GB2350753A

GB2350753A - Measuring channel characteristics in mobile communications networks

Info

Publication number: GB2350753A
Application number: GB9913097A
Authority: GB
Inventors: Behzad Mohebbi; Majid Boloorian
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1999-06-04
Filing date: 1999-06-04
Publication date: 2000-12-06
Anticipated expiration: 2019-06-04
Also published as: GB2350753B; GB9913097D0

Abstract

A mobile communications network includes respective first and second stations 1,2, one of which is a base station and the other of which is a mobile station. A first channel transmits signals from the first station to the second station and a second channel transmits signals from the second station to the first station. The network is one in which the uplink and downlink channel characteristics can validly be assumed to be the same or similar, for example a UTRA TD-CDMA network having a TDD mode. At the second station 2, channel estimation is carried out for the first channel. When a preselected change in communications-channel characteristics between the first and second stations is detected (e.g. based on the first channel FER), in response to such detection a predetermined control signal CS, embodying the channel estimate EST for the first channel, is transmitted from the second station to the first station. The control signal is received at the first station and employed to derive a channel estimate EST for the second channel. Thus, it is not necessary to provide the first station with its own channel estimation facility. In another embodiment (Fig. 6), when a change is detected in channel characteristics the predetermined control signal is transmitted via the second channel and comprises a known sequence of symbols such as a midamble and is used by a channel estimator in the first station to produce a channel estimate for the second channel directly. May be used for asymmetric data transmission, wherein the midamble is only added to the channel having the lowest data throughput requirement.

Description

2350753 -1 MEASURING CHANNEL CHARACTERISTICS IN MOBILE CQMMUNICATIQINJS

NETWORKS The present invention relates to measuring channel characteristics in mobile communications networks. In particular, but not exclusively, the present invention relates to methods of an apparatus for carrying out channel estimation in a mobile communications network such as a code-division multiple access (CDMA) communications network.

One mobile communications network currently under development by the European Telecommunications Standards Institute (ETSI) is referred to as a UTRA TD CDMA mode network. UTRA stands for Terrestrial Radio Access, UMTS stands for Universal Mobile Telecommunication System (a third generation mobile telecommunications system) and TD stands for time division. The UTRA TD-CDMA mode network is a CDMA/TDMA network using time division duplexing (TDD). TDMA stands for time-division multiple access. In such a TDD network, the available timeslots for two-way communication between a base station and a mobile station are divided into alternate downlink and uplink timeslots. The downlink timeslots are used for transmission of user data and/or control information from the base station to the mobile station, and the uplink timeslots are used for transmission of user data and/or control information from the mobile station to the base station. Effectively, such a TDD mode of operation still has respective uplink and downlink channels.

It is presently proposed that in the UTRA TD-CDMA network channel estimation should be performed both for the uplink channel and for the downlink channel. For this purpose, a known sequence of symbols is transmittedat intervals in both the uplink and downlink directions. The sequence of symbols is referred to as a "mid-amble" as it is normally.

transmitted in a middle portion of a timeslot, where there is less scope for transmission errors. At the receiving end of each channel, the impulse response of the transmission channel is measured by solving an equation involving a known matrix containing shifted versions of the mid-amble and a column matrix containing the received mid-amble sequence.

UMTS services, which must be supported by the UTRA TD-CDMA network, include so-called asymmetric -operation services in which a far greater capacity is required for transmitting user data and/or control information through one of the uplink and downlink channels between a base station and a mobile station than through the other of those channels. Such asymmetric -operation services include cell broadcast, wireless internet and file transfer. For such asymmetric -operation services the existing proposal to require channel estimation to be performed for both the uplink and downlink channels imposes a considerable burden on the network. In particular, as the mid-amble portion can occupy up to 2401 of the timeslot in which it is transmitted, the remaining portion of the timeslot available for transmission of user data and/or control information may be insufficient to maintain the required capacity for that one of the channels which must have the higher throughput. Also, it may be difficult to perform adequate channel estimation for the capacity- limited channel as, in the channel estimation process, the maximum delay spread which can be measured is dependent on the length of the mid-amble used. Thus, although it might be desirable from the throughput point of view to shorten the mid-amble, this makes it difficult to measure the delay spread properly in the channel estimation process.

Even when providing UMTS services which do not involve asymmetric operation, for example a normal telephone conversation, the existing proposal for channel estimation in both directions still has signif icant disadvantages. For example, to carry out channel estimation for the downlink channel it is necessary to solve a matrix equation. This requires a large amount of processing power at the mobile station, which in turn increases the power consumption of the mobile station, reducing the time available between battery charging operations.

According to a f irst aspect of the present invention there is provided a two-way communication method, for use in a mobile communications network including respective first and second stations and having a first channel for transmitting signals from the first station to the second station and a second channel for transmitting signals from the second station to the first station, one of the first and second stations being a base station of the network and the other station being a mobile station of the network, the method including: detecting a preselected change in communi cations -channel performance (communications -channel characteristics) between the first and second stations and, in response to such detection, transmitting a predetermined control signal from the second station to the first station; at the first station, receiving the predetermined control signal and employing that signal to derive a predetermined measure of performance (characteristics) of the said second channel.

Such a communication method makes it possible to avoid performing the channel estimation process continuously in the first station, irrespective of changes in the channel performance.

In one preferred embodiment of the first aspect of the invention, the channel estimation process is performed exclusively in the second station, and the predetermined control signal includes a predetermined measure of performance of the first channel, such as the channel estimate for the first channel, produced by the second station. In this case, the required measure of performance of the second channel (channel estimate) may be taken by the f irst station to be equal to the received measure of performance of the first channel.

In this case, it is not necessary for the first station to have any channel estimation capability of its own.

In another embodiment of the f irst aspect of the invention, the first station may have a channel estimation capability but only use it when a predetermined change in performance of one of the channels is detected. In this case, the predetermined control signal can be known information, required for channel estimation purposes in the first station, inserted into the second channel. In this case, when the known information is to be transmitted, the capacity of the second channel for carrying user data and/or control information is reduced, but the known information only needs to be transmitted when a change in channel performance is detected. Thus, for slow varying channels, there can be a significant increase in throughput for the second channel.

