Patent Description:
A cellular network comprises a group of base stations that defines the radio coverage areas (or cells) of the network. Typically, a non-line-of-sight (NLOS) radio propagation path exists between a base station (BS) and a subscriber station (or mobile station, mobile terminal, etc.) due to natural and man-made objects situated between the base station and the subscriber station. As a result, the radio waves propagate via reflections, diffractions and scattering. The arriving waves at the subscriber station in the downlink (and at the base station in the uplink) experience constructive and destructive additions because of different phases of individual waves. This is due the fact that, at the high carrier frequencies typically used in cellular wireless networks, small changes in the differential propagation delays introduce large changes in the phases of the individual waves.

If the subscriber station (SS) is moving or there are changes in the scattering environment, then the spatial variations in the amplitude and phase of the composite received signal will manifest themselves as time variations known as Rayleigh fading or fast fading. The time-varying nature of the wireless channel requires very high signal-to-noise ratio (SNR) in order to provide the desired bit error or packet error reliability.

Conventional wireless networks use various diversity techniques to combat the effect of fast fading. Diversity techniques provide the receiver (e.g., subscriber station) with multiple faded replicas of the same information-bearing signal. Assuming independent fading on each of the antenna branches, the probability that the instantaneous signal-to-noise ratio (SNR) is below a certain threshold on each of the branches is approximately pL, where p is the probability that the instantaneous SNR value is below the same threshold on each antenna branch.

Conventional diversity techniques generally fall into categories of space, angle, polarization, field, frequency, time and multipath diversity. Space diversity uses multiple transmit or receive antennas, where the spatial separations between the multiple antennas are chosen so that the diversity branches experience fading with little or no correlation. Transmit diversity uses multiple transmit antennas to provide the receiver with multiple uncorrelated replicas of the same signal.

Conventional transmit diversity schemes may be further divided into open-loop or closed-loop transmit diversity schemes. In an open-loop transmit diversity scheme, no feedback is required from the receiver. In one conventional closed-loop transmit diversity scheme, the receiver computes the phase and amplitude adjustment(s) that should be applied at the transmitter to maximize the received signal power at the receiver. In another conventional closed-loop transmit diversity scheme, referred to as selection transmit diversity (STD), the receiver provides feedback to the transmitter on antenna(s) to be used for transmission.

One well-known example of transmit diversity is the Alamouti 2x1 space-time diversity scheme. In this approach, during any symbol period, two data symbols are transmitted simultaneously from two transmit antennas. During a first symbol interval, the symbols transmitted from a first antenna (ANT1) and a second antenna (ANT2) are denoted as s(<NUM>) and s(<NUM>), respectively. During the next symbol period, the symbols transmitted from antennas ANT1 and ANT2 are -s*(<NUM>) and s*(<NUM>), respectively, where -s*(<NUM>) is the negative of the complex conjugate of s(<NUM>) and s*(<NUM>) is the complex conjugate of s(<NUM>). Signal processing in the subscriber station (SS) recovers the original symbols, s(<NUM>) and s(<NUM>). It is noted that the instantaneous channel gain estimates, g1 (for ANT1) and g2 (for ANT2), are required for processing at the SS receiver. Thus, separate pilot symbols are required for antennas ANT1 and ANT2 for channel gain estimation.

Another convention diversity technique commonly available in OFDM systems is frequency diversify. In an OFDM system exploiting frequency diversity, the subcarriers allocated for transmitting to a particular subscriber station may be uniformly distributed over the whole spectrum. For example, if an OFDM network allocates <NUM> out of N=<NUM> subcarriers to a first subscriber station, the network may allocate every eighth subcarrier (SC) to the first subscriber station starting at the first subcarrier (i.e., SC1, SC9, SC17,. Frequency diversity techniques are generally used for high mobility users and/or for delay-sensitive services.

Another conventional form of diversity is provided by Hybrid Acknowledgement Request (ARQ). Hybrid ARQ is a retransmission scheme whereby the transmitter sends the redundant coded information in small increments. In Hybrid ARQ, the transmitter first performs channel coding on an information packet P and then breaks the resulting coded bit stream into smaller subpackets (i.e., SP1, SP2, SP3,. The transmitter then transmits the first subpacket SP1 to the receiver.

The receiver initially tries to decode the entire information packet P using the first subpacket SP1. In case of unsuccessful decoding, the receiver stores subpacket SP1 and sends a NACK signal to the transmitter. After receiving the NACK signal, the transmitter transmits subpacket SP2. After receiving subpacket SP2, the receiver combines subpacket SP2 with the previously stored subpacket SP1 and tries to jointly decode the original information packet P. If decoding still fails, the receiver sends a NACK signal and the transmitter transmits additional subpackets (i.e., SP3, SP4,. At any point, if information packet P is successfully decoded, as indicated by a successful cyclic redundancy check (CRC), for example, the receiver sends an ACK signal to the transmitter.

Conventional networks also use beamforming techniques to transmit to multiple subscriber stations. The receiver in the subscriber station estimates the complex gains, g<NUM>, g<NUM>,. , gP, to be used from each transmit antenna of the base station. The base station uses these weights for transmission to the subscriber stations. However, the feedback information containing the complex gains represents a significant overhead and degrades the overall system spectral efficiency of the network.

<CIT>, discloses a method and apparatus for artificially providing diversity in an orthogonal frequency division multiplexing (OFDM) wireless communication system. In the method and apparatus disclosed in Application Serial No. <CIT>, diversity is artificially provided by generating a plurality of delayed symbols from a first symbol and then transmitting each of the delayed symbols from a different antenna. Each of the delayed symbols may also be scaled by a different gain factor.

