart 20 from the signature, and following the Walsh code de spreading the signal transmitted by the communication units is thus known. Hence, correlating the received signal with a local replica equivalent to the transmitted signal with varying time offsets will generate a number of correlation values. The magnitude of the correlation corresponds to the magnitude of 25 a signal component having the corresponding propagation delay, and the signal components are in the preferred embodiment simply determined as those resulting in a correlation value above a given threshold. However, any algorithm or method for determining a plurality of signal components may be used without detracting from the invention.

In step 305, the arrival time processor 221 determines the arrival times of each of the plurality of signal components. The time of arrival is in the

preferred embodiment simply determined as the relative time offsets between the signal components, as determined by the correlation process.

The time of arrival is in the preferred embodiment determined as a relative time of arrival. The relative time of arrival may be relative to the 5 time slot, e.g. to the beginning of the time slot, or to the arrival times of one or more of the other signal components, e.g. the first signal component, the largest signal component or the average time of arrival of all signal components.

10 In step 307, the delay interval processor 225 determines a maximum delay interval for a single access message. In the preferred embodiment, the maximum delay interval is predetermined based on knowledge about the typical propagation environment for the base station. Much research has been undertaken and published, and various models and values for delay 15 spreads in various propagation environments are well known in the art. In the preferred embodiment, the maximum delay interval is determined as the time interval beyond which a multi path signal has a probability of less than a given threshold of extending. A value of 30 1lsec is specifically suitable for a UMTS communication system. In slightly more complex 20 embodiments, different pre-defined values are stored in the base station for different environments such as rural or urban environments, and the appropriate value is chosen at installation of the base station depending on the environment in which the particular base station is installed.

25 In step 309, the access signal processor determines the number of access messages in the received signal in response to the maximum delay interval and the arrival times.

In a simple embodiment, it is first detected if any signal components have 30 been detected. If not, it is determined that the received signal contains no access messages and the method terminates for this time slot.

If one or more signal components are detected, it is investigated if all signal components have arrival times which can fit within a single time interval of a duration equal to the maximum delay interval. If so, it is determined that only one access message has been received with the given 5 signature, and the method continues in step 311.

If the signal components have arrival times which do not fit into a single time interval, it is investigated if the arrival times fit within trio separate time intervals. If so, it is determined that two access messages have been lO received, and the method continues in step 311. If unsuccessful, the process is repeated for increasing number of time intervals until all signal components fit into the time intervals.

Hence, the simple procedure provides a method of determining a minimum l5 number of received access messages. Thus, if the signal components spread beyond the maximum delay interval they are unlikely to originate at the same communication unit and therefore a minimum of two communication units must have transmitted access messages. Similarly, if the signal components do not fit within two maximum delay intervals, at 20 least three communication units must have transmitted access messages.

If two communication units are received such that their signal components are overlapping but together extend over more than one maximum delay interval, this simple embodiment will determine that two access messages 25 have been received. However, as they overlap it may not be possible to determine which signal components belong to which communication unit, and subsequent reception of further messages from these two communication units may thus be impaired.

30 Accordingly, in the preferred embodiment, the access signal processor 223 further determines a minimum time distance between arrival times of signal components of a first access message and arrival times of signal

( components of a second access message, and the determination of the number of access messages is further in response to minimum time distance. More specifically, the access signal processor 223 determines the number of access messages by grouping the signal components having 5 time of arrival signals within a time interval of size substantially equal to the maximum delay interval, and such that each group is separated by a time of arrival interval substantially equal to the minimum time distance.

In the preferred embodiment, the minimum time distance is set at a level that results in a suitable probability of ensuring that only signal lO components from the same communication unit are grouped together.

FIG. 4 illustrates an example of a distribution of received signal components for two access messages transmitted from two different communication units. In the example, eight signal components 401-415 15 have been received at different relative time of arrival estimates. The first four 401-407 fit within a single first time interval 417 of length equal to the maximum delay interval. The remaining four signal components 409-

415 do not fit within the first time interval 417, but all fit within a second time interval 419. In the example, the multi path spread 421 of the signal 20 received from the first communication unit is thus separated from the multi path spread 423 of the second communication unit by the distance 425. As can be seen from the figure, the first time interval can be located such that it is separated from the second time interval 419 by a value bigger than the threshold of the minimum user distance. Consequently, it 25 is considered that the signal components are grouped into groups corresponding to the originating communication units with sufficient reliability, and the probability of a signal component originating at the first communication unit being included in the group of the second time interval 419 is below a required value. Specifically, if the minimum user 30 distance is set equal to the maximum delay interval, the probability of signal components from one communication unit falling within two time intervals separated by the minimum user distance is close to zero. The

specific choice of a minimum user distance is a trade off between the risk of dividing signal components of the same communication unit into different time intervals and the risk of failing to detect multiple access signals. s In step 311, at least one access message has been detected which in the preferred embodiment of a UMTS RACH preamble comprises a given signature. In step 311, the receiver processor 227 controls the transmitter 215 to broadcast an acknowledgement message comprising the signature.

lO Upon receiving this, the communication units that transmitted the access messages will initiate the transmission of the RAClI information message.

