Patent Description:
Cellular networks are divided into many small geographic areas, called cells or sites. Each cell is adjacent to one or more cells. Collectively, the cells provide cellular service to a large geographic area. Each cell is typically served by one or more corresponding base stations. Within each cell, the one or more corresponding base stations serve one or more mobile terminals (or mobile stations) situated within the cell. As further detailed below, other equipment, such as a relay, which aids in base station - mobile terminal communications may also serve a cell.

Signals propagating through a given cell may include transmissions from equipment within the cell (e.g. mobile terminals and base stations within the cell), and may also include signals transmitted from adjacent cells. Thus, in certain instances, a mobile terminal may receive relatively strong signals from multiple transmitters. For example, while situated near the border of one cell and an adjacent cell, a mobile terminal may receive signals transmitted from both the base station serving the cell within which the mobile terminal is situated, and from the base station serving the adjacent cell. Moreover, a mobile terminal may receive signals from multiple sources within a given cell, such as relays and other base stations. Signals from these various sources may interfere, for example, by constructive superposition of signals from the various sites, with the signal expected or desired to be received by the mobile terminal.

Consequently, it would be advantageous if the various signals received by a mobile terminal could be combined in such a way as to transform what would otherwise be interference into a useful signal from the perspective of the mobile terminal. Various techniques of doing so have been proposed and generally include: cooperation between base stations and relays, between relays and base stations with distributed antennas, within the same cell; and between base stations of two or more different cells.

Open loop cooperation between the cooperating equipment typically involves transmit diversity schemes/techniques and spatial multiplexing schemes. Transmit diversity schemes included band switching transmit diversity wherein different sub-bands were allocated to a particular mobile terminal in the cooperating sites; phase delay diversity (PDD)/short cyclic delay diversity (CDD) wherein phase delay or cyclic delay was applied to the signal to generate spatial diversity through forward error correction; and space-time-frequency transmit diversity wherein different cooperating sites used the same resource using space-tone codes. In the spatial multiplexing scheme, different cooperating sites transmitted independent data streams to the receiver. A drawback of this known open loop cooperation technique was that it did not exploit channel state information (CSI) feedback to the cooperating site. Consequently, this technique was more useful for medium and high speed users and less useful for low speed users where channel state information could be exploited to provide better quality of service.

Another technique, interference alignment, employs multi-site multi-user MIMO (MU-MIMO) techniques wherein different MIMO cooperating sites transmitted sets of independent data to different users using the same shared resource. The cooperating sites aligned their induced interferences at all nodes. Drawbacks of this technique included that it could only be applied to high geometry users, required pairing up of two or more users serviced by the same base station and required knowledge by the transmitting site (e.g. base station) channel conditions. Since channel condition data or information from which channel conditions could be calculated by the transmitting site had to be transmitted from the mobile terminals, this technique resulted in higher feedback overhead. Consequently, this technique was largely applied to fixed or low speed users. "<NPL>, discloses cooperation scenarios and proposals for possible exploiting in cooperation between neighbouring sites in the LTE-A standard. "<NPL>, discloses techniques for a single-layer downlink joint processing.

A need therefore exists for improved techniques for multi-transmitting site cooperation.

In a first aspect of the invention, there is provided a method according to claim <NUM> According to a second aspect, there is provided a cellular communication network according to claim <NUM>.

In the figures which illustrate by way of example only, embodiments of the present disclosure,.

Like reference numerals are used in different figures to denote similar elements.

Referring to the drawings, <FIG> shows a base station controller (BSC) <NUM> which controls wireless communications within multiple cells <NUM>, which cells are served by corresponding base stations (BS) <NUM>.

In some configurations, each cell is further divided into multiple sectors <NUM> or zones (not shown). In general, each base station <NUM> facilitates communications using OFDM with mobile and/or wireless terminals <NUM>, which are within the cell <NUM> associated with the corresponding base station <NUM>. The movement of the mobile terminals <NUM> in relation to the base stations <NUM> results in significant fluctuation in channel conditions. As illustrated, the base stations <NUM> and mobile terminals <NUM> may include multiple antennas to provide spatial diversity for communications. In some configurations, relay stations <NUM> may assist in communications between base stations <NUM> and wireless terminals <NUM>. Wireless terminals <NUM> can be handed can be handed off <NUM> from any cell <NUM>, sector <NUM>, zone (not shown), base station <NUM> or relay <NUM> to an other cell <NUM>, sector <NUM>, zone (not shown), base station <NUM> or relay <NUM>. In some configurations, base stations <NUM> communicate with each and with another network (such as a core network or the internet, both not shown) over a backhaul network <NUM>. In some configurations, a base station controller <NUM> is not needed.

