GB2318026A

GB2318026A - Increasing capacity of TDMA channels

Info

Publication number: GB2318026A
Application number: GB9620702A
Authority: GB
Inventors: Paul Crichton; William Neil Robinson
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1996-10-04
Filing date: 1996-10-04
Publication date: 1998-04-08
Also published as: WO1998016025A1; GB9620702D0; AU4206997A

Abstract

Variations 1 in the offset between the start of a specified timeslot, allocated to a mobile station, and the receipt of a signal burst from that mobile station are monitored and the value of a statistical function, such as an average, related to the distribution of the burst about the centre of the timeslot is derived 2. This operation is repeated for other mobile stations linked to the base station and the mobile stations are then grouped 6 according to the ranges into which their statistical functions lie. A common carrier signal is assigned to the mobile stations within a group so that the base station can communicate with the mobile stations on a group basis, thereby enabling guard times to be reduced for mobiles of a similar distance from the base station. Weighted averages may be used for more accurate approximations of the offset time at any instant, whilst standard deviations may be used to assess their reliablility 3.

Description

Improvements in the Utilisation of Radio Communication Systems Field of the Invention The present invention relates to radio communication systems and, were specifically, to digital cellular radio communication systems.

Background of the Invention Cellular radio communication systems consist of a plurality of fixed base transceiver stations dispersed throughout the region served by the radio communication system. Each base station serves a limited region called a cell, and receives and transmits calls and communicates with mobile transceiver units or mobile stations with its own cell. In time distributed multiple access systems, individual mobile stations in a cell communicate with their associated base station at intervals, called slots which are multiplexed together.

The mobile stations will almost certainly be at differing distances from the base station. As radio signals travel at a given speed, there will be a delay between the transmission and reception of signals between a mobile station and its base station which is a function of the distance between the mobile station and the base station. For a cell of nominal radius 30km, this delay for a peripheral mobile station will be equal to approximately lOOIls.

Thus, unless preventative steps are taken, the slot received at the base station from a mobile station at the periphery of the cell could well overlap with that received at the base station from a mobile station which is closer to the base station.

Two forms of counter measure are employed currently. The first is the use of feed back signalling from the base station to the mobile stations telling them to advance, or retard their transmissions relative to one another depending upon their distances from the base station. This technique is known as timing advance control. Its major disadvantage is that it consumes capacity on the link between the base station and the mobile station (The down link).

The second technique which is employed is to use guard times which are inserted between the nominal slots. The guard times are periods during which no signals are transmitted from the mobile stations to the base station, and they may be used in conjunction with timing advance control. A main disadvantage of the guard time approach to the problem is that it reduces the total capacity of the link between a mobile station and a base station (The uplink) which can be used for the transmission of information.

For example, in the digital cellular radio communication system known as Trans European Trunked Radio (TETRA) the guard time for a normal uplink slot is assigned seven symbol durations out of a total time equal to 255 symbol durations. This represents a loss of 2.75% of the possible information transmission capacity of the uplink. In the case of a digital cellular radio communication of the type known as GSM, the guard time is allocated 8.25 bit durations out of a total time of 156.25 bit durations; this represents a loss of uplink information capacity of 5.28% of the total information transmission capacity. Furthermore, if for example, a time distributed multiple access carrier frequency is shared by say 10 mobile stations, a base station serving that group of mobile stations will schedule the mobile stations in such a way that each of the mobile stations communicate with it during a different slot.

Because of the differing distances between the mobile stations and the base station, each of the transmissions received by the base station will occur at some almost random offset from the time of commencement of its respective slot, the maximum value of the offset being determined by the guard time used. The whole of each transmission from the mobile station received by the base station is confined with the allocated slot appropriate to that mobile station and so the guard period has to such that it can accommodate the worst case condition. This will occur when one mobile station is at the periphery of the cell so that its signal transmission time is a maximum and the next mobile station to transmit to the base station is so close to the base station that its signal transmission time is to all intents and purposes zero.

The opposite case also can occur when each mobile station is diametrically opposite the other.

Typically, the base station responds to this variability in the offset of the useful information with the slots by using timing advance control to attempt to centralise the useful information from each mobile station within its respective slot. However, as stated above, the use of timing advance control reduces the downlink information capacity.

Summary of the Invention It is an object of the present invention to provide an improved method of compensating for the effects of mobile stations in a cellular radio communication system being at different distances from a base station serving the mobile stations.

According to the present invention there is provided a method of increasing the information carrying capacity of a time distributed multiple access cellular radio communication system in which information is transmitted in a continuous burst during time intervals of a predetermined duration between base stations and mobile stations forming the cellular radio communication system, wherein there is included the operations of monitoring variations in the offset between the start of a specified interval allocated by a base station to a given mobile station for that mobile station to communicate with the base station and the receipt of a signal burst from that mobile station, deriving the value of a statistical function related to the distribution of the burst about the centre of the said interval, repeating the operation for other mobile stations linked to the base station, grouping those mobile stations for which the said statistical function lies within specified limits, assigning a common carrier signal to the mobile stations within a group and causing the base station to communicate with the mobile stations on a group basis.