In UMTS, the RACH information message is synchronized to the acknowledgement message as it is transmitted a fixed period after the preamble, specifically 3 or 4 access slots, and therefore the communication 15 units will transmit the RACH information messages with the same relative time offset as for the pre-ambles. However, as the propagation environment typically will not have changed significantly, the signal components of the RACH information messages will be received at the base station with the same relative time offsets. Consequently, by using 20 the grouping of the signal components determined from the pre- amble, the signal components of the different communication units can be separated and thus both messages can be successfully received.

However, this requires that the necessary resource is allocated for two 25 messages including both air interface resource but more importantly the required resource of the base station 205. Specifically, demodulation resources need to be allocated for the demodulation of all detected access messages. Accordingly, in the preferred embodiment, the receiver processor 227, in step 313, for allocates a communication resource 30 appropriate for a number of communications corresponding to the number of access messages and in particular the communication resource is a demodulation resource.

i In one embodiment of the invention, the receiver processor 227 is operable to negatively acknowledging receipt of the received access messages.

Specifically the base station comprises a processor or controller for 5 negatively acknowledging receipt of the received access messages, if it is determined that more than one access message is received in the signal.

This is advantageous in situations where for example resources of the communication system are limited such that only one access message can successfully result in call setup. In other embodiments, a resource is 10 dynamically shared and a negative acknowledgement is transmitted if the number of access messages result in the resource requirement of a shared resource exceeding the available resource. A specific example is a RAKE receiver structure, where finger allocation and management is dynamically shared between a number of signals. In this case, if the 15 number of detected access messages, and in particular signal components in an access message, exceeds the remaining available fingers, a negative acknowledgement is transmitted. Otherwise the required number of fingers required for receiving the signals from all accessing communication units is reserved and a positive acknowledgement is 20 transmitted.

It is within the contemplation of the invention, that any suitable form of positioning the time intervals relative to the signal components can be used. Thus in very simple embodiments, the beginning of the first interval 25 is set at the earliest detected signal component. The second intervals is set at the first signal component (if any) occurring following the end of the first time interval. If the two time intervals are separated by a value higher than the minimum user separation, it is determined that two different access messages have been received and the signal components 30 comprised within the first interval are grouped as one access message, and the signal components of the second interval are grouped as a second

i, 22 access message. The process is repeated until all signal components have been allocated to a time interval.

In other embodiments, a more complex positioning of the time intervals is 5 used. Thus in one embodiment, the time interval of a group is determined so as to have the largest accumulated signal energy for signal components in the interval. Specifically, the access signal processor 223 determines the largest signal component in the received signal. A first time interval is then positioned around the largest signal component such that it includes 10 the signal components resulting in the largest accumulated signal energy within the interval. The process then continues to determine the largest signal component (if any) not included in the first window. A second time interval is then positioned around this signal component such that it includes the signal components resulting in the maximum signal energy 15 being comprised in the interval. If the resulting distance between the two time intervals is higher than the minimum user separation, it is determined that two different access messages have been received, and the signal components comprised within the two intervals are grouped as two different access messages. The process is repeated for any remaining 20 signal components.

It is further within the contemplation of the invention that any suitable method for grouping the signal components into different access messages can be used.

In the preferred embodiment, the base station determines a signature of each access message, and determines the number of access messages in the signal having the same signature. Specifically, the process is performed in parallel on signals which have been de-spread using the 30 signature Walsh code, and therefore the signature of each access message is known for each detected signal. Thus, the number of access messages determined in each parallel operation corresponds to the number of access

( 23 messages having the same signature. In communication systems such as UMTS, it is not possible to individually acknowledge access signals received in the same time slot and having the same signature. Therefore, a positive acknowledgement of the signature may result in a plurality of 5 communication units continuing to transmit to the base station simultaneously. It is a significant advantage of the described embodiment that it allows for determination of the number of communication units accessing the base station simultaneously with the same signature, as it can reserve the required demodulation resource and thereby enable 10 receipt of all signals. Thus rather than a conflict and resulting failure of call setups, the present approach allows for a plurality of calls being successfully setup, thereby significantly increasing the success rate for call setup, lowering the call setup delay and thereby improving the service to the user and the capacity of the communication system.

In one embodiment, the receiver processor 223 further derives an information content of an access message from the signal components comprised in a group out of the plurality of groups. Specifically, if the access message comprises information data, the receiver processor 223 20 combines the signals grouped together as one access message and demodulates and decodes the data using RAKE receiver techniques, as will be apparent to the person skilled in the art.

It is within the contemplation of the invention that any suitable method 25 for determining the maximum delay interval can be used. In the preferred embodiment, as described above, the maximum delay interval is simply pre-determined from measurements of typical propagation environments.