With reference to <FIG>, an example of a base station <NUM> is illustrated. The base station <NUM> generally includes a control system <NUM>, a baseband processor <NUM>, transmit circuitry <NUM>, receive circuitry <NUM>, multiple antennas <NUM>, and a network interface <NUM>. The receive circuitry <NUM> receives radio frequency signals bearing information from one or more remote transmitters provided by mobile terminals <NUM> (illustrated in <FIG>) and relay stations <NUM> (illustrated in <FIG>). A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams.

The baseband processor <NUM> processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. As such, the baseband processor <NUM> is generally implemented in one or more digital signal processors (DSPs) or application-specific integrated circuits (ASICs). The received information is then sent across a wireless network via the network interface <NUM> or transmitted to another mobile terminal <NUM> serviced by the base station <NUM>, either directly or with the assistance of a relay <NUM>.

On the transmit side, the baseband processor <NUM> receives digitized data, which may represent voice, data, or control information, from the network interface <NUM> under the control of control system <NUM>, and encodes the data for transmission. The encoded data is output to the transmit circuitry <NUM>, where it is modulated by one or more carrier signals having a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signals to the antennas <NUM> through a matching network (not shown). Modulation and processing details are described in greater detail below.

With reference to <FIG>, an example of a mobile terminal <NUM> is illustrated. Similarly to the base station <NUM>, the mobile terminal <NUM> will include a control system <NUM>, a baseband processor <NUM>, transmit circuitry <NUM>, receive circuitry <NUM>, multiple antennas <NUM>, and user interface circuitry <NUM>. The receive circuitry <NUM> receives radio frequency signals bearing information from one or more base stations <NUM> and relays <NUM>. A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams.

The baseband processor <NUM> processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. The baseband processor <NUM> is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs).

For transmission, the baseband processor <NUM> receives digitized data, which may represent voice, video, data, or control information, from the control system <NUM>, which it encodes for transmission. The encoded data is output to the transmit circuitry <NUM>, where it is used by a modulator to modulate one or more carrier signals that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas <NUM> through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the mobile terminal and the base station, either directly or via the relay station.

In OFDM modulation, the transmission band is divided into multiple, orthogonal carrier waves. Each carrier wave is modulated according to the digital data to be transmitted. Because OFDM divides the transmission band into multiple carriers, the bandwidth per carrier decreases and the modulation time per carrier increases. Since the multiple carriers are transmitted in parallel, the transmission rate for the digital data, or symbols, on any given carrier is lower than when a single carrier is used.

OFDM modulation utilizes the performance of an Inverse Fast Fourier Transform (IFFT) on the information to be transmitted. For demodulation, the performance of a Fast Fourier Transform (FFT) on the received signal recovers the transmitted information. In practice, the IFFT and FFT are provided by digital signal processing carrying out an Inverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform (DFT), respectively. Accordingly, the characterizing feature of OFDM modulation is that orthogonal carrier waves are generated for multiple bands within a transmission channel. The modulated signals are digital signals having a relatively low transmission rate and capable of staying within their respective bands. The individual carrier waves are not modulated directly by the digital signals. Instead, all carrier waves are modulated at once by IFFT processing.

In operation, OFDM is preferably used for at least downlink transmission from the base stations <NUM> to the mobile terminals <NUM>. Each base station <NUM> is equipped with "n" transmit antennas <NUM> (n >=<NUM>), and each mobile terminal <NUM> is equipped with "m" receive antennas <NUM> (m>=<NUM>). Notably, the respective antennas can be used for reception and transmission using appropriate duplexers or switches and are so labelled only for clarity.

When relay stations <NUM> are used, OFDM is preferably used for downlink transmission from the base stations <NUM> to the relays <NUM> and from relay stations <NUM> to the mobile terminals <NUM>.

With reference to <FIG>, an example of a relay station <NUM> is illustrated. Similarly to the base station <NUM>, and the mobile terminal <NUM>, the relay station <NUM> will include a control system <NUM>, a baseband processor <NUM>, transmit circuitry <NUM>, receive circuitry <NUM>, multiple antennas <NUM>, and relay circuitry <NUM>. The relay circuitry <NUM> enables the relay <NUM> to assist in communications between a base station <NUM> and mobile terminals <NUM>. The receive circuitry <NUM> receives radio frequency signals bearing information from one or more base stations <NUM> and mobile terminals <NUM>. A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams.

For transmission, the baseband processor <NUM> receives digitized data, which may represent voice, video, data, or control information, from the control system <NUM>, which it encodes for transmission. The encoded data is output to the transmit circuitry <NUM>, where it is used by a modulator to modulate one or more carrier signals that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas <NUM> through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the mobile terminal and the base station, either directly or indirectly via a relay station, as described above.