Embodiments of the first aspect of the present invention are particularly advantageous in situations where asymmetric - operation services are being provided such that the second channel is more throughput critical than the first channel.

According to a second aspect of the present invention, there is provided a two-way communication method, for use in a mobile communication network including a base station and a mobile station and having a downlink channel for transmitting signals from the base station to the mobile station and an uplink channel for transmitting signals from the mobile station to the base station, the method comprising:

determining which of the said downlink and uplink channels is the less capacity- limited channel; and performing measurement of channel characteristics (e.g.

channel estimation) only for the determined less capacity-limited channel.

In such a method, it is determined, for example for each particular type of service being provided by the network, which one of the uplink and downlink channels is the less capacity-limited. Once this determination has been made, channel estimation can be carried out for the less capacity-limited channel at the receiving end of that channel, and that channel estimate can be employed to derive a channel estimate for the more capacity- limited channel without the need to perform channel estimation at the receiving end of the more capacity-limited channel. The determination of the less capacity- limited channel can be made at the time the service is set up and maintained for the duration of that particular service. Alternatively, the determination could be made at intervals and changed in dependence upon the instantaneous capacity requirements of the uplink and downlink channels.

According to a third aspect of the present invention there is provided a two-way communication method, for use in a mobile communications network including a base station and a mobile station and having a downlink channel for transmitting signals from the base station to the mobile station and an uplink channel for transmitting signals from the mobile station to the base station, the method including:

calculating in the base station a channel impulse response of the uplink channel; carrying out a matrix inversion operation in the base station to derive from the uplink channel impulse response an inverse. of a matrix containing an assumed downlink channel impulse response; transmitting data of the inverse matrix from the base station to the mobile station; and in the mobile station, receiving the inverse-matrix data and using it to derive a downl ink- channel performance measure.

Such a method can enable a useful simplification of the design of the mobile station, as the mobile station does not need to perform the matrix inversion operation for downlink channel estimation purposes. In addition to reducing mobile station (handset) complexity, power consumption in the mobile station can be reduced by avoiding carrying out the complex matrix inversion operation in the mobile station.

According to a fourth aspect of the present invention there is provided a mobile communications network including respective first and second stations and operable when in use to provide a first channel for transmitting signals from the first station to the second station and a second channel for transmitting signals from the second station to the first station, one of the f irst and second stations being a base station of the network and the other station being a mobile station of the network; the network also including detection means for detecting a preselected change in communications-channel performance between the f irst and second stations; the second station comprising control signal transmission means operable, in response to such detection, to transmit a predetermined control signal to the first station; and the first station comprising control signal receiving means for receiving the predetermined control signal, and also comprising performance measuring means for employing the received control signal to derive a predetermined measure of performance of the said second channel.

According to a f if th aspect of the present invention there is provided a mobile communications network including a base station and a mobile station and operable when in use to provide a downlink channel for transmitting signals from the base station to the mobile station and an uplink channel for transmitting signals from the mobile station to the base station, the network comprising: determining means for determining which of the said downlink and uplink channels is the less capacitylimited channel; and channel estimation means operable to perform channel estimation only for the determined less capacity limited channel.

is According to a sixth aspect of the present invention there is provided a mobile communications network including a base station and a mobile station and operable when in use to provide a downlink channel for transmitting signals from the base station to the mobile station and an uplink channel for transmitting signals from the mobile station to the base station, the base station including: calculating means for calculating a channel impulse response of the uplink channel; matrix inversion means for carrying out a matrix inversion operation to derive from the uplink channel impulse response an inverse of a matrix containing an assumed downlink channel impulse response; and transmission means for transmitting data of the inverse matrix from the base station to the mobile station; and the mobile station including receiving means for receiving the inverse-matrix data, and also including performance measuring means for employing the received inverse-matrix data to derive a downlink- channel performance measure.

According to a seventh aspect of the present invention there is provided a first station of a mobile communications network, the network further including a second station and being operable when in use to provide a first channel for transmitting signals from the first station to the second station and a second channel for transmitting signals from the second station to the first station, one of the first and second stations being a base station of the network and the other station being a mobile station of the network; the first station comprising: control signal receiving means for receiving a predetermined control signal transmitted thereto by the second station in response to detection of a preselected change in communications -channel performance between the first and second stations; and performance measuring means for employing the received control signal to derive a predetermined measure of performance of the said second channel.

According to an eighth aspect of the present invention there is provided a second station of a mobile communications network, the network further including a first station and being operable when in use to provide a first channel for transmitting signals from the first station to the second station and a second channel for transmitting signals from the second station to the first station, one of the first and second stations being a base station of the network and the other station being a mobile station of the network; the second station comprising: performance measuring means operable to produce a predetermined measure of performance of the first channel; and control signal transmission means operable, in response to detection by the network of a preselected change in communications -channel performance between the first and second stations, to transmit to the first station a predetermined control signal including the said predetermined measure of first-channel performance.

According to a ninth aspect of the present invention there is provided the base station of a mobile communications network embodying any one of the sixth to eighth aspects of the present invention.

According to a tenth aspect of the present invention there is provided the mobile station of a mobile communications network embodying any one of the sixth to eighth aspects of the present invention.

Reference will now be made, by way of example, to the accompanying drawings, in which:

Fig. 1 shows a schematic view of parts of a mobile communications network embodying the present invention; Fig. 2 shows parts of a base station and a mobile station according to a first embodiment of the present invention; Fig. 3 shows parts of a base station and a mobile station according to a second embodiment of the present invention.

Fig. 4 shows a first example of signals transmitted in respective downlink and uplink timeslots in the Fig. 2 embodiment; Fig. 5 shows a second example of signals transmitted in respective uplink and downlink timeslots in the Fig. 2 embodiment; Fig. G shows parts of a base station and a mobile station according to a third embodiment of the present invention; and Fig. 7 shows parts of a base station and a mobile station according to a fourth embodiment of the present invention.

