MIMO (Multiple-Input-Multiple-Output) is a type of radio communication that exploits multipath propagation to increase the capacity and/or performance of the network. It is a mature subject in both industry and academia and is an essential element of various wireless communication standards.

A MIMO network could be, for example, a cellular network in which a base station has multiple antennae for communication with mobile stations within its cell. In known MIMO systems, precoding is performed on signals prior to transmission in order to improve signal quality by addressing interference (through interference cancellation and/or beamforming).

Although the use of MIMO provides increased capacity and/or performance, it has certain problems. In particular, providing a high signal to noise ratio is challenging, due to resource scarcity, as is providing a low error rate. These issues may be particularly important to some users.

It is desirable to provide a method which addresses some or all of the above-mentioned and/or other disadvantages of the prior art.

According to a first aspect of the invention there is provided a method of processing a plurality of multiple-input multiple-output (MIMO) signals for transmission over a plurality of communication channels, the method comprising: precoding the plurality of signals prior to transmission, the precoding incorporating channel state information in relation to the plurality of communication channels, the precoding further incorporating: a priority ranking of a first channel of the plurality of communication channels relative to a second channel of the plurality of communication channels; and/or an error rate required by a channel of the plurality of communication channels; and/or an amount of noise experienced by a channel of the plurality of communication channels.

An advantage of this method is that it enables resources to be diverted to channels based on their priority, bit error ratio demands and/or the amount of noise the channels are experiencing. Taking these factors into account during the precoding stage, as opposed to other stages, enables resources to be allocated more effectively.

The error rate may be a bit error rate and may be a bit error ratio. A bit error ratio is the ratio of corrupted bits to successfully received bits.

The method may be performed at a MIMO transmitter which may be located within a base station of a cellular network and/or within a mobile station of the cellular network. The method may comprise transmitting the plurality of signals over the plurality of channels to one or more receivers. The one or more receivers may be located in mobile stations and may be within the cellular network. The one or more receivers may operate as conventional MIMO receivers. Alternatively, or in addition, the receiver may be located in the base station.

The method may comprise a preliminary step of encoding the plurality of signals and/or mapping the plurality of encoded signals onto constellation points. The encoding step may be performed by a symbol encoder and the mapping step may be performed by a mapper.

Following precoding, the method may further comprise performing an inverse discrete Fourier transform on the plurality of signals. The plurality of signals may then be passed to an analogue front end of the transmitter and transmitted.

The method may further comprise determining a channel matrix containing the channel state information. Elements of the channel matrix may comprise the channel coefficients associated with the plurality of channels. The method may comprise determining a precoding matrix by modifying the channel matrix using an indication of the priority ranking and/or an indication of the bit error ratio and/or an indication of the amount of noise experienced by the channel of the plurality of channels. This may be performed at a single component within the transmitter. The single component may be a management component.

The method may further comprise using the precoding matrix to precode the plurality of signals to be transmitted and may comprise multiplying the signal to be transmitted by the precoding matrix. This may be performed at a precoder which may be a discrete component of the transmitter and may be separate from the management component.

The step of modifying the channel matrix using the indication of priority ranking may comprise re-ordering the elements of the channel matrix in accordance with the indication of the priority ranking. The step of modifying the channel matrix may comprise using the indication of priority ranking and may comprise reducing one or more channel coefficients associated with a highest-ranked channel or highest- ranked plurality of channels, preferably to a value of zero.

The step of modifying the channel matrix using the indication of bit error ratio may comprise modifying one or more elements in accordance with the bit error rate required by a channel of interest. This may involve modifying one or more elements of the channel matrix associated with the channel of interest and may comprise reducing the value of the one or more elements. The one or more elements may be indicative of the interference caused to the channel of interest by one or more of the other channels. The step of modifying the one or more elements may comprise multiplying the one or more elements with a gap function associated with the channel of interest. The gap function may be defined as: where ber is the bit error rate required by the channel of interest and q is the lowest QAM modulation order.