With reference to <FIG>, a logical OFDM transmission architecture will be described. Initially, the base station controller <NUM> will send data to be transmitted to various mobile terminals <NUM> to the base station <NUM>, either directly or with the assistance of a relay station <NUM>. The base station <NUM> may use the channel quality indicators (CQIs) associated with the mobile terminals to schedule the data for transmission as well as select appropriate coding and modulation for transmitting the scheduled data. The CQIs may be directly from the mobile terminals <NUM> or determined at the base station <NUM> based on information provided by the mobile terminals <NUM>. In either case, the CQI for each mobile terminal <NUM> is a function of the degree to which the channel amplitude (or response) varies across the OFDM frequency band.

Scheduled data <NUM>, which is a stream of bits, is scrambled in a manner reducing the peak-to-average power ratio associated with the data using data scrambling logic <NUM>. A cyclic redundancy check (CRC) for the scrambled data is determined and appended to the scrambled data using CRC adding logic <NUM>. Next, channel coding is performed using channel encoder logic <NUM> to effectively add redundancy to the data to facilitate recovery and error correction at the mobile terminal <NUM>. Again, the channel coding for a particular mobile terminal <NUM> is based on the CQI. In some implementations, the channel encoder logic <NUM> uses known Turbo encoding techniques. The encoded data is then processed by rate matching logic <NUM> to compensate for the data expansion associated with encoding.

Bit interleaver logic <NUM> systematically reorders the bits in the encoded data to minimize the loss of consecutive data bits. The resultant data bits are systematically mapped into corresponding symbols depending on the chosen baseband modulation by mapping logic <NUM>. Preferably, Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key (QPSK) modulation is used. The degree of modulation is preferably chosen based on the CQI for the particular mobile terminal. The symbols may be systematically reordered to further bolster the immunity of the transmitted signal to periodic data loss caused by frequency selective fading using symbol interleaver logic <NUM>.

At this point, groups of bits have been mapped into symbols representing locations in an amplitude and phase constellation. When spatial diversity is desired, blocks of symbols are then processed by space-time block code (STC) encoder logic <NUM>, which modifies the symbols in a fashion making the transmitted signals more resistant to interference and more readily decoded at a mobile terminal <NUM>. The STC encoder logic <NUM> will process the incoming symbols and provide "n" outputs corresponding to the number of transmit antennas <NUM> for the base station <NUM>. The control system <NUM> and/or baseband processor <NUM> as described above with respect to <FIG> will provide a mapping control signal to control STC encoding. At this point, assume the symbols for the "n" outputs are representative of the data to be transmitted and capable of being recovered by the mobile terminal <NUM>.

For the present example, assume the base station <NUM> has two antennas <NUM> (n=<NUM>) and the STC encoder logic <NUM> provides two output streams of symbols. Accordingly, each of the symbol streams output by the STC encoder logic <NUM> is sent to a corresponding IFFT processor <NUM>, illustrated separately for ease of understanding. Those skilled in the art will recognize that one or more processors may be used to provide such digital signal processing, alone or in combination with other processing described herein, The IFFT processors <NUM> will preferably operate on the respective symbols to provide an inverse Fourier Transform. The output of the IFFT processors <NUM> provides symbols in the time domain. The time domain symbols are grouped into frames, which are associated with a prefix by prefix insertion logic <NUM>. Each of the resultant signals is up-converted in the digital domain to an intermediate frequency and converted to an analog signal via the corresponding digital up-conversion (DUC) and digital-to-analog (D/A) conversion circuitry <NUM>. The resultant (analog) signals are then simultaneously modulated at the desired RF frequency, amplified, and transmitted via the RF circuitry <NUM> and antennas <NUM>. Notably, pilot signals known by the intended mobile terminal <NUM> are scattered among the sub-carriers. The mobile terminal <NUM>, which is discussed in detail below, will use the pilot signals for channel estimation.

Reference is now made to <FIG> to illustrate reception of the transmitted signals by a mobile terminal <NUM>, either directly from base station <NUM> or with the assistance of relay <NUM>. Upon arrival of the transmitted signals at each of the antennas <NUM> of the mobile terminal <NUM>, the respective signals are demodulated and amplified by corresponding RF circuitry <NUM>. For the sake of conciseness and clarity, only one of the two receive paths is described and illustrated in detail. Analog-to-digital (A/D) converter and down-conversion circuitry <NUM> digitizes and downconverts the analog signal for digital processing. The resultant digitized signal may be used by automatic gain control circuitry (AGC) <NUM> to control the gain of the amplifiers in the RF circuitry <NUM> based on the received signal level.