Fig. 1 shows parts of a mobile communications network to which embodiments of the present invention can be applied. The network includes a mobile station MS which, when the network is in use, is in two-way communication with a base station BS. The mobile station MS and base station BS have an uplink channel for transmitting signals (user data and/or control signals) from the mobile station to the base station, and a downlink channel for transmitting signals (user data and/or control signals) from the base station to the mobile station. In some networks, the mobile station and base station have more than one uplink and/or downlink channel for two-way communication purposes. For example, there may be separate data and control channels in one or both directions.

The base station BS is also in two-way communication with a base station controller BSC of the network. Usually, several base stations are in communication with the same base station controller.

The base station controller BSC is in turn in two-way communication with a mobile switching centre MSC.

The base station controller BSC serves to manage the radio resources of its connected base stations, for example by performing hand-off and allocating radio channels. The mobile switching centre MSC serves to provide switching functions and coordinates location registration and call delivery.

Fig. 2 shows parts of respective first and second stations embodying the first aspect of the present invention. One of the first and second stations is a base station of a mobile communications network such as the Fig. 1 network and the other of the first and second stations is a mobile station of such a network.

In embodiments of the first aspect of the invention, which of the base station and mobile station is the first station, and which is the second station, is predetermined (fixed), for example at the time the base station and mobile station are manufactured.

The first station 1 comprises a control unit 10 and a control signal receiving unit 12. The control signal receiving unit 12 is connected to the control unit 10 for supplying thereto channel estimation data EST.

The second station 2 comprises a control unit 20, a channel estimation unit 22, a frame error rate (FER) monitoring unit 24 and a control signal generating unit 26. The channel estimation unit 22 is connected to the control unit 20 and to the control signal generating unit 26 for supplying thereto channel estimation data EST. The FER monitoring unit 24 is connected to the control unit 20 for supplying thereto data FER relating to a frame error rate, and the control signal generating unit is connected to the control unit 20 for receiving therefrom an activation signal SEND. The control signal generating unit 26 is also connected within the second station to transmitter circuitry thereof (not shown) for supplying to the transmitter circuitry a predetermined control signal CS.

operation of the Fig. 2 first and second stations will now be described.

In the use, the first and second stations 1 and 2 are in two-wa"y communication using respective first and second channels provided by the mobile communications network in which they are operating. The first channel provides for the transmission of signals from the first station 1 to the second station 2. The second channel provides for transmission of signals back from the second station to the first station 1. Thus, it will be appreciated that when the first and second stations are a base station and a mobile station respectively, the first channel is a downlink channel and the second channel is an uplink channel. When, on the other hand, the first and second stations are a mobile station and a base station respectively, the first channel is an uplink channel and the second channel is a downlink channel.

In this example, it will be assumed that the mobile communications network is a UTRA TD-CDMA network in which the f irst and second channels are provided on a time-division duplexing basis. In such a network, particularly when operating in an indoor environment (e.g. with Doppler spread of around 20Hz leading to 50msec channel coherence time), the variations of the f irst and second channels over a period of one or several frames (each frame is 10msec) are relatively small. Furthermore, it can be assumed, to a reasonable approximation, that the channel characteristics and variations for one of the first and second channels will be the same for the other of those two channels.

Taking advantage of the above channel characteristics, in the Fig. 2 embodiment channel estimation is carried out in the second station only, is and channel estimation data produced in the second station is transmitted to the first station, thereby avoiding the need for the first station to have its ow-n channel estimation function.

The channel estimation in the second station is performed for the first channel in conventional manner by the channel estimation unit 22. For example, information known to the second station (a known sequence of symbols) is inserted periodically, for example every timeslot, into the information stream of the first channel by transmitter circuitry (not shown) in the first station 1. The known information is usually included in a middle portion of the timeslot in which it is transmitted, so as to reduce the scope for transmission errors. In this case, the known sequence of symbols may be referred to as a "mid-amble".

Receiving circuitry (not shown) in the second station 2 detects the received sequence of symbols (mid-amble) and applies it to the channel estimation unit 22. The channel estimation unit 22 measures the impulse response of the first channel by solving an equation involving a known matrix containing shifted versions of the mid-amble and a column matrix containing the received mid-amble sequence. Further information on the channel estimation process may be found, for example in "Uplink channel estimation in synchronous CDMA mobile radio systems with joint detection", B. Steiner & T. Jung, Proceedings of International Symposium on Personal, Indoor and Mobile Radio Communications, September 1993, Yokohama, Japan, pp. 123-127.

The channel estimation unit 22 produces channel estimation data EST relating to the first channel. The channel estimation data includes data specifying, for example, a delay imposed by the first channel and a distortion imposed by the first channel.

The FER monitoring unit 24 continuously monitors a frame error rate (FER) of frames received at the second station. The monitoring data FER is received by the control unit 20. In this embodiment, the control unit employs the received data FER to detect a preselected change in the performance (characteristics) of the first channel, for example a change of more than a predetermined percentage in the frame error rate in a certain time period or when the frame error rate exceeds a predetermined threshold level.

Upon detecting that the preselected change in the performance of the first channel has occurred, the control unit 20 applies the activation signal SEND to the control signal generating unit 26. In response to the SEND signal, the control signal generating unit 26 generates a predetermined control signal CS which includes the channel estimation data EST produced by the channel estimation unit 22 in the second station 2.

This signal CS is applied to transmitter circuitry (not shown) in the second station which causes the predetermined control signal CS to be transmitted to the first station. The control signal may be transmitted to the first station via the second channel itself or via a separate control channel, if available, such as the common control channel CCH in the case in which the second station is a base station).

In the first station 1, such a transmitted predetermined control signal CS is detected by the control signal receiving unit 12 which extracts from the control signal the channel estimation data EST included therein. The extracted data EST is supplied to the control unit 10 in the first station 1 and is made available by the control unit 10 to receiving circuitry in the first station for use in carrying out receive processing in relation to the second channel.

Accordingly, there is no need for the first station 1 to include its own channel estimation unit.

