Adaptive radio communications system

This invention relates to radio communication networks and more particularly, but not exclusively, to radio communication networks using multiple access techniques. A system and method for adaptively changing the characteristics of a signal transmitted across the network is provided. In one embodiment, the communications network includes at least two transceivers; wherein at least one of the transceivers is capable of sending a feedback signal to the other transceiver after receiving a signal transmitted over the network from the other transceiver after analysis of the transmitted signal and in the event that the signal characteristics of the system need to be varied.

FIELD OF THE INVENTION
 This invention relates to radio communications and in particular relates to
 a method for assigning radio frequency bearers in a radio communications
 system which is subject to slow temporal fading.
 BACKGROUND TO THE INVENTION
 In radio communications systems such as GSM digital mobile radio protocol,
 the communications channel hops from one frequency band to another
 according to a specified routine. The system overcomes the effects of
 fading, scattering and other transmission problems on a particular channel
 by swapping channels and providing an average of the signal strength of
 the channels available, which will provide a sufficient signal. Obstacles
 in a signal path, such as buildings in built-up areas and hills in rural
 areas, act as signal scatterers and can cause signalling problems. These
 scattered signals interact and their resultant signal at a receiving
 antenna is subject to deep and rapid fading and the signal envelope often
 follows a Rayleigh distribution over short distances, especially in
 heavily cluttered regions.
 A receiver moving through this spatially varying field experiences a fading
 rate which is proportional to its speed and the frequency of the
 transmission. Since the various components arrive from different
 directions, there is also a Doppler spread in the received spectrum. If
 the channel allocation was static, then as the subscriber, for example,
 moved to an urban environment where signal reflections affected the
 particular frequency in which the channel was operating more than other
 frequencies, then the channel which was previously best then becomes poor.
 In fact such movement may produce a break in communications. In fixed
 radio applications, the problems of fading still exist but are not so
 rapid; in a fixed system, the best channel would be likely to stay the
 best signal for a period of time. Some mobile radio protocols are
 similarly inflexible; in the case of DECT, dynamic channel assignment only
 applies when a call is set up.
 In fixed radio applications, the problems of fading still exist but are not
 so rapid; in a fixed system, the best channel would be likely to stay the
 best signal for a period of time. Frequently, the fading follows a
 Rayleigh distribution.
 In radio communications, signals are transmitted at a particular frequency,
 in a frequency band or in several frequency bands. The signals may be
 modulated in a variety of fashions using techniques such as Time Division
 Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and a
 multitude of other techniques. Nevertheless there are a finite number of
 available individual communications channels for separate sets of parties
 to communicate with each other.
 A radio communications system of the TDMA-TDD type is designed so that a
 multiple frame is made up of a plurality of frames each divided into a
 plurality of time slots; each base station selects an idle time slot of a
 multiple frame for transmission of the control-channel signal to send
 control-channel information to the associated receiver at intervals of a
 multiple frame period. FIG. 1 is a timing chart showing the relationship
 of the transmission and reception of signals between an base station (BS)
 and an outstation (OS).
 In FIG. 1, a block of up-link signal time slots and a block of down-link
 signal time slots have four slots respectively. The time slots of each
 frame are divided into a block of down-link signal (for communication from
 the base station to the out station) slots 10 (down-link signal slot block
 10) and a block of up-link signal (for communication from the out station
 to the base station) slots 20 (up-link signal slot block 20), and the
 aforementioned slot for transmission of the control-channel signal
 directed to the out station (which slot will be referred to as the
 down-link control-channel slot, hereinafter) is selected from the
 down-link signal slots of the block of a frame (for example, a time slot 4
 in FIG. 1 is selected).