Preferably the groups are so arranged that the said offsets vary progressively from one group to another.

A suitable statistical function is a running average derived according to the equation: n-y Running average for mobile station; (RAi) = E;0 n/y+1 n n = the nth interval in which mobile stations has transmitted.

where:- on = the offset of the information burst within the boundary of interval n and y = an integral number of intervals over which the running average has been taken.

Alternatively, there can be used a weighted running average offset in which more importance is given to later observed offsets in a sequence. Such a weighted running average would be given by the equation: n-y WRAi = on/wn/y+l n Where wn is a weighting factor and the other symbols have the same meaning as before.

Brief Description of the Drawings The invention will now be described and explained by way of example, with reference to the accompanying drawings, in which: FIG. 1 shows, schematically, a distribution of ten mobile transceiver stations about a base transceiver station of a cellular radio communication system.

FIG. 2 shows how the offset of useful information as received from the mobile stations of FIG. 1 at the base transceiver station may vary within the boundaries of its nominal slot; FIG. 3 shows a worst case condition for the arrival of useful information at the base transceiver station of FIG. 1.

FIG. 4 shows the opposite extreme position to that of FIG. 3.

FIG. 5 is a flow diagram showing the steps of a method embodying the invention for improving the utilisation of a cellular radio communication system.

FIG. 6 shows an improved distribution of the offsets of useful information and within slots received from the mobile stations of FIG. 1 achieved by the use of the present invention, and FIG. 7 shows a second improved distribution of offsets of useful information within slots received from the mobile stations of FIG. 1 achieved by the use of the present invention.

Description of a Preferred Embodiment Referring to FIG. 1, a cell 10 of a cellular radio communication system has a central base transceiver station 11 which is surrounded by a distribution of mobile transceiver stations 12 in communication with it.

Each mobile station 12 communicates with the base transceiver station 11 during a period of time called a slot, part of which is dedicated to the transmission of useful information and the remainder of which forms a guard period the purpose of which, as described above, is to separate the transmissions of one mobile station from those of another. As previously explained, because of the limited, although high, speed of transmission of electromagnetic signals and the varying distances of the mobile stations 12 from the base transceiver station 11, the useful information transmission periods of each mobile station 11 is displaced, or offset, from the beginning of its slot by a different amount, as shown in FIG. 2. The two limiting cases are shown in FIG. 3 and FIG. 4. That of FIG. 3 occurs when one mobile station 11 is at the periphery of the cell 10 and the next one to communicate with the base transceiver station 12 is adjacent the base transceiver station 12. In this case, there is no interval between the useful information received by the base transceiver 12 station in slot n and that received in slot n+ 1 with the result that some confusion between the transmissions from the mobile stations could occur. In practice, the guard intervals have to be so chosen that there is some minimal offset even for a mobile station 11 which is virtually on top of the base station 12. The case of FIG. 4 occurs when two mobile stations 11, though adjacent in their communication slots, are opposite each other on the periphery of the cell 10. In this case there is an excessively large interval between the periods of useful information received at the base transceiver station 12.

FIG. 5 illustrates the steps of the method by means of which the present invention seeks to ensure that the two extreme cases do not occur so that the portion of the slots used as guard periods can be reduced, so increasing the portion of the slots which can be used for the transmission of useful information between the mobile stations 11 and the base transceiver station 12. Referring to FIG. 5, for each mobile station 11, the base transceiver station 12 monitors the offset of the period of useful information transmission during the slot allocated to that mobile station (Stage 1) and determines a running average value for the offset, according to the equation: n-y RA = l; n / y+1 . (1) n where RA i = running average offset for mobile stations y = an integral number of slots over which the running average is taken n = the nth slot, in which the mobile station is transmitted, and on = the offset of the useful information within the boundaries of slot n (stage 2) Using standard statistical methods, a running standard deviation for the running average is determined (stage 3) and compared with a value which indicates an acceptable standard of reliability (stage 4). If the running average offset is considered to be unreliable, then the mobile station concerned is placed on a carrier reserved for such cases and treated according to the existing methods previously described (stage 5). If the running average offset is considered to the reliable, then the mobile station is assigned to the carrier relevant to a group mobiles which have running average offsets within a given range (stage 6). There are as many of these groups of mobile stations as there are carriers available at the base transceiver station.

The effectiveness of the method can be increased by calculating a weighted running average for the offsets instead of the simple running average given by equation 1, more importance being given to later observed offsets in a sequence. A suitable equation for calculating a weighted running average is: n-y VVRA mobile: = S n wn / y+1 ....

n Where wn is a weighting factor and the other symbols have the same meanings as before.

Another way in which the effectiveness of the method can be increased is to order the mobile stations within each group such that the offsets either increase or decrease progressively within the limits associated with that group. The results of these orderings are shown in FIG. 5 and FIG. 6, respectively.