Initially, the digitized signal is provided to synchronization logic <NUM>, which includes coarse synchronization logic <NUM>, which buffers several OFDM symbols and calculates an auto-correlation between the two successive OFDM symbols. A resultant time index corresponding to the maximum of the correlation result determines a fine synchronization search window, which is used by fine synchronization logic <NUM> to determine a precise framing starting position based on the headers. The output of the fine synchronization logic <NUM> facilitates frame acquisition by frame alignment logic <NUM>. Proper framing alignment is important so that subsequent FFT processing provides an accurate conversion from the time domain to the frequency domain. The fine synchronization algorithm is based on the correlation between the received pilot signals carried by the headers and a local copy of the known pilot data. Once frame alignment acquisition occurs, the prefix of the OFDM symbol is removed with prefix removal logic <NUM> and resultant samples are sent to frequency offset correction logic <NUM>, which compensates for the system frequency offset caused by the unmatched local oscillators in the transmitter and the receiver. Preferably, the synchronization logic <NUM> includes frequency offset and clock estimation logic <NUM>, which is based on the headers to help estimate such effects on the transmitted signal and provide those estimations to the correction logic <NUM> to properly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready for conversion to the frequency domain using FFT processing logic <NUM>. The results are frequency domain symbols, which are sent to processing logic <NUM>. The processing logic <NUM> extracts the scattered pilot signal using scattered pilot extraction logic <NUM>, determines a channel estimate based on the extracted pilot signal using channel estimation logic <NUM>, and provides channel responses for all sub-carriers using channel reconstruction logic <NUM>. In order to determine a channel response for each of the sub-carriers, the pilot signal is essentially multiple pilot symbols that are scattered among the data symbols throughout the OFDM sub-carriers in a known pattern in both time and frequency. Continuing with <FIG>, the processing logic compares the received pilot symbols with the pilot symbols that are expected in certain sub-carriers at certain times to determine a channel response for the sub-carriers in which pilot symbols were transmitted. The results are interpolated to estimate a channel response for most, if not all, of the remaining sub-carriers for which pilot symbols were not provided. The actual and interpolated channel responses are used to estimate an overall channel response, which includes the channel responses for most, if not all, of the sub-carriers in the OFDM channel.

The frequency domain symbols and channel reconstruction information, which are derived from the channel responses for each received path are provided to an STC decoder <NUM>, which provides STC decoding on both received paths to recover the transmitted symbols. The channel reconstruction information provides equalization information to the STC decoder <NUM> sufficient to remove the effects of the transmission channel when processing the respective frequency domain symbols.

The recovered symbols are placed back in order using symbol de-interleaver logic <NUM>, which corresponds to the symbol interleaver logic <NUM> of the transmitter. The de-interleaved symbols are then demodulated or de-mapped to a corresponding bitstream using de-mapping logic <NUM>. The bits are then de-interleaved using bit de-interleaver logic <NUM>, which corresponds to the bit interleaver logic <NUM> of the transmitter architecture. The de-interleaved bits are then processed by rate de-matching logic <NUM> and presented to channel decoder logic <NUM> to recover the initially scrambled data and the CRC checksum. Accordingly, CRC logic <NUM> removes the CRC checksum, checks the scrambled data in traditional fashion, and provides it to the de-scrambling logic <NUM> for de-scrambling using the known the known base station de-scrambling code to recover the originally transmitted data <NUM>.

In parallel to recovering the data <NUM>, a CQI, or at least information sufficient to create a CQI at the base station <NUM>, is determined and transmitted to the base station <NUM>. As noted above, the CQI may be a function of the carrier-to-interference ratio (CR), as well as the degree to which the channel response varies across the various sub-carriers in the OFDM frequency band. For this embodiment, the channel gain for each sub-carrier in the OFDM frequency band being used to transmit information is compared relative to one another to determine the degree to which the channel gain varies across the OFDM frequency band. Although numerous techniques are available to measure the degree of variation, one technique is to calculate the standard deviation of the channel gain for each sub-carrier throughout the OFDM frequency band being used to transmit data.

Referring to <FIG>, an example SC-FDMA transmitter <NUM>(a) and receiver <NUM>(b) for single-in single-out (SISO) configuration is illustrated provided in accordance with one embodiment of the present application. In SISO, mobile stations transmit on one antenna and base stations and/or relay stations receive on one antenna. <FIG> illustrates the basic signal processing steps needed at the transmitter and receiver for the LTE SC-FDMA uplink. In some embodiments, SC-FDMA (Single-Carrier Frequency Division Multiple Access) is used. SC-FDMA is a modulation and multiple access scheme introduced for the uplink of 3GPP Long Term Evolution (LTE) broadband wireless fourth generation (<NUM>) air interface standards, and the like. SC-FDMA can be viewed as a DFT pre-coded OFDMA scheme, or, it can be viewed as a single carrier (SC) multiple access scheme. There are several similarities in the overall transceiver processing of SC-FDMA and OFDMA. Those common aspects between OFDMA and SC-FDMA are illustrated in the OFDMA TRANSMIT CIRCUITRY and OFDMA RECEIVE CIRCUITRY, as they would be obvious to a person having ordinary skill in the art in view of the present specification. SC-FDMA is distinctly different from OFDMA because of the DFT pre-coding of the modulated symbols, and the corresponding IDFT of the demodulated symbols. Because of this pre-coding, the SC-FDMA sub-carriers are not independently modulated as in the case of the OFDMA sub-carriers. As a result, PAPR of SC-FDMA signal is lower than the PAPR of OFDMA signal. Lower PAPR greatly benefits the mobile terminal in terms of transmit power efficiency.