The step of modifying the channel matrix using the indication of noise may comprise modifying one or more elements in accordance with the noise experienced by a channel of interest. This may involve modifying one or more elements of the channel matrix associated with the channel of interest and may comprise reducing the value of the one or more elements. The one or more elements may be indicative of the interference caused to the channel of interest by one or more of the other channels.

The step of determining a channel matrix containing the channel state information may comprise transmitting training signals into each channel and measuring the return signals. This may be performed at a discrete component of the transmitter which may be a channel state information component.

The method may further comprise receiving information relating to the indication of priority ranking and/or the indication of bit error ratio and/or the indication of noise. This information may be received at a service information component and may be received from one or more users of the plurality of channels. The service information component may be a discrete component within the transmitter. The method may further comprise using this received information to determine the indication of priority ranking and/or the indication of bit error ratio and/or the indication of noise.

The precoding may comprise channel multiplexing and/or beamforming.

According to a second aspect of the invention there is provided a system for processing a plurality of multiple-input multiple-output (MIMO) signals for transmission over a plurality of communication channels, the system comprising: a precoder adapted to precode the plurality of signals prior to transmission, the precoding incorporating channel state information in relation to the plurality of communication channels, the precoding further incorporating: a priority ranking of a first channel of the plurality of communication channels relative to a second channel of the plurality of communication channels; and/or an error rate required by a channel of the plurality of communication channels; and/or an amount of noise experienced by a channel of the plurality of communication channels.

The precoder may be adapted to receive channel state information in relation to the plurality of communication channels from the channel state information component.

The system may further comprise a management unit adapted to provide precoding information to the precoder, the precoding information incorporating the channel state information and/or the indication of a priority ranking and/or the indication of a bit error ratio and/or the indication of noise. The management unit may be adapted to receive the channel state information from the channel state information component. The management unit may be adapted to receive the indication of a priority ranking and/or the indication of a bit error ratio and/or the indication of noise from the service information component.

A specific embodiment of the invention will now be described, for illustration only, and with reference to the enclosed drawings, in which:

Fig 1 is a schematic view of a network in accordance with the invention;

Fig 2 is a schematic view of a transmitter adapted to perform a method in accordance with the invention;

Fig 3 is a flow chart showing a method in accordance with the invention.

Fig. 1 shows a cellular network in accordance with the invention. The network is shown generally at 4. In particular there is a cellular base station 5 and mobile stations 1 , 2 and 3. Mobile stations 1 , 2 and 3 are located within the cell of the base station 5 and communicate with base station 5.

Fig 2 shows a MIMO transmitter 5 in accordance with the invention. The transmitter 5 is located within base station 5 of Fig 1 . The transmitter 5 comprises an array of N symbol encoders and mappers 6. Three components of the array are shown for illustration. The symbol encoders encode users’ bit stream data using a given coding scheme e.g. convolutional, LDPC or turbo. This is done to combat errors arising from the background noise of the channel. The mappers map subsets of the encoded data into constellation points with distinct phase and amplitude. The length of the subsets is determined via the modulation order and can be optimised via bit loading.

The transmitter 5 comprises a MIMO channel entity 7. The MIMO channel entity 7 performs a channel estimation. It does so by transmitting training signals (not shown) on the channels and measuring the effect the channel has on the training signals. From this a channel matrix H is determined. H comprises channel coefficients. An example of H is: The elements hij are channel coefficients, and are a measure of the interference experienced on channel i as a result of the signal transmitted on channel j. The MIMO channel entity 7 measures the channel coefficients, constructs channel matrix H, and sends H to a MIMO Management Unit 8.

There is also a traffic demands entity 11 . This obtains a measure of the demands on the channels. For example, it obtains prioritisation information that enables it to rank the different users (ie mobile stations 1 , 2 and 3 in Fig 1) in order of importance. An ambulance, for example, may be considered a user of high importance and given a high ranking. It also obtains information relating to the bit rate required by each user. A streaming service, for example, may require a higher bit rate than other services. This information is passed to the MIMO Management Unit 8.