The situation is a dynamic one, and should a previously deemed unreliable running average offset of a mobile station become reliable, or vice versa, the mobile station concerned can be re-assigned to the appropriate group or general carrier.

As the extreme conditions shown in FIG. 3 and FIG. 4 no longer occur, the amount of guard time per slot can be reduced. A suitable guard time which will be satisfactory for 99% of the time is 3.25 running standard deviations.

The reduction in guard time can be used to add additional time for the transmission of useful information or to increase the rate at which the useful information is transmitted.

In all cases, the base transceiver station must indicate to each of its associated mobile stations in which slot it is allowed to transmit and how much useful information may be transmitted to the base transceiver station in one signal burst. Alternatively, the base transceiver station must inform all of the mobile stations of the rate at which the uplink slots are being scheduled and relate the timing of these slots to the down link.

As an example, applying the method of the invention to a digital cellular radio communication system operating under the specifications of the body known as Groupe Systeme Mobile, or similar. (A GSM system) and having cells with a nominal radius of 30km, we have: a) The one way delay, or transit time, of a signal from the boundary of the cell to the base transceiver station at the centre of the cell is 10011sects.

b) Without timing advance control 100KLsecs is the maximum offset within the slot boundaries of the useful information, and 0 llsecs is the minimum offset possible.

c) If 32 mobile stations are uniformly distributed in range from the base transceiver station, they can be grouped into 4 groups according to their distances from the base transceiver station as modelled by the above running average technique.

d) The mobile stations in the Groups are assigned to a slot on one of 4 different GSM like carriers according to the group they are in.

e) The maximum variation in offset of the useful information within the slot boundary for each carrier is 100/4 llsecs, i.e. 25 Asecs, so 25 KLsecs is the maximum guard time required.

f) The current guard time for a GSM type system is 30.5 llsecs.

In practice, because the distribution of the mobile stations about the base transceiver stations is both irregular and constantly changing, the amount of guard time which can be released for information transfer also changes, but nonetheless, a reduction in the guard time of about ten per cent is achievable.

Claims

1 A method of increasing the information carrying capacity of a time distributed multiple access cellular radio communication system in which information is transmitted in a continuous burst during time intervals of a predetermined duration between base stations and mobile stations forming the cellular radio communication system, wherein there is included the operations of monitoring variations in the offset between the start of a specified interval allocated by a base station to a given mobile station for that mobile station to communicate with the base station and the receipt of a signal burst from that mobile station, deriving the value of a statistical function related to the distribution of the burst about the centre of the said interval, repeating the operation for other mobile stations linked to the base station, grouping those mobile stations for which the said statistical functions lie within specified limits, assigning a common carrier signal to the mobile stations within a group and causing the base station to communicate with the mobile stations on a group basis.

2. A method according to claim 1 wherein the said affects vary progressively from one group to another.

3. A method according to claim 1 or claim 2 wherein the said offsets vary progressively within a group.

4. A method according to claim 3 wherein the said offsets within a group increase progressively.

5. A method according to claim 3 wherein the said affects decrease progressively within a group decrease progressively.

6. A method according to any preceding claim wherein the statistical function is a running average of the offsets.

7. A method according to claim 6 wherein the statistical function is a running average derived according to the equation: n-y Running average for mobile station; (RA) i= E; on/y+1 n n = the nth interval in which mobile stations has transmitted.

where:- n = the offset of the information burst within the boundary of interval n and y = an integral number of intervals over which the running average has been taken.

8. A method according to claim 6 wherein the statistical function is a weighted running average derived according to the equation.

n-y WRA mobile; = l; n wn / y+ 1 n where WRA mobile = the weighted running average for mobile station n = nth interval in which mobile stations has transmitted on = the offset of the information ?with the boundary of interval n y = an integral number of intervals over which the running average has been taken, and wn = a weighting factor.

9. A method according to claim 8 wherein the weighting factor wn places more importance upon offsets observed later in a series of transmissions from the ith mobile station.

10. A method according to any preceding claim wherein there is included the operation of assessing the reliability of the determination of the value of the said statistical function.

11. A method according to claim 10 wherein the reliability of the determination of the value of the said statistical function is assessed by determining the standard deviation of the value of the statistical function.

12. A method according to claim 11 wherein the determination of the value of the said statistical function is deemed reliable if the standard deviation is below 3.25.

13. A method according to any of claims 10 to 12 wherein those mobile stations for which the determination of the value of the statistical function is deemed unreliable are assigned to a specific carrier

14. A method according to claim 13 wherein should the determination of the value of the statistical function relating to any of the said mobile stations become deemed reliable, then that mobile station is re-assigned to an appropriate one of the other groups and vice versa.

15. A method according to any preceding claim wherein the cellular radio communication system is a digital cellular radio communication system.

16. A method according to claim 15 wherein the digital radio communication system is a GSM system.

17. A method of increasing the information carrying capacity of a time divided multiple access cellular radio communication system as hereinbefore described and with reference to the accompanying drawings.