<FIG> provide one specific example of a communication system that could be used to implement embodiments of the application. It is to be understood that embodiments of the application can be implemented with communications systems having architectures that are different than the specific example, but that operate in a manner consistent with the implementation of the embodiments as described herein.

<FIG> depict three exemplary cooperation scenarios. Specifically, <FIG> depicts cooperation between base station <NUM> and relay <NUM> in serving mobile terminal <NUM> within a given cell <NUM>. <FIG> depicts transmission by base station <NUM> to two relays <NUM>, and cooperation between relays <NUM> to serve mobile terminal <NUM> in a given cell <NUM>. <FIG> depicts cooperation between base station 14a in cell 12a and base station 14b in cell 12b to serve mobile terminal <NUM> situated in cell 12a. Hereinafter, the cooperating equipment may also be generally referred to as "cooperating sites".

As will be further detailed below, cooperating sites may cooperate in different ways. However, at minimum, cooperation may require that some information be shared between the cooperating sites (e.g. to coordinate transmission to a particular target mobile terminal <NUM>). In this regard, the cooperating sites may be controlled by a base station controller (e.g. base station controller <NUM>) which is in communication with the cooperating sites. Alternatively, cooperating sites may be interconnected via a network, e.g. backhaul network <NUM> or other network, such as the internet. Conveniently, in the techniques described below requiring coordination between cooperating sites, such coordination may take place via base station controller <NUM> and/or other network.

As previously discussed, a cellular network may include a plurality of base stations <NUM> and relays <NUM> serving a plurality of mobile terminals <NUM> in a plurality of cells. Thus, a given mobile terminal <NUM> may be within reception range of a number of base stations <NUM> and relays <NUM>. Moreover, a given base station <NUM> or relay <NUM> may be within transmission/reception range of one or more other base station(s) <NUM> or relay(s) <NUM>. Thus, it would be desirable if two or more base stations <NUM> and/or relays <NUM> (hereinafter also referred to as "transmitting sites") could cooperate in serving mobile terminals <NUM>. As further detailed below, such cooperation may provide increased quality of service to mobile terminals <NUM>. Accordingly, the present application discloses a number of schemes whereby base stations and relays cooperate in serving one or more mobile terminals <NUM>.

In overview, both closed loop cooperation and semi-closed loop cooperation schemes are disclosed in the present application. In the disclosed closed loop scheme, cooperating transmitters (e.g. a base station <NUM> or relay <NUM>) have knowledge of partial or full channel state information (CSI). Their target (e.g. mobile terminal <NUM>) may constructively add the transmissions received from the various transmitting sites. In the disclosed semi-closed loop scheme, all or some of the cooperating sites may perform closed loop techniques within each site.

In an exemplary embodiment of the present application, all cooperating sites transmit the same signal to a target mobile device <NUM>. One of the cooperating sites, typically the equipment such as a base station <NUM> serving the cell in which the target mobile device <NUM> is situated, is identified as the serving site. Due to the differences in the paths between a given serving base station <NUM> and target mobile device <NUM>, the cooperating sites may be required to carry out timing/distance adjustments. More specifically, and as further detailed below, the sites cooperate using multi-site beamforming; in addition to using multi-site beamforming, the sites may cooperate using one or more of multi-site closed loop precoding and heterogeneous closed loop.

In order to carry out the multi-site beamforming technique, two or more sites (e.g. base station <NUM>/relay <NUM>) transmit as an array. Each site beams signals containing identical data. In FDD, the beam is formed using the intended mobile terminal's uplink Angle of Arrival (AoA). At the target mobile device <NUM>, constructive superposition may be employed to combine the received signals. To this end, a superposition dedicated pilot signal may be employed for channel estimation. As may be appreciated, constructive superposition requires timing and distance adjustment between the cooperating sites as the signals from the different cooperating sites may arrive at the target mobile terminal <NUM> at different times due to differences in path characteristics (e.g. physical distances) to the target. Thus, the transmitted signals may be linearly phase shifted. According to the invention, to compensate, conveniently, the target mobile terminal <NUM> considers one of the transmitting sites as a reference site, and reports timing differences to the other transmitting sites. Each of the other transmitting sites, in turn, adjusts timing of its transmissions so that the signals arriving at the target mobile terminal <NUM> may be constructively combined. It has been observed that different signal frequencies may be phase shifted by different amounts. Thus, in order to detect such phase differences, orthogonal pilot signals may be employed. Upon detecting a phase difference(s), the target mobile terminal <NUM> may report the phase difference(s) to the corresponding transmitting sites, using known techniques. Based on this feedback, the transmitting sites may then perform appropriate phase correction using known phase correction techniques (e.g. codebook phase correction). Moreover, the transmitting sites may perform opportunistic phase correction for nomadic target mobile terminals <NUM> wherein the different transmitting sites apply random phase sequences. Based on CQI reports, the best phase combination may be evaluated and thereafter employed.