Although the Fig. 2 embodiment detects a change in the performance of the first channel based on a frame error rate at the second station 2, it will be understood that a change in the first channel performance could be detected in many other ways. For example, a change could be detected based on a bit error rate (BER), or on a received signal strength (RSS) measure, or on the basis of a signal-to-noise and-interference (SNIR) measure, or on the channel estimate for the first channel itself, or based on a combination of any of the foregoing measures.

It is also not necessary to base the change detection on the performance of the first channel, as will now be described with reference to Figure 3.

Indeed, the change detection could be based on any channel between the first and second stations, not necessarily either of the first and second channels.

For example, if the first and second channels are traffic channels, the change detection could be based on a separate control channel, if available.

Fig. 3 shows a further embodiment in accordance with the first aspect of the present invention. In Fig. 3, parts which are the same as or correspond closely to parts already described with reference to the Fig. 2 embodiment are denoted by the same reference numerals as in Fig. 2.

The Fig. 3 embodiment includes a first station 11 and a second station 21, one of which is a base station and the other of which is a mobile station, as in the Fig. 2 embodiment.

In the Fig. 3 embodiment, the first station 1 1 includes, in addition to the control signal receiving unit 12 and a control unit 101 (generally similar to the control unit 10 in the Fig. 2 embodiment), a frame error rate (FER) monitoring unit 14 and a request signal generating unit 16. The FER monitoring unit 14 is connected to the control unit 10, for supplying frame error rate (FER) data thereto. The request signal generating unit 16 is connected to the control unit 101 for receiving an activation signal REQUEST therefrom. The request signal generating unit 16 is also connected to transmitter circuitry (not shown) in the first station for applying thereto a predetermined request control signal RCS.

The second station 21 does not include the FER monitoring unit 24 present in the second station 2 of the Fig. 2 embodiment. The remaining components of the Fig. 2 second station 2 are still present. The second station 21 in Fig. 3 further includes a request signal receiving unit 28 which is connected to the control unit 201 for applying thereto an activation signal REQUEST.

The Fig. 3 embodiment operates in generally the same way as the Fig. 2 embodiment in that channel estimation is performed exclusively in the second station and the channel estimation data EST produced in the second station is transmitted to the first station -16 so as to avoid the need for a channel estimation function in the first station. However, in the Fig. 3 embodiment the channel estimation data is transmitted from the second station to the first station when a change in performance of the second channel is detected by the control unit 101 in the first station 11 based on FER data for the second channel produced by the FER monitoring unit 14.

When the control unit 101 detects such a change, it applies the activation signal REQUEST to the request signal generating unit 16 in the first station 11. In response to the REQUEST signal the request signal generating unit generates the predetermined request control signal RCS which is applied to transmitter is circuitry in the first station. The RCS signal is then transmitted by the transmitter circuitry of the first station 11 to the second station 21 This transmission may be via the f irst channel itself or via a separate control channel, if available.

At the second station 21, the RCS signal is detected by the request signal receiving unit 28 which, in response thereto, applies the activation signal REQUEST to the control unit 20'. The control unit 201 applies the activation signal SEND to the control signal generating unit 26 onreceiving the REQUEST signal. Accordingly, as described previously with reference to the Fig. 2 embodiment, the predetermined control signal CS, embodying the channel estimation data EST produced by the channel estimation unit 22 for the first channel, is transmitted to the first station 19.

Fig. 4 shows an example of the signals transmitted via the f irst and second channels in the Fig. 2 embodiment in the case in which the f irst station is a base station and the second station is a mobile station, in other words when the f irst channel is a downlink channel and the second channel is an uplink channel. In this example, it is assumed for simplicity that there are three users, each with a mobile station (second station), in two-way communication with the base station (first station).

As shown in Fig. 4, each downlink timeslot (first channel transmission) includes a mid-amble portion sandwiched between respective start and end data portions. User data intended for the three users is sent in the start and end data portions using CDMA techniques to enable the data for the different users to be separated.

When no change is detected by a particular mobile station (second station) in its downlink channel performance (downlink FER) the entire uplink timeslot is utilised for transmitting user data. Only when the preselected change in channel performance is detected at a particular mobile station does that mobile station transmit the predetermined control signal CS embodying the channel estimation data EST for its downlink channel as determined in the mobile station concerned.

It will be appreciated that the Fig. 4 signalling is particularly advantageous for providing asymmetric operation services in which the uplink channel (second channel) is more capacity- limited than the first channel (downlink channel), for example file transfer (uploading). It is not necessary to reserve a mid amble portion of uplink timeslots, leaving the entire uplink timeslot duration available for transmission of data.

In addition, even if the uplink channel has little or no spare capacity for channel estimation purposes, the mobile station (second station) in this case can use the entire mid-amble portions of the non-capacity limited downlink timeslots to produce the required downlink channel estimate (channel impulse response), and the length of these mid-amble portions is really limited only by the duration of the downlink timeslots.

The delay spread which can be measured for the downlink can therefore be as long as the mid-amble portion used.

This enables the mobile station to estimate a wider range of delay spreads.

Fig. 5 shows an example of the signals transmitted via the first and second channels in the Fig. 2 embodiment in the case in which the f irst station is a mobile station and the second station is a base station. Again, as in the Fig. 4 example it is assumed that there are three mobile stations (first stations) in two-way communication with the same base station (second station).

In the Fig. 5 example, an uplink timeslot for each user (mobile station) contains a mid-amble portion sandwiched between respective start and end data portions. The mid-amble portions can contain different user-specific sequences of symbols for the different users in this case.

When no preselected change in the uplink channel performance from a particular mobile station is detected by the base station, the downlink timeslot for that particular mobile station is utilised entirely for the transmission of data to that particular mobile station. When, on the other hand, such a preselected change is detected, the downlink timeslot is utilised to send the predetermined control signal including the uplink channel estimate for that particular mobile station.

The Fig. 5 signalling is particularly advantageous when providing asymmetric-operation services in which the downlink channel (second channel) is more capacity limited than the uplink channel (first channel).