 The transmission of the control-channel signal from the mobile station to
 the base station is carried out at one (which will be referred to as the
 up-link signal slots of each frame having a corresponding positional
 relationship with the above down-link control-channel slot. e.g., located
 as shifted by a half frame from the down-link control-channel slot. For
 example, when the time slot 4 in FIG. 1 is used as the down-link
 control-channel signal slot, a time slot 8 shifted by a half frame from
 the time slot 4 is used as the up-link control-channel slot. The remaining
 slots (time slots 1, 2, 3, 5, 6 and 7 in FIG. 1) of the up and down-link
 signal slot blocks of each frame other than the up and down-link
 control-channel signal slots are used as slots for communication of data
 information between the base station and out station.
 Each base station transmits the control-channel signal at intervals of the
 multiple frame period with use of a signal carrier of an identical
 frequency commonly used by the other base stations and also with use of
 the down-link control-channel slot of the specific frame selected by its
 own base station. With respect to the frames of each multiple frame other
 than the specific frame, ones of the down-link signal slots located to
 correspond to the down-link signal slots located to correspond to the
 down-link control-channel slot, e.g. located as shifted by one frame are
 not effectively used. Each base station assigns specific up-and down-link
 traffic-channel slots of each frame to each of the out stations under the
 jurisdiction of the base station and assigns a frequency to one selected
 from a plurality of predetermined channels. Accordingly, each out station
 communicates with the base station and another out station via the base
 station at intervals of each of the frames of the multiple frame with use
 of the traffic channel slots specified by the base station.
 A disadvantage of employing such schemes, however, is that the numbers of
 time slots for actual transmission of data are reduced by the presence of
 these control-channel slots which represent large overheads, and
 inevitably reduce system capacity. These control-channel slot overheads
 detract from the gains in efficiency achieved by the use of adaptive
 techniques. Where training sequences are employed over an asymmetrical
 channel only an approximation of the forward channel characteristics can
 be determined, further reducing the optimisation that can be achieved.
 Where training sequences have not been employed, systems have tended to
 rely on each transmitter analysing the characteristics of received signals
 transmitted from the other end of the circuit. However during data
 transmission, the majority of information tends to flow in one direction,
 e.g. during transmission of a large data file. Where transmission time is
 long, the channel conditions may change sufficiently that the
 characteristics of the transmitted signal are no longer optimal. However,
 as the majority of information flows in one direction only, the
 transmitter does not receive information relating to required changes in
 signal characteristics.
 A slow adaptive modulation System (AMS) has been proposed for future
 multi-media communication systems. This slow adaptive modulation system
 assigns modulation parameters, as well as the number of slots, in each
 TDMA frame according to the average channel conditions, such as the
 average carrier-to-noise plus interference power ratio (C/(N+I)), and the
 average delay spread.
 In this system, when the average (C/(N+1) during a call is high, and the
 average delay spread is small, a higher modulation level and/or higher
 symbol rate, and a smaller number of slots are assigned to increase system
 capacity without degrading the transmission quality. Conversely, when the
 average C/(N+I) is low, or average delay spread is large, a lower
 modulation level and/or lower symbol rate, and a larger number of slots,
 are assigned to improve transmission quality. As a result, the slow AMS
 can increase system capacity by mitigating the effect of spatially
 distributed electric field strength variation. Various dynamic channel
 assignment (DCA) algorithms have been proposed to effectively assign
 channels in microcellular systems. In DCA, the whole of the channel is
 shared by all the base stations (BSs), and any channel with an average
 C/(N+I) larger than a threshold value is available for each terminal. As a
 result, DCA can increase system capacity by mitigating spatially and
 temporally distributed traffic. Adaptive modulation dynamic channel
 assignment schemes (AMDCA) take up valuable overhead space and, at
 present, are unlikely to be widely implemented as such.
 OBJECT OF THE INVENTION
 It is an object of the present invention to provide a system and method
 which overcomes the above disadvantages and seeks to provide an improved
 form of channel re-allocation in a radio communications system.
 SUMMARY OF THE INVENTION
 According to a first aspect of the present invention, there is provided a
 communications network including at least two stations for transmitting
 and receiving; wherein at least one of the stations is capable of sending
 a control signal to the other station after receiving a signal transmitted
 over said network from said other station after analysis of the
 transmitted signal, which control signal acts as a feedback signal and is
 transmitted as a data packet independent of the other data and overhead
 signals; and, in the event that the signal characteristics of the system
 need to be varied, the signalling characteristics are varied in a
 sequential fashion.