The MIMO Management Unit 8 combines the prioritisation and bit rate requirements obtained from the traffic demands entity 11 with the channel matrix H obtained from the MIMO channel entity 7. How this is done will now be described.

The example system has three channels. The prioritisation order of the three channels is 3, 2, 1 , with 3 being the most important. The prioritisation can be represented as a row vector (where the operator T provides its transpose):

[3 2 1] ^T

The MIMO Management Unit 8 constructs a binary matrix U which reflects the priority rankings. If the channels were of equal importance, U would take the form of the identity matrix I:

However, in this example the channels have the ranking 3, 2, 1. This causes the rows U to shuffle, resulting in:

0 0 1'

U = 0 1 0

.1 0 0. The MIMO Management Unit 8 now uses U to modify H to reflect the channel rankings.

This is done using the formula:

This re-orders the elements in H. Now the elements on bottom row of H relate to the effect of the channel on the signals on channel 3. The elements on the bottom row of the modified H matrix are reduced to zero:

H is then transmitted from the MIMO Management Unit 8 to a MIMO Precoder Unit 9. The MIMO Precoder Unit 9 uses H to determine a precoder matrix to be applied to the signal X _N it receives from the Symbol Encoder and Mapper array. This is done using the following formula:

P = H ^H ( HH ^H ) ¹

Where H ^H represents the Hermitian of H.

Incorporating H into this formula gives:

The MIMO Precoder Unit 9 then re-orders the resulting precoder matrix P so that each element matches its respective antenna:

P33 P23 0 0 P21 P31

P = Up P32 P22 0 Up ^T = 0 P22 P32

P31 P21 0 .0 p ₂₃ p ₃₃ . The resulting precoder matrix P has therefore been modified to take account of the relative priorities of the three channels. The MIMO Precoder Unit 9 then multiplies P by the signals XN it receives from the Symbol Encoder and Mapper Array 6. This results in the transmission signals X _N ’. The signals X _N ’ are then passed to an array of Inverse Discrete Fourier Transform components/Analogue Front End 10 and are transmitted from antennae to users 1 , 2 and 3. This is conventional in MIMO systems and so will not be described in detail here. Users 1 , 2 and 3 receive the signals and process them in a manner which is conventional and so will not be described in detail here.

A second embodiment will now be described which, in addition to taking user priority into account, also takes the user service requirements into account. It may be, for example, that some users require particularly high data rates. Furthermore, some users require the data to have very few errors and so require a low bit error rate (ber). A quantity known as the Gap value T is relevant in this context. The Gap value is defined as the minimum allowed separation between the signal and noise levels to achieve a given ber and can be expressed mathematically as: where q is the lowest QAM modulation order.

In the first embodiment described above, the MIMO Management Unit 8 modified the channel matrix H to incorporate the different user priorities, resulting in H. In the present embodiment, H is further modified, at the MIMO Management Unit 8, to incorporate the ber requirement. This is done using the further processing step:

In this step, H is multiplied by a diagonal matrix containing the relevant Gap values to produce H. H is then transmitted from the MIMO Management Unit 8 to the MIMO Precoder Unit 9 as described above in relation to the first embodiment. The MIMO Precoder Unit 9 then uses H to determine a precoder matrix to be applied to the signal XN it receives from the Symbol Encoder and Mapper array 6 in the manner described above in relation to the first embodiment.

In a third embodiment, H is further modified to lower the priority level of a user if communication with that user is significantly obstructed by, for example, noise, being a large distance from the base station, the fact that the user is moving, or there are physical obstructions between the user and the base station. This is done at the MIMO Management Unit 8 using the following step:

Where Hi represents the noise on the various channels. As in embodiments 1 and 2, this modified channel matrix H is passed to the MIMO Precoder Unit 9 and used to determine a precoder matrix to be applied to the signal X _N .

Fig 3 is a flow chart showing the method of the invention.