Because it is not necessary to reserve a mid-amble portion of each downlink timeslot for channel estimation purposes, the entire duration of each downlink timeslot. is made available for transmission of user data. Only when a preselected change in the channel performance is detected is a transmission of channel estimation data through the downlink channel required. This is particularly useful in relation to services such as cell broadcast, wireless internet and file transfer (downloading).

The channel estimation data can be transmitted from the base station to each mobile station on a common control channel CCH or via the downlink traffic channel TCH (first channel) itself.

Although, as described above in relation to Figs.

4 and 5, a mid-amble portion may be included in each first-channel timeslot, this may not be necessary in some situations, for example where the channel characteristics vary very slowly. In this case, the inclusion of a mid-amble portion in every first-channel timeslot is not necessary and the interval of inclusion could be increased to one mid-amble per several timeslots, resulting in a further capacity increase, but in this case for the first channel. It would even be possible for the transmission of the mid-amble to be requested by the second station in an intelligent dynamic manner, based on actual need, for example by monitoring the received FER and/or BER at the second station. Alternatively, the period between successive mid-amble transmissions could be negotiated by the base station and/or mobile station with the network at the time the service is set up.

A further advantage of the Fig. 5 signalling is that it can enable the mobile station complexity (handset complexity) to be reduced. As the 3S availability of processing power is not a major issue at the base station, it is highly attractive to confine the evaluation of the channel characteristics to the base station. If the downlink channel impulse response can validly be assumed to be a replica of the uplink channel impulse response, the uplink channel impulse response can be used to form at the base station the inverse matrix required for the downlink. The result may then be transmitted (via a CCH or a dedicated control channel DCCH or the TCH) to the mobile station to be used in its joint detection calculations. In this way, the need to perform power-hungry complex matrix inversion at the mobile station is eliminated, resulting in a less-complex mobile station handset.

According to a further aspect of the present invention, the inverse matrix data produced for the downlink in the base station can be transmitted at intervals independent of any detected change in the channel performance between the base station and mobile station, for example at fixed intervals, or at intervals negotiated between the network and base station/mobile station.

Figure 6 shows a third embodiment in accordance with the first aspect of the present invention. In Figure 6, parts which are the same as or correspond closely to parts already described with reference to the Figures 2 and 3 embodiments are denoted by the same reference numerals as in those figures.

The Figure 6 embodiment includes a first station and a second station 32, one of which is a base station and the other of which is a mobile station, as in the preceding embodiments.

In the Figure 6 embodiment, the first station 30 includes a control unit 31, a control signal receiving unit 34 and a channel estimation unit 36. The control signal receiving unit 34 is connected to the control unit 31 for applying an activation signal ACT thereto.

The control signal receiving unit 34 is also connected to the channel estimation unit 36 for applying thereto the activation signal ACT and received sequence data RSEQ. The channel estimation unit 36 is connected to the control unit 31 for applying channel estimation data EST thereto.

The second station 32 in the Figure 6 embodiment includes a channel estimation unit 22 and a FER monitoring unit 24, similar to the Figure 2 embodiment.

The second station 32 further includes a control unit 33 and a control signal generating unit 38. Unlike in the Figure 2 embodiment, the channel estimation data EST produced by the channel estimation unit 22 is not supplied to the control signal generating unit 38.

Instead, in the Figure 6 embodiment, the control signal generating unit 38 is connected to receive predetermined sequence information SEQ. As in the Figure 2 embodiment, the control signal generating unit 38 is connected to the control unit 33 for receiving therefrom an activation signal SEND and is also connected to transmitter circuitry (not shown) of the second station for applying a predetermined control signal CS to the transmitter circuitry.

The Figure 6 embodiment operates as follows.

Channel estimation for the first channel is carried out in conventional manner by the channel estimation unit 22 in the second station. The resulting channel estimation data EST for the first channel is supplied to the control unit 33 and is used exclusively in the second station 32 in this embodiment.

As in the Figure 2 embodiment, the FER monitoring unit 24 monitors the performance of the first channel and produces data FER representing the monitoring results. The control unit 33 employs the data FER to detect when a preselected change occurs in the first channel performance. As in the Figure 2 embodiment this change can be, for example, a change in the FER of more than a predetermined percentage in a given period, or could be when the FER exceeds a predetermined threshold value.

In response to detection of such a preselected change in the first-channel performance, the control unit 33 produces the activation signal SEND. In response to the SEND signal, the control signal generating unit 38 produces the predetermined control signal CS which, in this embodiment, embodies the predetermined sequence data SEQ instead of the channel estimation data EST produced by the channel estimation unit 22. The sequence data SEQ is, for example, a sequence of symbols known to the first station and usable thereby for channel estimation purposes.

The predetermined control signal is transmitted at a known time in a second-channel timeslot (such as a mid-amble portion) to the first station 30 via the second channel by the transmitter circuitry in the second station 32.

In the first station 30, the control signal receiving unit 34 detects the predetermined control signal CS in the received stream of symbols produced by receiver circuitry (not shown) of the first station.

Upon detecting such a predetermined control signal, the control signal receiving unit 34 produces the activation signal ACT which is applied both to the control unit 31 and to the channel estimation unit 36 in the first station 30. When the ACT signal is asserted the control signal receiving unit 34 also applies to the channel estimation unit 36 received sequence data RSEQ representing the sequence data SEQ included in the control signal CS as received by the first station.

In response to the ACT signal the channel estimation unit 36 is activated to perform channel estimation based on the received sequence data RSEQ and on knowledge of the data content of the sequence data SEQ transmitted from the second station 22. The channel estimation unit 36 produces estimation data EST for the second channel in conventional manner.

It will be appreciated that in the Figure 6 embodiment, the predetermined control signal CS directly embodies the sequence data needed for channel estimation purposes in the first station, and the predetermined control signal is transmitted via the second channel itself.