 The data packet is transmitted independently of the rate and type of the
 other data and overhead signals, although could take, for example a data
 slot normally reserved for a TDMA transmission. The control signal could
 also be transmitted as an overhead on an infrequent basis. In this
 fashion, the control signal is transmitted by way of a data packet whereby
 no specific system overheads are required, thereby providing increased
 system capacity.
 By providing a system wherein the signalling characteristics are varied in
 a known, sequential fashion, the data relating to changes of frequency and
 other parameters, need not be retained within the control portions of data
 signals. This reduces the overheads of the signalling protocol, thereby
 providing increased system capacity.
 In accordance with another aspect of the present invention, there is
 provided a multi-bearer communications network comprising carrier
 allocation means, feedback means for a multi-bearer communications network
 response to channel variation; wherein responsive to a degradation in
 signal quality identified by a feedback mechanism, the signal
 characteristics of the link are varied in a sequential fashion.
 Preferably, the signalling techniques that are varied can be selected from
 a range of frequency changing schemes, adaptive modulation techniques and
 coding techniques. Preferably, the feedback signal is provided as a data
 packet, which can be a slot in a multiple access system. The feedback
 signal can be transmitted as a random access slot. The feedback signal can
 advise the transmitter to reduce or increase signal power, change
 modulation scheme, frequency or other types of signal parameters. The
 communications system can be a radio communications system, but can find
 applicability in other communications systems employing other signalling
 media.
 According to a still further aspect of the present invention, there is
 provided a method of communicating over a communications network including
 at least two stations; said method comprising the steps of:
 1) transmitting a signal from a first station of the communications network
 to a second station of the communications network;
 2) receiving at the second station a signal transmitted over said circuit
 by a transmitting end;
 3) analysing the transmission characteristics of said transmitted signal;
 4) determining whether the signal characteristics need to be varied and;
 5) in the case that the signal characteristics need to be varied
 transmitting a control signal as a feedback signal independent of other
 data and overhead signals to the first station in response to said
 analysis of said received signal characteristics whereby the transmitter
 is instructed to change the signalling characteristics in a sequential
 fashion.
 Preferably, the feedback signal is provided as a data packet, which can be,
 for example, a slot in a multiple access system. The feedback signal can
 be transmitted as a random access slot. The feedback signal can advise the
 transmitter to reduce or increase signal power, change modulation scheme,
 frequency or other types of signal parameters. The communications system
 can be a radio communications system, but can find applicability in other
 communications systems employing other signalling media.
 In accordance with another aspect of the invention, there is provided a
 method of communicating over a multi-bearer communications network
 wherein, responsive to a degradation in signal quality identified by a
 feedback mechanism, the signal characteristics of the link are varied in a
 sequential fashion.
 The signal characteristic that is varied can be a modulation type such as
 quadrature phase shift keying or quadrature amplitude modulation. The
 signal characteristic that is varied could relate to the slot size or
 repetition rate. The frequency or frequency band of operation is a further
 signal characteristic that can be varied.
 The invention is particularly suited to situations where slow temporal
 fading is present as the main form of fading such as a fixed radio access
 telecommunications environment: the fading should not be faster than the
 adaptation time constant associated with the system, which inter alia is
 dependent upon the processing rate of the system and the data rate.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring now to FIG. 2, there is shown a block diagram of one particular
 system mode in accordance with the present invention. In the system
 described, a voice/data signal (herein after referred to as the signal) is
 transmitted across a channel where it is received by a receiving end of
 the circuit.
 At the receiving end the transmitted signal, the signal is analysed to
 determine the transmission characteristics of the signal. Parameters such
 as attenuation, noise interference levels etc. are assessed. At the
 receiving end the transmitted signal, the signal is analysed to determine
 the transmission characteristics of the signal. If it is determined that
 the signal quality is varying outside an acceptable boundary or beyond an
 acceptable threshold, then a feedback signal is transmitted back to the
 transmitter.
