GB2372407A

GB2372407A - An improved signalling method for mobile telephones

Info

Publication number: GB2372407A
Application number: GB0105810A
Authority: GB
Inventors: David Thomas; Simon Davis; Anthony Peter Hulbert
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2000-05-05
Filing date: 2001-03-09
Publication date: 2002-08-21
Also published as: GB0010786D0; GB0105810D0

Abstract

In a communication system comprising a base station 5 tasked to allocate communication resource to a plurality of mobile stations 4, a method of requesting the resource from the base station 5 to the mobiles 4 comprising the step of sending a signal 6 on a channel common to other peripheral stations and wherein the request signals 6 can be sent at the same time. The signal is preferably orthogonal such as a Walsh code.

Description

Improved signalling method for mobile phones This invention relates to communication systems whereby a server allocates communication services to a plurality of terminals such as radio transceivers. The invention has particular but not exclusive application to mobile telecommunications whereby a base station functions to provide facility to a number of mobile handsets.

In a basic configuration, a communication channel needs to be constantly open during a conversation between a mobile and base station for voice traffic; i. e. one channel needs to be dedicated to each user during conversation. However such a system is inefficient because during human speech there are periods of inactivity on behalf of the speaker, or viewed alternatively, the communication comprises a number of so called"talk-spurts"1 ; as illustrated in figure la.

Pauses 2, in the speech can be up to 50% or more, and thus this proportion of the resource may be wasted due to inactivity.

It is known to increase the efficiency of such systems, by utilising the periods of inactivity in the voice signal from one mobile and "stuffing"conversation slots 3, into the pauses, as shown in figure lb.

This known technique is referred to in the art as"statistical multiplexing". In this way, a plurality of mobiles can share the same channel, (usually in addition to other channels) during live communication.

It is a requirement in statistical multiplexing, for the mobile station to detect voice activity during speech and subsequently send a request

signal to the base station so as to allocate resources. This signal must be fast-a target delay of 40ms or les,, is often considered as an acceptable limit for cut-off in human conversation; i. e. the maximum delay which would otherwise cause problems in relation to delays in telephone speech communications.

A problem arises however, if more than one mobile tries to send request signals at the same time. In order to avoid such collisions, socalled collision detection and"back-off"algorithms are known in the art which prevent this. The problem with such methods is that one needs to maintain a low signalling channel load otherwise there is increased chances of overload and the number of consequentially necessary re-trys increases, which tends to lead to increased delays and consequent decrease of efficiency.

At present there is no way to co-ordinate more than one requests for facility to the base station at the same time. It is an object of the invention to overcome these problems and to make it possible for mobile stations to send simultaneous signals to the base station without confusing the base station and provide for a good delay response and high efficiency.

The invention consists of, in a communication system comprising a server tasked to allocate communication resource to a plurality of peripheral stations, a method of requesting said resource from the server by said peripheral stations comprising the step of sending a

signal on a channel/carrier common to other peripheral stations and wherein said request signals can be sent at the same time.

The term"at the same time"means that the peripheral station, e. g. a mobile phone, does not have to consider when another peripheral station may also send a request signal on the same channel and that the request signal can overlap to any degree. The term should thus be construed as such.

Preferably, the signal is sent on a specific signalling channel, which is shared by all mobiles when sent uplink from a mobile. Signals from more than one mobile can thus be sent at the same time within this signalling channel.

Preferably, this is achieved by using a form of Code Division Multiple Access (CDMA). The signals from mobiles having codes which are orthogonal to each other.

According to a yet preferred embodiment of the invention, the codes are Walsh codes. Walsh codes, which are used in CDMA, comprise a series of 1's and 0's, and can be decoded such that all other codes, apart from the one being searched for, appear as background noise.

The invention will now be described by way of example and with reference to the following figures of which: Figure 2 shows an embodiment of the invention.

Figure 3 shows a representation of an uplink FSRACH burst according to an embodiment of the invention.

Figure 4 shows the resolution of FSRACH collisions in code space according to the figure 3 embodiment.

Figure 5 shows the time relation ship between the request signal (FSRACH) and the acknowledgement signal (FSACH).

Figure 6 shows how the request signal is produced according to a preferred embodiment.

Example I Figure 2 illustrates an embodiment of the invention when applied to a GSM system. It shows the procedure of a mobile telephone 4 in communication with a base station 5, the latter acting to serve the mobile station with telecommunication services. The mobile is one of number of mobiles which are served by the base station. In the figure one assumes that a link between the mobile and the base station has been established. Other mobile stations may also be in communication with the base station using the same channel. When the mobile requires radio resources for the next talk-spurt, it will send a signal 6, (requesting resource) including a code identifying the mobile. This signal is sent in a single burst on the shared Fast Spread Random Access Channel (FSRACH) in the next available timeslot. It should be

noted that in this example is based on the GSM system where it is assumed a common signalling channel is utilised on one of the 8 possible timeslots on one carrier. This channel is common to a plurality of mobiles. One or more other mobiles may also send request signals with their identifying codes at the same time, i. e. in the same time slot. The base station will therefore receive zero, one or more access requests in each timeslot. The identifying codes are sent as the Walsh codes, which being orthogonal to each other means that the base station can decode several request from mobiles at the same time.