It will be appreciated that, in comparison with the proposed UTRA TD-CDMA mode network in which channel estimation is performed continuously for both the uplink and downlink channels, the Figure G embodiment can save bandwidth for the second channel by only performing channel estimation for the second channel when the preselected change in channel performance is detected. As in the previous embodiments, it is not necessary for the change in channel performance to be detected based on the first channel performance, although this is convenient for reducing the signalling required. The preselected change in channel performance could be detected in the first station 30 based on the performance of the second channel, as in the Figure 3 embodiment. In this case, in similar manner to the Figure 3 embodiment, a request signal could be transmitted from the first station 30 to the second station 32 to cause the second station control unit 33 to bring about transmission of the predetermined control signal including the sequence data SEQ. Alternatively, the preselected change in channel performance could be a change in performance of a channel other than the first and second channels.

In the Figure G embodiment, in addition to the control unit 33 in the second station asserting the activation signal SEND in response to the predetermined change in channel performance, the second station control unit 33 could also, as a backup measure, automatically assert the SEND signal a predetermined period after the last control signal CS was sent if, within that period, no preselected change in the channel performance has been detected at the second station.

Another aspect of the present invention will now be described with reference to Fig. 7.

Fig. 7 shows parts of a base station 40 and a mobile station 42 embodying the second aspect of the present invention. The base station 40 includes a control unit 50, a determining unit 52, a channel estimation unit 54, a FER monitoring unit 56, a control signal generating unit 58 and a control signal receiving unit 60. The determining unit 52 is connected to the control unit 54 for applying thereto a determining signal SELECT. The channel estimation unit 54 is connected to the control unit 50 and to the control signal generating unit 58 for applying thereto uplink channel estimation data UPEST. The FER monitoring unit is connected to the control unit 50 for applying thereto uplink frame error rate data FERUP.

The control signal generating unit 58 is connected to the control unit 50 for receiving therefrom an activation signal SENDDOWN. The control signal receiving unit 60 is connected to the control unit 54 for supplying thereto downlink channel estimation data DOWNEST.

The units 54, 56, 58 and 60 are also connected to the control unit 50 for receiving therefrom an enabling signal UPENABLE. The units 54, 56 and 58 have respective normal (active-high) enable inputs to which the UPENABLE signal is applied, but the unit Go has an active-low enable input to which the UPENABLE signal is applied.

The mobile station 42 includes a control unit 70, a channel estimation unit 72, a FER monitoring unit 74, a control signal generating unit 76, and a control signal receiving unit 78. The units 72, 74, 76 and 78 in the mobile station 42 correspond respectively to the units 54, 56,' 58 and 60 of the same name in the base station. The channel estimation unit 72 in the mobile station 42 is connected to the control unit 70 and to the control signal generating unit 76 in the mobile station for applying thereto downlink channel estimation data DOWNEST. The FER monitoring unit 74 is connected to the control unit 70 for supplying downlink frame error rate data (FERDOWN) thereto. The control signal generating unit 76 is connected to the control unit 70 for receiving therefrom an activation signal SENDUP. The control signal receiving unit 78 is connected to the control unit 70 for receiving therefrom uplink channel estimation data UPEST.

The units 72, 74, 76 and 78 are also connected in common to the control unit 70 for receiving therefrom an enabling signal DOWN ENABLE. The units 72, 74 and 76 have respective normal (active-high) enable inputs to which the DOWNENABLE signal is applied. The control signal receiving unit 78, on the other hand, has an active-low enable input to which the DOWNENABLE signal is applied.

operation of the Fig. 7 base station and mobile station will now be described.

When a new service is set up between the base station 40 and the mobile station 42, the determining unit 50 determines, according to the type of service being set up, which of the uplink and downlink channels between the base station and mobile station will be the less capacity- limited in use. For example, there are various kinds of asymmetric - operation service for which it can be predicted, at the time the service is set up that one or other of the uplink and downlink channels will have the lower data throughput. For example, in the case of a cell broadcast or wireless internet service, the uplink channel will be the less capacity limited channel. on the other hand, in the case of a file transfer (uploading), the downlink channel will be the less capacity-limited channel.

The determination signal SELECT, identifying the determined less capacitylimited channel, is supplied to the control unit 50. Based on the determination, the control unit 50 decides whether channel estimation is to be performed in the base station or in the mobile station.

When the less capacity-limited channel is determined to be the uplink channel the control unit 50 decides that channel estimation should be performed exclusively in the base station and activates (sets high) the UPENABLE signal. This enables operation of the channel estimation unit 54, the FER monitoring unit 56 and the control signal generating unit 58 in the base station. The control signal receiving unit 60 is disabled. In addition, the base station control unit informs the mobile station control unit 70, for example via the downlink traffic channel TCH itself or via a separate control channel such as the common control channel CCH, of its decision that the base station is to perform the channel estimation exclusively. On receipt of this information, the control unit 70 deactivates (sets low) its DOWNENABLE signal with the result that the channel estimation unit 72, the FER monitoring unit 74 and the control signal generating unit 76 in the mobile station are disabled.

The control signal receiving unit 78 in the mobile station, on the other hand, is enabled. The mobile station control unit 70 also configures its transmitter circuitry (not shown) to include, in each uplink timeslot or in suitably-spaced uplink timeslots, a mid amble portion or other known information needed by the channel estimation unit 54 in the base station for channel estimation purposes.

Once the UPENABLE and DOWNENABLE signals have been set in this way, channel estimation is performed for the uplink channel only by the channel estimation unit 54 in the base station. The base station control unit monitors the uplink frame error rate using the uplink frame error rate data FERUP produced by the FER monitoring unit 5G. The base station control unit 50 employs the FERUP data to detect a preselected change in uplink channel performance. When such a change occurs the control unit generates the SENDDOWN signal to activate the control signal generating unit 58 which then transmits a predetermined control signal CSDOWN including the uplink channel estimation data UPEST produced by the channel estimation unit 54. This downlink control signal CSDOWN is transmitted by transmitter circuitry (not shown) in the base station to the mobile station 42.

As explained previously in relation to the Fig. 2 embodiment, the signal CSDOWN may be transmitted via the downlink traffic channel TCH itself or via a separate control channel, if available.