 This control signal is transmitted as a data packet independent of the
 other data and overhead signals, although could take, for example a data
 slot normally reserved for a TDMA transmission. The control signal could
 also be transmitted as an overhead on an infrequent basis. The parameters
 are communicated via the transmit path of the receiving end of the
 circuit. The transmitter then adapts its scheme of transmission on a
 sequential basis. In this fashion, the control signal is transmitted by
 way of a data packet whereby no specific system overheads are required,
 thereby providing increased system capacity.
 The advantages provided by this system are that the capacity of the system
 can be improved by the reduction of overheads in the signalling protocols.
 The system can improve capacity on a cell-by-cell basis. The system is
 particularly appropriate to communications networks which operate under
 conditions of slow fades and the like, as is common in fixed radio
 communications systems. The system also provides flexibility in the number
 of communications that may be conducted within a cell.
 The signal characteristic that is varied can be a modulation type such as
 quadrature phase shift keying or quadrature amplitude modulation. The
 signal characteristic that is varied could relate to the slot size or
 repetition rate. The frequency or frequency band of operation is a further
 signal characteristic that can be varied.
 The invention is particularly suited to situations where slow temporal
 fading is present as the main form of fading such as a fixed radio access
 telecommunications environment: the fading should not be faster than the
 adaptation time constant associated with the system, which inter alia is
 dependent upon the processing rate of the system and the data rate.
 The transmitting end of the circuit will receive these parameters and then
 adapt the characteristics of the transmit path. To analyse the signals,
 standard signal analysis techniques can be used to derive signal
 characteristics.
 The feedback signal can be transmitted by way of a data packet whereby no
 specific system overheads, such as training sequences and the like are
 required. This contrasts with the techniques used, for example, in GSM,
 where a dedicated portion of each data packet is allocated to signal
 administration functions. This simplifies the signalling and increases the
 system capacity. In another embodiment, the feedback packets could be
 transmitted on a contention basis with a reduced overhead scheme.
 The use of network/system information and/or commands can be used to assist
 with determining whether trade offs between signal parameters can achieve
 acceptable network/system performance.
 FIG. 3 is a block diagram of a network employing multiple access techniques
 typically a radio frequency multiple access system. The diagram assumes
 that the out station which could be a mobile unit (MU) or a residential
 subscriber unit (RSS) is transmitting to a base station. Signal analysis,
 however, could be carried out in a residential subscriber unit or mobile
 unit but this could increase outstation unit cases. The out station could
 also provide the base station with transmit signal optimisation
 instructions.
 Signal characteristics can be agreed upon by both parties during call set
 up or may be a system default process. These parameters are stored in a
 transmitted signal parameters memory detailed in FIG. 3. Where default
 signal characteristics are used, these characteristics are similarly
 stored. During a signal optimisation procedure, the system may have
 reference to these signal characteristics when generating optimising
 instructions. It is preferred that this store be updated with optimised
 characteristics when optimisation instructions are issued to the
 transmitter. By retaining and updating a central store of transmitted
 signal characteristics, the system reduces the need to perform channel
 modelling and signal optimisation routines.
 Allocation of circuit instructions to particular frames could follow a
 number of methods according to the systems priorities with regard to
 optimisation. For example, a system may be aiming to achieve maximum data
 throughput during a period of heavy data use. It may be desirable to
 modify the encoding algorithms used to achieve this. In this case, the
 system may allocate slots on successive frames in order of heaviest user
 to lightest user. Alternatively, high system quality may be preferred. In
 such an instance, slots on successive frames may be allocated to those
 circuits with highest quality transmission before improvements in poor
 quality signals may be effected. Other types of allocation techniques
 could be used. For example, a circuit may request to transmit optimisation
 information only when it is determined that the transmitted signal falls
 outside optimum signal parameters. In such an instance, the optimisation
 information may be allocated to contention slot transmission.