At the same time as sending the request, the MS will start a fast access timer. If an appropriate Fast Assignment message is not received in the Fast Assignment channel before the timer expires the original request will be re-sent.

Continuing when the base station receives a request it will attempt to allocate an uplink radio resource for the mobile station in question. If this is successful, a Fast Assignment message 7, is sent on the common downlink Fast Spread Assignment Channel (FSACH). The message will contain the Statistically Multiplexed Mobile Identifier SMMI to uniquely identify the mobile in question and the details of radio resource to be assigned. If there is no currently available radio resource, a negative acknowledgement is sent to the MS in order to avoid a flood of re-transmits. When a radio resource becomes available, a positive acknowledgement is sent with the new radio resource assignment. The time relationship between the uplink FSRACH and downlink FSACH is illustrated in figure 5. Timeslots in GSM e. g. are numbered 0-7 in both uplink and downlink directions

but are skewed in time by three timeslots. If timeslot (TS) 3 is being used in the uplink for FSRACH then timeslot 3 should be used in the downlink for FSACH. This is purely preferably and applicable for GSM systems as telephone channels are allocated in pairs of the same number timeslots. If you used TS 3 on uplink and TS 2 on downlink for the signalling channels you would block TS3 downlink and TS2 uplink from being used for voice calls). The term"same timeslot" should thus be construed as explained above with reference to this application. The same timeslot therefore is preferably used on the FRACH and FSACH in order to provide maximum usage for GSM circuit switched channels. This leaves five timeslots in which to apply the signal processing (2.98 ms) in order to recover several fast requests received in the same timeslot.

Upon receipt by the mobile of the FSACH message the mobile will then further send an acknowledgement to the base station. Preferably this acknowledgement is implicit to save on extra control bandwidth.

This means that if the base station receives some coded speech on the assigned radio resource from the correct mobile station, it takes it to be an acknowledgement.

In the same way as the MS protects against loss of the FSRACH request message with a timer, the base station also starts a timer upon sending the assignment message. This timer is cancelled upon receipt of an acknowledgement. If it expires, the fast assignment message will be resent up to a maximum number of times.

Therefore in this example when a voice call is established, or hand over procedures are initiated, the base station will allocate the mobile station with three pieces of information related to the fast uplink access :-. i) The radio resources to use for the fast uplink channel and fast assignment downlink channel. ii) A unique code identifying the mobile which is used in downlink messages called the statistically Multiplexed Mobile Indicator iii) A 128 bit Walsh code for the uplink access (called the Access code) Preferably the format of the uplink FSRACH burst includes guard periods which flank the Walsh codes, which allows for inconsistencies in the timing advance. This is shown in figure 3.

In order to allow some timing mis-alignment between incoming FSRACH bursts, the Walsh codes are protected by a pseudo-random sequence. Suitable modulation is shown in Figure 6.

In general, the multiple access mechanism is based on a twodimensional scheme of timeslots and orthogonal codes as illustrated in Figure 4. As several requests might be received in the same timeslot they are separated with orthogonal codes. Using a 128 bit codes allows up to 128 separate statistically multiplexed voice users to be identified, but also provides as raw coding gain of 21 db.

In general, a number of techniques can be used by the base station to decode several requests received from a plurality of mobile stations in the same time slot including joint detection and interference calculation. Other requests simply appear as noise. This means that the capacity of the fast request channel can be greatly enhanced over that of a normal random access channel. In addition no time related back-off procedures are required. Fast requests that are errored can simply be re-transmitted in the next available slot thus minimising access delays and further increasing the fast request channel capacity.

By providing an out-of band common uplink random access channel some of the problems associated with using an in-band scheme can be avoided. These include having to avoid transmission on certain frame numbers and having to decode spread messages on top of nonorthogonal voice data.

Claims

1. In a communication system comprising a server tasked to allocate communication resource to a plurality of peripheral stations, a method of requesting said resource from the server by said peripheral stations comprising the step of sending a signal on a channel common to other peripheral stations and wherein said request signals can be sent at the same time.

2. A method as claimed in claim 1 wherein said signals are arranged to be sent in the same dedicated timeslot.

3. A method as claimed in claim I or 2 where said server is a base station and said peripheral stations are mobile telephones.

4. A method as claimed in any preceding claim wherein said signal includes a code which is orthogonal to those of other mobiles.

5. A method as claimed in claim 4 wherein said code is a Walsh code.

6. A method as claimed in claims 4 or 5 wherein said code is protected by a pseudo-random sequence.

7. A method as claimed in any preceding claim including the additional step of step of the server station sending an acknowledgement signal to the requesting peripheral station to indicate whether or not said resource is available.

8. A method as claimed in claim 7 wherein said acknowledgement signal is sent in the same timeslot.

9. A method as claimed in claims 7 or 8 wherein, on receipt of said acknowledgement signal, said peripheral station sends coded speech on the appropriate assigned resource and this taken by the base station to be an implicit acknowledgement.

10. A mobile phone adapted for the method as claimed in any preceding claim.