To carry out the multi-site closed loop precoding, different MIMO transmitting sites may form the same beams to the target mobile device <NUM> using precoders. When employing FDD, a target mobile terminal <NUM> may report/specify which precoder to use to the transmitting site(s), and may report different precoders to different transmitting sites. At a given site, a precoder set may be re-used. In this manner, a signal may be improved by phase correction according to one of the exemplary methods detailed above. Alternatively, the precoder may be selected taking into consideration all transmitting sites, thus obviating the need for further phase correction techniques. While timing and distance adjustments may still be required, the signals transmitted in this manner may be less susceptible to timing differences than when transmitted using the multi-site beamforming method previously detailed above. Notably, to carry out this technique, orthogonal common pilots may be required for precoder selection; orthogonal common pilots or superpositioned dedicated pilots may be required for demodulation.

For the heterogeneous multi-site closed loop/beamforming technique, array and MIMO sites may send the same data stream to the target mobile terminal <NUM>. Precoder selection may be employed for the MIMO site(s). Timing and distance adjustments as well as phase correction (similar to the multi-site beamforming method detailed above) may be required between the sites. Conveniently, the MIMO site(s) may carry extra data streams to improve throughput of the system. A common pilot for FDD precoder selection may be employed in addition to a superposition dedicated pilot for demodulation.

As previously explained, timing/distance differences in signals transmitted from the various transmitting sites may result in linear phase shifts of the signals in the frequency domain. Thus, in order to achieve constructive interference of the arriving signals at the target mobile terminal <NUM>, the transmitted signals may be corrected. Accordingly, in an embodiment of the present application, target mobile terminal <NUM> may measure the timing mismatch of arriving signals using arrival time estimation or channel estimation techniques. One transmitting site is considered the reference site and timing differences of the different arriving signals are reported to the non-reference transmitting sites by target mobile terminal <NUM>. Each transmitting site then applies linear phase correction techniques in the frequency domain to correct for the detected timing differences. Specifically, only those tones assigned to the intended target mobile terminal <NUM> may be linear phase adjusted, the transmitter (e.g. base station <NUM>) may be also transmitting to other mobile terminals. Therefore, a phase correction of all signals transmitted by that transmitter may result in a degradation of the signals intended for the other mobile terminals. Also, notably, phase is adjusted in the frequency domain as opposed to the in the time domain because adjustment in the time domain may result in timing mismatch to mobile terminals that the transmitting site is serving, other than target mobile terminal <NUM>.

As discussed above, phase correction may be achieved by a number of different techniques, including, in particular, codebook-based phase correction. In an exemplary embodiment of the present application, variations on codebook-based phase correction may be employed to achieve cooperation between cooperating sites. Generally, the specific technique may vary by the number of cooperating sites, as follows.

For two cooperating transmitting sites, a two transmitter constant amplitude codebook may be employed. The non-serving transmitting site may be used as the phase reference. A fixed precoder may be assigned to the non-serving transmitting site. The target mobile terminal <NUM> may report a preferred precoder to the serving transmitting site for phase correction. In this technique, only phase correction feedback need be provided to the serving transmitting site.

Cooperation between greater than two transmitting sites (i.e., M><NUM> where M is the number of transmitting sites) may be achieved also by employing a two transmitter constant amplitude codebook. Specifically, the non-serving transmitting site may be used as the phase reference and a fixed precoder may be pre-assigned to the non-serving site. M-<NUM> precoders may be reported to the serving transmitting site and all other non-serving transmitting sites for phase correction. Alternatively, cooperation between M><NUM> sites may be achieved by using a M-transmitter constant amplitude codebook. Specifically, the non-serving transmitting site may be used as the phase reference and pre-assigned a fixed precoder. One precoder may be reported to the serving transmitting site and all other non-serving transmitting sites for phase correction.

An example of codebook-based phase correction for a two transmitter LTE codebook for two transmitting site cooperation is as follows. Each of the two sites may have an antenna array or MIMO antennas. The precoder set may be {[<NUM>,<NUM>]T, [<NUM>,j]T, [<NUM>,-<NUM>]T, [<NUM>,-j]T } where the first transformation in the set represents a <NUM>° phase shift, the second represents a <NUM>° phase shift, the third represents a <NUM>° phase shift and the last represents a <NUM>° phase shift. The serving transmitting site may be assigned to port <NUM> and the helping transmitting site to port <NUM>. The best precoder that aligns the phases of the two sites (i.e. maximizes the received power of the signals) may be determined. This determined precoder may then be reported to the serving transmitting site. However, there may be no need to report the precoder to the helping site (because the helping site is transmitting on port <NUM>, and the first element of the precoding matrices is always a "<NUM>"). As may be appreciated, codebook-based techniques may restrict phase shifting to certain pre-defined amounts therefore, it may be expected that system performance may be improved with a bigger codebook that provides the ability to phase shift by a greater number of pre-defined amounts.