In the mobile station, the transmitted CSDOWN signal is detected by the control signal receiving unit 78. The unit 78 extracts the uplink channel estimation data UPEST from the transmitted CSDOWN signal and applies it to the mobile station control unit 70 for use by receiving circuitry (not shown) in the mobile station.

When the determination made by the determining unit 52 at the time a service is set up is that the downlink channel will be the less capacitylimited channel, the base station control unit 50 decides that channel estimation should be performed exclusively in the mobile station 42 and the results of the channel estimation in the base station should be reported to the base station. In this case, the base station control unit 50 deactivates (sets low) the UPENABLE signal so as to disable the channel estimation unit 54, the FER monitoring unit 56 and the control signal generating unit 58 in the base station and so as to enable the control signal receiving unit 60 in the base station. The base station control unit 50 also informs the mobile station control unit 70 of its decision and the mobile station control unit 70 activates (sets high) enables its DOWNENABLE signal so as to enable operation of the channel estimation unit 70, FER monitoring unit 74 and control signal generating unit 76 in the mobile station and so as to disable the control signal receiving unit 78 in the mobile station.

Thereafter, operation proceeds analogously to the operation described above in which channel estimation is performed exclusively in the basestation, but in this case channel estimation is performed exclusively in the mobile station.

A number of variations on the Fig. 7 embodiment are possible.

It is not necessary for the determining unit to be in the base station 40. It could be in the mobile station 42 or elsewhere in the network, for example in a base station controller BSC (Fig. 1). Also, the function of the determining unit could be distributed across the base station, mobile station and other parts of the network, as appropriate.

In the Fig. 7 embodiment, the station which performs the channel estimation also detects the change in channel performance used to trigger transmission of channel estimation data to the other station. However, as described previously in relation to the Fig. 3 embodiment, the change in channel performance could be detected in the station that is not performing the channel estimation. In this case, as in the Fig. 3 embodiment, the station which performs the detection would send a request signal to the station carrying out the channel estimation, requesting that station to send it updated channel estimation data.

Furthermore, in the Fig. 7 embodiment it is not necessary for transmission of the channel estimation data to occur based on detecting a change in the channel performance. It would be possible to simply configure the control unit in the station performing the channel estimation to send the channel estimation data at regular intervals which could be fixed intervals or intervals negotiated in the service set-up process.

It would also be possible for the determining unit to operate at other times than when a service is set up. For example, there may be certain kinds of information transfer service which, at one time, call for a higher throughput on one channel (e.g. the downlink channel) and at another time call for higher throughput on the other (uplink) channel. By having the determining unit operate continuously during provision of the service, it would be possible to switch over the station performing the channel estimation so as to achieve the highest possible throughput of data at all times.

Although the present invention has been described above in relation to a UTRA network, it will be appreciated that it can also be applied to any other networks in which it is possible to assume that, to a reasonable approximation, the uplink and downlink channel characteristics will be the same as or similar to one another, for example any network which have a TDD mode of operation. These networks could be, or could be adapted f rom, other CDMA networks such as a wideband CDMA (W-CDMA) network or an IS95 network.

These networks could also be, or be adapted f rom, other mobile communication networks not using CDMA, for example networks using one or more of the following multiple-access techniques: time-division multiple access (TDMA), wavelength-division multiple access (WDMA), frequencydivision multiple access (FDMA) and space-division multiple access (SDMA), provided that it is possible to assume that, to a reasonable approximation, the uplink and downlink channel characteristics will be the same as or similar to one another.

Although embodiments of the present invention have been described as having distinct "units" such as the channel estimation unit, those skilled in the art will appreciate that a microprocessor or digital signal processor (DSP) may be used in practice to implement some or all of the functions of the base station and/or mobile station in embodiments of the present invention.

Claims

CLAIMS:

1 A two-way communication method, for use in a mobile communications network including respective first and second stations and having a first channel for transmitting signals from the first station to the second station and a second channel for transmitting signals from the second station to the first station, one of the first and second stations being a base station of the network and the other station being a mobile station of the network, the method including:

detecting a preselected change in communications channel performance between the first and second stations and, in response to such detection, transmitting a predetermined control signal from the second station to the first station; at the first station, receiving the predetermined control signal and employing that signal to derive a predetermined measure of performance of the said second channel.

2. A method as claimed in claim 1, comprising:

at the second station, producing a predetermined measure of performance of the first channel and including that measure of first-channel performance in the transmitted control signal; and at the first station, deriving the predetermined measure of second-channel performance from the measure of first-channel performance included in the transmitted control signal.

3. A method as claimed in claim 2, wherein the said predetermined measure of performance of the second channel is made equal to the predetermined measure of performance of the said first channel included in the transmitted control signal.

4. A method as claimed in 2 or 3, wherein one or each said measure of performance is an estimate of channel impulse response.

S. A method as claimed in claim 1, wherein the predetermined control signal is transmitted via said second channel and includes predetermined control information known to the first station, and the first station employs the known information and the control information included in such a control signal received via said second channel to derive the said predetermined measure of second-channel performance.

G. A method as claimed in claim 5, wherein the predetermined control signal, when transmitted, constitutes a midamble portion of a time slot of said second channel.

7. A method as claimed in any preceding claim, wherein, in the event that no such preselected change in commund cations -channel performance is detected user data and/or control information other than said predetermined control signal is transmitted by the second station to the first station.

8. A method as claimed in any preceding claim, wherein the first station is a mobile station, the second station is a base station, the first channel is a downlink channel and the second channel is an uplink channel.

9. A method as claimed in claim 8, comprising the steps of:

calculating in the base station a channel impulse response of the uplink channel; carrying out a matrix inversion operation in the base station to derive from the uplink channel impulse response an inverse of a matrix containing an assumed downlink channel impulse response; including data of the inverse matrix in the predetermined control signal transmitted by the base station to the mobile station; and in the mobile station, receiving the inverse matrix data and using it to derive a downl ink- channel performance measure.

10. A method as claimed in any preceding claim, wherein the said preselected change in communications channel performance is detected at the said second station.

