GB2367455A

GB2367455A - Interference reduction in a cellular communication system having a beamed downlink

Info

Publication number: GB2367455A
Application number: GB0023855A
Authority: GB
Inventors: Stephen William Wales
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2000-09-29
Filing date: 2000-09-29
Publication date: 2002-04-03
Also published as: GB0023855D0; WO2002027970A1; EP1320947A1

Abstract

A cellular communications system comprises a number of base stations serving respective system cells; each base station is capable of communicating with many mobile terminals within its cell and with at least some mobile terminals in adjacent cells. In accordance with the invention, transmission of comparative data received by a mobile terminal from its serving base station and from at least one other base station are communicated to the serving base station and utilised therein to adaptively influence at least the direction of transmission of the downlink beam. By this means, the downlink beam can be directed accurately at the intended mobile terminal, and the risk of interference, whether from a third party station or terminal into the downlink beam or from the downlink beam into a third party station or terminal, is substantially reduced.

Description

CELLULAR COMMUNICATION SYSTEM

This invention relates to cellular communication systems, such as mobile telephone systems, and it relates in particular to such systems conforming to UTRA specifications.

Such systems are basically short-range cordless telephony systems, based upon the notional overlaying of a geographic area, representing the system's coverage limits, with a more-or-less regular tessellation of spatial cells, each served by at least one base station which primarily communicates with mobile terminals in its cell, but also communicates with base stations and mobile terminals in adjoining cells to perform various house-keeping activities, such as providing hand-over functions which allow mobiles to move freely from cell to cell.

The demands placed upon such systems are continually increasing, in terns not only of the number of mobiles to be supported, but also of the amounts of data traffic handled per mobile. These latter demands arise at least in part because of major increases in downlink traffic (i. e. data passed from base stations to mobiles) as access to major information centres and databases grows wider. Increased downlink traffic not only presents straightforward capacity problems; it also gives rise to data handling problems associated with the asymmetry between uplink and downlink traffic.

Various expedients have been used to address the problems associated with the above-mentioned demands, and indeed some proposed solutions have involved the use of hybrid systems combining

the technologies associated with Frequency Division Duplex (FDD) and Time Division Duplex (TDD), both of which are well established for use with cellular communication systems. Typically, FDD is preferred for operating environments involving communications over relatively large distances, since it is not as susceptible as TDD to round-trip delays and Doppler fading. TDD can be useful, however, in so-called communications"hot-spots", such as offices and public buildings, and it has thus been proposed to include, within certain FDD cells, smaller cells (sometimes referred to as"micro-cells") centred upon communications hot-spots and operating in accordance with TDD principles.

This expedient is effective to a degree. However, the more mobiles that are accommodated by a system, the greater are the difficulties of keeping inter-cell and intra-cell interference down to tolerable levels.

This invention seeks to address the problem of interference in cellular communications systems and, according to the invention from one aspect, there is provided a cellular communications system comprising a plurality of base stations serving respective system cells, each base station being capable of communicating with a plurality of mobile terminals within its cell and with at least some mobile terminals in adjacent system cells; wherein data are beamed from a base station to a mobile terminal in its cell, and wherein transmission of comparative data received by a mobile terminal from its serving base station and at least one other base station are communicated to the

serving base station and utilised therein to adaptively influence at least the direction of transmission of said beam.

By this means, the downlink beam can be directed accurately at the intended mobile terminal, and the risk of interference, whether from a third party station or terminal into the downlink beam or from the downlink beam into a third party station or terminal, is substantially reduced.

Preferably, the said comparative data include time difference signals relating to the time of arrival, at the mobile terminal, of signals from different base stations. This is advantageous, since these signals are already utilised within existing systems to enable the location of the mobile terminal to be calculated.

The comparative data may also include signal strength and interference measurements which, again, are in current use for handover purposes.

The invention also encompasses a base station for a cellular communications system; the station being capable, in response to its receipt of data indicative of the position of a mobile terminal, of adaptively directing its transmitter beam at the mobile terminal, to enable the mobile terminal to resist the reception of interfering data from other base stations or mobile terminals.

In order that the invention may be clearly understood and readily carried into effect, one embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 shows, schematically, the occurrence of intra-cell interference; Figure 2 shows, schematically, the occurrence of inter-cell interference; and Figure 3 shows, in block diagrammatic form, a cellular communications system in accordance with one embodiment of the invention.