A further example of codebook-based phase correction, in which the serving site is represented by the subscript "<NUM>", and the other cooperating site by subscript "<NUM>", to send a one-layer data X, is as follows. <MAT> Where.

In the above example, joint selection of P<NUM>, P<NUM> and Ppc may improve performance of a closed-loop system. However, a potential drawback of this technique is higher codebook search complexity since there are three codebooks (i.e. it may be more complex to determine the optimum combination of the three codebooks). Moreover, for multi-layer data transmission, one phase correction precoder per layer may be reported by the mobile to the base station.

In some scenarios, in order to limit feedback overhead and precoder set size, virtual antenna precoders may be applied. Notably, a multiple antenna array may be formed between antennas at transmitting sites (e.g. base stations <NUM>/relays <NUM>) that are situated sufficiently close together to be able to exchange information required to form a multiple-antenna transmitter. Accordingly, in an exemplary embodiment of the present application, cooperation between two sites each with four transmit antennas to transmit one data stream to a target mobile terminal <NUM> may be achieved with the following three exemplary techniques. Each technique involves a pre-coding matrix index (PMI) report. In the first exemplary technique, PMI1 is sent to transmitting site <NUM> and PMI2 is sent to site <NUM> wherein both PMI1 and PMI2 are from a four-transmitter codebook. Phase correction may be required. In the second exemplary technique, one PMI is reported to both transmitting sites from an eight-transmitter codebook. Transmitting site <NUM> may use the upper half of the precoder and transmitting site <NUM> may use the lower half. The third technique uses a virtual antenna. Each transmitting site is treated as the equivalent of a two-antenna transmitter, and one PMI is sent to both antennas. The virtual antenna precoder may be derived using AoA. It has been observed that the second example has the highest gain and the third example has lower overhead. It may be noted that the second example requires a larger codebook and that example one requires three PMI feedbacks (i.e. one for each site in additional to the cooperation codebook).

In each of the above exemplary embodiments, closed-loop and semi¬ closed loop schemes may be employed. It may be appreciated that each has advantages and disadvantages. Specifically, some advantages of semi-closed loop schemes include easy implementation in that a single site feedback signally may be reused; there may be no need for fine timing adjustments; there may be no need for beam phase correction; cooperation between MIMO and array transmitting sites (i.e. heterogeneous cooperation) may be facilitated. Moreover, semi-closed loop schemes may be more robust against channel aging since channel coefficients from the same site age in the same way especially at LoS (line of sight) conditions and/or array sites. Semi-closed loop schemes may also be more robust against carrier frequency synchronization errors. Benefits of closed-loop schemes involve better performance by exchanging channel state information (CSI) between transmitting sites.

More specifically, the following semi-closed loop (CL) schemes may be employed in conjunction with the above-described exemplary embodiments. In a first exemplary semi CL scheme, multi-site CL transmit diversity may be employed wherein different MIMO sites transmit the same CL stream(s) to a target mobile terminal <NUM> using a transmit diversity scheme such as Alamouti. Alternatively, multi-site beamforming (BF) transmit diversity may be employed wherein different array sites transmit the same BF stream(s) to the user using a transmit diversity scheme such as Alamouti. In a further alternative, multi-site CL spatial multiplexing (SM) may be employed wherein different MIMO sites transmit independent CL streams to the target mobile terminal <NUM>. In this alternative, precoder selection may minimize inter-layer interference, and the same precoder set may be reused. In yet a further alternative, multi-site BF spatial multiplexing may be employed wherein different array sites may transmit independent beams to the target mobile terminal <NUM>.

The following multi-site closed loop transmit diversity scheme may be employed in conjunction with the above-described exemplary embodiments. Different transmitting MIMO sites may each send a CL stream to the target mobile terminal <NUM>. Moreover, the different transmitting sites may form a transmit diversity scheme between them. This scheme may require orthogonal common pilots for precoder selection, as well as orthogonal common pilots or orthogonal dedicated pilots for demodulation. The transmit diversity scheme may include band switching, tone switching and space-tone coding. For example, cooperation between two four-transmitter FDD sites may be carried out as follows. The target mobile terminal <NUM> may report two independent precoders to the transmitting sites and the two sites may send two streams of Alamouti (SFBC or STBC) to the target mobile terminal <NUM>. The same approach may be followed for transmit diversity schemes of rate greater than one.