However, in more complex embodiments, the delay interval processor 225 dynamically determines the maximum delay interval from on-going 30 measurements of the received signals. In one embodiment, the delay spread between the first and last signal component of each group is determined for all received access message. The resulting delay spreads

( 24 are filtered in a low pass filter, and the maximum delay interval is determined as the resulting average delay spread multiplied by a proportionality factor. In more complex embodiments, a more advanced statistical analysis is performed on the determined delay spreads to 5 determine the maximum delay interval. For example, the maximum delay interval may be determined as three times the standard variation of the determined delay spreads.

Although the above description has focussed on a base station, it will be

10 clear that it can be applied to any suitable component of a cellular communication system including the Radio Network Controller (RNC).

Likewise, although the description of the preferred embodiment has

focussed on a UMTS communication system, it will be apparent that the invention is equally applicable to many other communication systems.

For clarity and brevity, only the functionality of the communication system, and specifically the communication units and base stations, required for a description of a preferred embodiment of the invention has

been described, and it will be clear to the person skilled in the art that 20 further well known features, algorithms and functionality will typically be part of a cellular communication system.

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. However, 25 preferably, the invention is implemented as computer software running on one or more data processors. The elements and components of an embodiment of the invention may be located in the core network, the radio access network or any suitable physical or functional location. Indeed the functionality may be implemented in a single unit, in a plurality of units 30 or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally

distributed in the network. Specifically, all digital processing is preferably performed in a single Digital Signal Processor of a base station.

It will follow from the above, that the invention tends to provide a number 5 of advantages singly or in combination, including the following: It allows detection of the number of access messages received simultaneously. It provides a method for resolving conflicts of access messages 10 An improvement in call setup reliability is achieved.

Call setup delay is significantly reduced as more access messages are successful.

Resource requirements can be allocated to meet the number of call setups. 15. Capacity of the communication system is improved as fewer access messages are on average required for setting up a call.

Claims

( Claims

1. An apparatus for detecting multiple access signals in a cellular communication system having at least one base station supporting a 5 plurality of users, the apparatus comprising: means for receiving a signal comprising at least one access message; means for determining a plurality of signal components of the signal; means for determining arrival times of each of the plurality of 10 signal components of the signal; means for determining a maximum delay interval for a single access message; and means for determining the number of access messages in the signal in response to the maximum delay interval and the arrival times.

2. An apparatus as claimed in claim 1 further comprising means for grouping the signal components into a plurality of groups each group corresponding to one access message.

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3. An apparatus as claimed in claim 1 further comprising means for deriving an information content of an access message from the signal components comprised in a group out of the plurality of groups.

4. An apparatus as claimed in claim 2 or 3 wherein each group of the 25 plurality of groups comprises the signal components having arrival times within a time interval determined in response to the maximum delay interval.

5. An apparatus as claimed in claim 4 wherein the time interval is 30 substantially equal to the maximum delay interval.

6. An apparatus as claimed in any of the claims 1 to 5 further comprising means for determining a minimum time distance between arrival times of signal components of a first access message and arrival times of signal components of a second access message and the means for S determining the number of access messages is operable to further determine the number of access messages in response to the minimum time distance.

7. An apparatus as claimed in claim 6 wherein the means for 10 determining the number of access messages is operable to determine the number of access messages by grouping the signal components having time of arrival signals within a time interval substantially equal to the maximum delay interval and such that each group is separated by a time of arrival interval substantially equal to the minimum time distance.

8. An apparatus as claimed in any of the claims 2,3,4,5 or 7 wherein the time interval of a group is determined so as to have the largest accumulated signal energy for signal components in the interval.

20

9. An apparatus as claimed in any previous claim further comprising means for allocating a communication resource appropriate for a number of communications corresponding to the number of access messages.

10. An apparatus as claimed in claim 9 wherein the communication 25 resource is a demodulation resource.

11. An apparatus as claimed in any of the previous claims further comprising means for determining a signature of each access message and wherein the means for determining the number of access messages is 30 operable to determine a number of access messages in the signal having the same signature.

12. An apparatus as claimed in any of the previous claims further comprising means for acknowledging receipt of the received access messages. 5

13. An apparatus as claimed in any of the previous claims further comprising means for negatively acknowledging receipt of the received access messages.

14. A base station for a cellular communication system comprising an 10 apparatus as claimed in any of the previous claims.

15. A UMTS cellular communication system comprising a base station as claimed in claim 14.

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16. A method for detecting multiple access signals in a cellular communication system having at least one base station supporting a plurality of users, the method comprising the steps of receiving a signal comprising at least one access message; determining a plurality of signal components of the signal; 20 determining arrival times of each of the plurality of signal components of the signal; determining a maximum delay interval for a single access message; and determining the number of access messages in the signal in 25 response to the maximum delay interval and the arrival times.