Referring now to Figures 1, a cell I of a cellular communications system is served by a base station 2. The base station 2 primarily transmits down-link data to, and receives up-link data from, a plurality of mobile terminals in its cell. In Figure 1, two mobile terminals 3 and 4 are shown by way of example only, and the intention is that each mobile terminal can communicate independently with the base station 2 over respective paths 232 and 242. However, it is possible for downlink data intended for mobile terminal 3 to interfere with data being received by mobile terminal 4, and vice versa, as well as for uplink data from one of the mobile terminals to interfere with up-link data from the other. The down-link interference is the more acute difficulty however, because as mentioned above, downlink data volumes tend to be significantly higher than on the up-link, and of course the power transmitted by the base station significantly exceeds that emitted from the mobile terminals.

Likewise, as shown in Figure 2, in which features similar to those in Figure I bear identical reference numerals, intra-cell interference can occur between the base station I serving cell I and a

mobile terminal 5 in an adjacent cell 6 served by another base station 7.

It will be appreciated that both intra-cell and inter-cell interference can be influenced by a number of factors, among which are the transmission characteristics of the geographic area in which the cells are located, the duplexing system used and other operational criteria, such as the occurrence of widely differing asymmetries in upand down-link data as between different users of the system. In any event, however, such interference problems are exacerbated by accepting increased user number capacities.

This invention seeks to address the problem of capacity-induced interference by incorporating into the system the concept of smart antennas, i. e. antennas coupled to processors that are capable, when provided with information about the location of a target mobile terminal, of adaptively focusing the transmission/reception beam associated with an antenna upon the target mobile terminal. Increased user capacity is accommodated because the reduced susceptibility to interference of a properly directed beam allows greater use of the orthogonal code space (in both TDD and FDD modes) and (in the TDD mode) through reduced inter-cell interference.

Use of the invention also offers enhanced range capabilities, by exploiting beamforming gains.

On the uplink, beamforming can be achieved using algorithms that seek to maximise the signal to noise plus interference ratio after de-spreading. The complex weights generated by such algorithms, however, give rise to uncontrolled sidelobes, rendering the re-use of

the uplink beam for downlink traffic unsuitable, because sidelobes potentially exist in directions other than that of the mobile terminal.

Ideally, the downlink beam should direct the energy at the mobile terminal, whilst minimising the transmission of energy in other directions which could potentially cause interference to other mobile terminals. In practice, multi-path propagation may cause the optimum direction of arrival of energy at the mobile terminal to differ from the direction based on purely geographic considerations. Provided the downlink beamwidth is compatible with the azimuthal spread of the uplink signals, however, the geographic criteria should suffice to direct the beam adequately. In practice it is unlikely that the array order will be sufficiently high that beamwidths are significantly less than the azimuthal spread.

With the objective of locating the mobile terminal, so as to permit the base station's transmission beam to be steered appropriately towards it, it is possible to obtain information as to the angle of arrival of incoming (uplink) data by examining the covariance matrix of the received uplink signal from each of the base station's antennas and using direction-finding algorithms, such as MUSIC and ESPRIT, which exploit spatial correlations. However, these algorithms typically require high signal to noise ratios to obtain reliable directional information, and the array size has to be compatible with the number of signals present, and the complex signal environment encountered in systems such as UTRA, where both inter-cell and intra-cell interference is experienced, and with all signals being additionally

subject to time-dispersion, renders the task of direction finding extremely difficult.

One simple ad-hoc approach to estimating the direction of a mobile terminal is to form a number of receiving beams spanning, between them, the entire azimuthal extent of the base station's coverage.

For each mobile terminal, the beam receiving the maximum signal to noise plus interference ratio could be identified, thus to form a crude estimate of the direction of that mobile terminal from the base station. This estimate will, however, be affected by interference conditions, and for example a strong interfering signal received in a sidelobe null could affect the estimation process.

It is thus intended that a more reliable procedure be adopted, and to this effect the inventor has taken into account that mobile terminals are required, by operating protocols for the system, to perform a number of measurements for purposes such as location services and handover. For location services, the time difference of arrival measurement between signals received from different base stations is reported to the serving base station, in order to allow the location of the mobile terminal to be connected. For handover purposes, signal strength and interference measurements are also reported to the serving base station, and all measurements are reported at regular intervals.