11. A method as claimed in any one of claims 1 to 9, wherein the said preselected change in communications -channel performance is detected at the said first station, the method comprising the further step of transmitting a request signal from the first station to the second station following detection in the first station of the preselected change and transmitting the said predetermined control signal from the second station to the first station in response to the request signal.

12. A two-way communication method, for use in a mobile communications network including a base station and a mobile station and having a downlink channel for transmitting signals from the base station to the mobile station and an uplink channel for transmitting signals from the mobile station to the base station, the method comprising:

determining which of the said downlink and uplink channels is the less capacity- limited channel; and performing channel estimation only for the determined less capacity- limited channel.

13. A method as claimed in claim 12, wherein a channel estimate for the determined less capacity limited channel is produced by the station at the receiving end of that channel, and information relating to that channel estimate is transmitted at intervals to the station at the transmitting end of the less capacity- limited channel and is employed in that station to produce a channel estimate for the more capacity-limited channel.

13. A method as claimed in claim 12, wherein the said intervals are fixed intervals.

14. A method as claimed in claim 12 or 13, wherein the said intervals are negotiated by the base station and/or mobile station with the said network when one or both of the said channels are set up.

15. A method as claimed in claim 12, wherein the said intervals are varied in dependence upon a measure of communications-channel performance between the base station and the mobile station.

16. A method as claimed in claim 12, wherein the said information relating to the channel estimate for the less capacity- limited channel is transmitted upon detecting a predetermined change in communications channel performance between the base station and the mobile station.

17. A method as claimed in any one of claims 11 to 16, wherein the determination of the less capacity limited channel is carried out when a service between the base station and the mobile station is set up.

18. A method as claimed in any one of claims 11 to 17, wherein the determination of the less capacity limited channel is repeated at intervals during the course of providing a service between the base station and the mobile station.

19. A method as claimed in any one of claims 1 to 11 or 16, wherein the said preselected change in communications -channel performance is detected based on one or on a combination of the following measures of performance of one or more communications channels between the base station and the mobile station: a frame error rate; a bit error rate; a received signal strength; a signal - to-noise -and- interference ratio; a channel estimate; a channel impulse response.

20. A two-way communication method, for use in a mobile communications network including a base station and a mobile station and having a downlink channel for transmitting signals from the base station to the mobile station and an uplink channel for transmitting signals from the mobile station to the base station, the method including:

calculating in the base station a channel impulse response of the uplink channel; carrying out a matrix inversion operation in the base station to derive from the uplink channel impulse response an inverse of a matrix containing an assumed downlink channel impulse response; is transmitting data of the inverse matrix from the base station to the mobile station; and in the mobile station, receiving the inverse matrix data and using it to derive a downlink- channel performance measure.

21. A mobile communications network including respective first and second stations and operable when in use to provide a first channel for transmitting signals from the first station to the second station and a second channel for transmitting signals from the second station to the first station, one of the first and second stations being a base station of the network and the other station being a mobile station of the network; the network also including detection means for detecting a preselected change in communications channel performance between the first and second stations; the second station comprising control signal transmission means operable, in response to such detection, to transmit a predetermined control signal to the first station; and the f irst station comprising control signal receiving means for receiving the predetermined control signal, and also comprising performance measuring means for employing the received control signal to derive a predetermined measure of performance of the said second channel.

22. A mobile communications network including a base station and a mobile station and operable when in use to provide a downlink channel for transmitting signals from the base station to the mobile station and an uplink channel for transmitting signals from the mobile station to the base station, the network comprising:

determining means for determining which of the said downlink and uplink channels is the less capacity limited channel; and channel estimation means operable to perform channel estimation only for the determined less capacity-limited channel.

23. A network as claimed in claim 22, wherein the base station and the mobile station each includes channel estimation means and associated control means, and the respective control means of the base station and the mobile station are co-operable, in dependence upon the determination made by the determining means, to disable channel estimation by the channel estimation means of the station at the transmitting end of the determined less capacity- limited channel and to enable channel estimation by the channel estimation means of the station at the receiving end of that channel.

24. A mobile communications network including a base station and a mobile station and operable when in use to provide a downlink channel for transmitting signals from the base station to the mobile station and an uplink channel for transmitting signals from the mobile station to the base station, the base station including:

calculating means for calculating a channel impulse response of the uplink channel; matrix inversion means for carrying out a matrix inversion operation to derive from the uplink channel impulse response an inverse of a matrix containing an assumed downlink channel impulse response; and transmission means for transmitting data of the inverse matrix from the base station to the mobile station; and the mobile station including receiving means for receiving the inverse-matrix data, and also including performance measuring means for employing the received inverse-matrix data to derive a downlink- channel performance measure.

25. A first station of a mobile communications network, the network further including a second station and being operable when in use to provide a first channel for transmitting signals from the first station to the second station and a second channel for transmitting signals from the second station to the first station, one of the first and second stations being a base station of the network and the other station being a mobile station of the network; the first station comprising:

control signal receiving means for receiving a predetermined control signal transmitted thereto by the second station in response to detection of a preselected change in communications -channel performance between the first and second stations; and performance measuring means for employing the received control signal to derive a predetermined measure of performance of the said second channel.

26. A second station of a mobile communications network, the network further including a first station and being operable when in use to provide a f irst channel for transmitting signals from the first station to the second station and a second channel for transmitting signals from the second station to the first station, one of the first and second stations being a base station of the network and the other station being a mobile station of the network; the second station comprising:

performance measuring means operable to produce a predetermined measure of performance of the first channel; and control signal transmission means operable, in response to detection by the network of a preselected change in communicat ions -channel performance between the first and second stations, to transmit to the first is station a predetermined control signal including the said predetermined measure of first-channel performance.

27. The base station of a mobile communications network as claimed in any one of claims 21 to 24.

28. The mobile station of a mobile communications network as claimed in any one of claims 21 to 24.

29. A two-way communication method substantially as hereinbefore described with reference to the accompanying drawings.

30. A mobile communications network substantially as hereinbefore described with reference to the accompanying drawings.

31. A base station substantially as hereinbefore described with reference to the accompanying drawings.

32. A mobile station substantially as hereinbefore described with reference to the accompanying drawings.