The following multi-site beamforming (BF) transmit diversity scheme may be employed in conjunction with the above-described exemplary embodiments. Different array sites may each send a beam to the target mobile terminal <NUM>, and the different beams may form a transmit diversity scheme (e.g. band switching, tone switching and space-tone coding) between them. In this scheme, orthogonal dedicated pilots for demodulation may be used.

The following multi-site closed-loop SM scheme may be employed in conjunction with the above-described exemplary embodiments. Different MIMO sites may transmit independent CL streams to the target mobile terminal <NUM>. Different precoders may be selected for different sites. A single-site precoder codebook may be reused. Furthermore, the precoder may be selected to minimize inter-layer interference. This may increase spectral efficiency for high geometry users. Orthogonal common pilots for FDD precoder selection, and orthogonal common pilots or orthogonal dedicated pilots for demodulation may be used. In a related alternative, multi-site beamforming SM may be employed wherein different array sites may transmit independent beams to the target mobile terminal <NUM>. AoAmay be used for beamforming. Likewise, orthogonal dedicated pilots may be used for demodulation.

The following heterogeneous multi-site SM / transmit diversity schemes may be employed in conjunction with the above-described exemplary embodiments, in particular, for heterogeneous multi-site SM, array and MIMO sites may transmit independent data streams to the target mobile terminal <NUM>. Precoder selection may minimize the interlayer interference between the two sites. A common pilot for FDD precoder selection may be employed and orthogonal pilots between the transmitting site may be used for demodulation. To achieve heterogeneous mutli-site transmit diversity, array and MIMO sites may transmit data streams of a transmit diversity scheme to the target mobile terminal <NUM>. A common pilot for FDD precoder selection may be used. Likewise, orthogonal pilots between the sites for demodulation may be used.

In summary, notable aspects of the above-described embodiments include cooperation between two or more transmitting sites (e.g. base station <NUM>/relay <NUM>) to serve one or more target mobile terminals. To this end, the transmitting sites may also cooperate in a closed-loop manner to send data to a target mobile terminal. Constructive superposition of the different received signals may occur at the target mobile terminal. Closed-loop operation may also be based on MIMO channel coefficients, MIMO precoders or beamforming.

As detailed above, signals may be linearly phase shifted in the frequency domain during transmission from the transmitting site(s) to the target mobile terminal. Consequently, some of the above-described embodiments include linear phase correction techniques in the frequency domain to cancel the effect of timing mismatches in the arriving signals at the target mobile terminal. Moreover, as described, phase correction may also be achieved using codebook phase correction. In particular, a two-transmitter codebook may be employed for cooperation between two transmitting sites, whereas a two-transmitter or M-transmitter (where M><NUM>) codebook may be used for cooperation between M sites.

Also as detailed above, virtual antenna closed-loop precoding may be employed to limit feedback overhead for aggregate precoding matrix reporting. Conveniently, the number of antenna ports may be reduced at each transmitting site by using virtual antenna techniques.

Lastly, also as detailed above, semi-closed loop cooperation between the sites may be employed. Specifically, within each site closed-loop techniques may be employed, whereas open loop (OL) cooperation may occur between sites. Even more specifically, OL cooperation techniques may be employed in sending the same data from different transmitting sites using a transmit diversity scheme, for example, frequency shifting or Alamouti (semi-CL transmit diversity). Moreover, OL techniques may be employed in sending different data streams from different sites (semi-CL SM). CL operation within each site may be based on MIMO channel coefficients, MIMO precoders or beamforming. In this manner, cooperation between heterogeneous sites may be facilitated.

Claim 1:
A method of serving a data stream to a target mobile terminal (<NUM>) in a cellular communications network comprising a plurality of transmitting sites (<NUM>, <NUM>), the method comprising:
designating a first transmitting site (<NUM>) and a second transmitting site (<NUM>) as cooperating sites;
at said first transmitting site, transmitting said data stream to said mobile terminal in accordance with a beamforming technique; and
at said second transmitting site, transmitting said data stream to said mobile terminal in accordance with a beamforming technique, wherein beams transmitted by said second transmitting site are adjusted to result in constructive addition of beams arriving at said target mobile terminal from said first and said second transmitting sites;
wherein one of said first and second transmitting sites comprises an antenna array and wherein said transmitting comprises transmitting the same said data stream from each antenna of said antenna array;
the method characterized in further comprising:
at a primary antenna of said antenna array, receiving channel quality information, CQI, feedback from said target mobile terminal;
responsive to said CQI feedback, providing an indicator of a linear phase adjustment to each other antenna of said antenna array; and
phase shifting beams transmitted by each of said other antennas by said indicated linear phase adjustment to thereby result in constructive addition of beams arriving at said target mobile terminal from said first and second transmitting sites;
wherein said indicator comprises an indicator of a timing mismatch of arriving beams at said target mobile terminal; and
wherein one of said first and second transmitting sites is considered as reference site, and the timing mismatch is reported by the target mobile terminal to the other one of said first and second transmitting sites.