The measurements for location services are utilised at the base station to direct the downlink beam towards the mobile terminal.

Angle information is calculated from the time difference of arrival

measurements, knowing the locations of the two base stations in question.

If desired, more sophistication can be incorporated by combining the time difference of arrival measurements with direction information derived from the received uplink signals and possibly downlink power control. Treating these as the outputs from sensors, the information can be combined, perhaps using data fusion techniques, to determine the desired direction and beamwidth of the downlink transmission. The beamwidth can be related to the uncertainty in the direction estimate, such that, in the event of a large uncertainty in direction, a larger beamwidth is used. Changes in beamwidth are, where necessary, accompanied by adjustments in the power of the downlink transmissions to ensure that sufficient signal strength is received at the mobile terminal.

Thus, as shown by way of example only in Figure 3, in a system in accordance with one embodiment of the invention, a base station 10 comprises a receiving means 11, including antenna and receiving circuit components as will be known to those skilled in the art. The base station 10 also includes circuits 12 coupled to the circuit components of the receiving means 11 and capable of extracting, from the received data, the time of arrival measurements relating to a given mobile terminal 13. The measurements extracted by circuits 12 are applied to a digital signal processor 14 which calculates, using known algorithms, the preferred azimuthal angle to be adopted by the transmitted beam 15 of the base station 10 for communication with the mobile terminal 13 and applies, to the downlink signals to be

transmitted along the beam 15, appropriate phasing to orient the beam correctly.

If it is desired to also control the width or some other characteristic of the beam 15, or to fine-tune the beam alignment, so as to further improve the selectivity with which the base station 10 can communicate with the mobile terminal 13, the signal processor 14 may perform further operations upon the time of arrival measurements and/or it may be supplied with further information, such as direction information derived from the received uplink signals by means of a circuit 16.

This expedient may be particularly effective if delayed until the beam 15 has been properly aligned with the mobile terminal 13, thereby ensuring that the signal to noise ratio of received uplink data is substantially optimised before these data are used in the adaptive alignment and/or beam-shaping process. As mentioned previously, the signal processor 14 may with advantage use data fusion techniques to combine the information applied thereto from the circuits 12 and 16.

Although the invention has been described herein with respect to particular embodiments, it is not intended to be limited by such description. The principle of the adaptive alignment and/or shaping of the transmitter beam of a base station serving at least one mobile terminal in response to comparative data, received by the mobile terminal from its serving base station and at least one other base station, and communicated to the serving base station being utilised therein to adaptively influence at least the direction of transmission of said beam may be implemented in other ways that will be evident to those skilled in the art.

Claims

CLAIMS :

1. A cellular communications system comprising a plurality of base stations serving respective system cells, each base station being capable of communicating with a plurality of mobile terminals within its cell and with at least some mobile terminals in adjacent system cells; wherein downlink data are beamed from a base station to a mobile terminal in its cell, and wherein transmission of comparative data received by a mobile terminal from its serving base station and at least one other base station are communicated to the serving base station and utilised therein to adaptively influence at least the direction of transmission of said beam.

2. A system according to Claim 1 wherein the said comparative data include time difference signals relating to the time of arrival, at the mobile terminal, of signals from different base stations.

3. A system according to Claim 2 wherein said time difference means are processed in a signal processor to determine a preferred direction of transmission from said base station to said mobile station.

4. A system according to Claim 3 wherein the signal processor combines said comparative data with information relating to the angle of reception at said base station of uplink data from said mobile terminal.

5. A system according to Claim 4 wherein said signal processor utilises data fusion techniques to effect the combination of said comparative data with said information relating to the angle of reception at said base station of uplink data from said mobile terminal.

6. A system according to any preceding claim wherein the beamwidth of the downlink beam is related to uncertainty as to a preferred direction of transmission of said beam whereby, in the event of a large uncertainty in said preferred direction, a larger beamwidth is used.

7. A system according to Claim 6 wherein changes in beamwidth of said downlink beam are accompanied by adjustments in the power of the downlink transmissions to ensure that sufficient signal strength is received at the mobile terminal.

8. A cellular communications system substantially as herein described with reference to the accompanying drawings.

9. A base station for a cellular communications system; the station being capable, in response to its receipt of data indicative of the position of a mobile terminal, of adaptively directing its transmitter beam at the mobile terminal, to enable the mobile terminal to resist the reception of interfering data from other base stations or mobile terminals.