In an adaptive cyclic delay diversity scheme, the delay values can be different for different subscriber stations depending upon the subscriber station's channel profile, velocity, and other factors. For example, a large delay value may be chosen for a high-speed subscriber stations requiring frequency-diversity benefit while a small delay value may be adopted for a low-speed subscriber stations that may potentially benefit from frequency-selective multi-user scheduling. Moreover, if the channel is already sufficiently frequency selective, a small delay value may be sufficient even for frequency-diversity mode transmission for high Doppler subscriber stations.

In an exemplary base station that implements adaptive cyclic delay diversity (ACDD) transmitter using (P+<NUM>) transmit antennas, the cyclic delay values on antenna ANT1 through antenna ANTP for each subscriber station m may be designated Dm1, Dm2, and DmP, respectively. A non-delayed signal (Dm0=<NUM>) is transmitted from the first antenna, designated antenna ANT0. In a more general form, different complex gains, g<NUM>, g<NUM>,. gP, may also be applied to signals transmitted from different transmit antennas. The transmission of the same OFDM symbol from different antennas artificially provides frequency-selective fading. The frequency-selectivity may then be exploited by either using frequency-selective multi-user scheduling for low-speed to medium-speed subscriber stations or frequency-diversity for high-speed subscriber stations.

By using adaptive cyclic delay diversity (ACDD), the reception in the subscriber station receiver resembles multipath transmission from a single transmit antenna. The composite channel response, Hmc(k), on subcarrier k can be written as: <MAT> where Hmn(k) is the channel response for subscriber station m on antenna n, and k is the subcarrier index. In this case, it is assumed that the complex antenna gains, g<NUM>, g<NUM>,. gP, are all unity.

Alternatively, the adaptive cyclic delay diversity operation may be performed directly in the frequency domain. A weight of <MAT> may be applied to subcarrier k transmitted from antenna p to subscriber station m, where DmP is the cyclic delay value on antennas p for subscriber station m.

In one example of resource portioning in an OFDM network, a total of <NUM> OFDM subcarriers may be divided into eight (<NUM>) groups (or subbands) of <NUM> subcarriers each. A given subscriber station may be allocated one or more of these subbands. In an exemplary embodiment of adaptive cyclic delay diversity, the <NUM> subcarriers of a first OFDM symbol may be transmitted from a first antenna with no phase shift (i.e., no delay), while the <NUM> sucarriers of the first OFDM symbol may be transmitted from a second antenna with a delay of one sample period. A one sample delay results in a weight of <MAT> applied to the kth subcarrier. A phase shift of 2π/N is applied to the first subcarrier and phase shift of 2π is applied to the last subcarrier, respectively, where N=<NUM>. Therefore, the phase shift applied to each subcarrier increases linearly with the subcarrier index (i.e., from subcarrier <NUM> to subcarrier <NUM>).

It is noted that a complete cycle of phase shifts from 2π/N to 2π happens over the whole bandwidth. The phase shift increments by 2π/N from one subcarrier to the next. The phase shift applied to the subbands transmitted from the second antenna happens in increments of 2πM/N, where M is the number of subcarriers in a subband. In case of a cyclic delay of D samples, D cycles of phase shift from 2π/N to 2π happen over the whole bandwidth. The benefits provided by cyclic delay diversity may be achieved by applying a different random phase shift to different subcarriers. The receiver obtains the benefits of frequency-diversity because different subcarriers combine constructively and destructively, depending upon the random phase shift applied.

Cyclic delay diversity as well as other forms of transmit diversity schemes, such as space-time diversity (or STD) suffer from performance loss in the case of correlated antennas or correlated channels because there is little or no diversity present in the channel that can be exploited. Additionally, the frequency-selectivity introduced due to delayed transmissions from multiple antennas in case of adaptive cyclic delay diversity (ACDD) may result in loss in performance relative to transmission from a single antenna with no transmit diversity. It is noted that ACDD actually translates spatial or antenna diversity into frequency-diversity. When there is no spatial or antenna diversity present due to correlated antennas, ACDD cannot create any frequency-diversity. However, the delayed transmissions from multiple antennas create frequency-selectivity without frequency diversity, which results in performance loss. The antenna correlations may result from closely spaced antennas, lack of scattering, or both.

Therefore, there is a need for improved wireless networks that implement adaptive cyclic delay diversity. In particular, there is a need for wireless network s that implement adaptive cyclic delay diversity in correlated antenna and correlat ed channels conditions without performance loss. <CIT> relates to a method and system in which, a transmitter demultiplexes an input data stream into M substreams, where each of the M substreams have a data rate lower than a data rate of the input data stream. The substreams are transmitted as signals from a transmit array of M antennas to be received by a receiver array at a receiver in the system, based on a set of transmission parameters. The transmitter adjusts at least one of the transmission parameters based on a condition experienced by the receiver so as to increase data throughput in the system. The adjustable parameters include the number of antennas to employ, the on/off patterns for the antennas, and eigenmode values for the antennas used in order to determine transmit power. Accordingly, system data throughput may be improved while maintaining transmit power and system bandwidth constant. <CIT> provides a transmit diversity apparatus and method for adaptively providing a transmit diversity gain or a beamforming gain depending on changes in a radio channel undergoing multipath fading in a mobile communication system using multiple antennas. A transmitter forms as many fixed beams as the number of transmit antennas and a receiver selects a fixed beam having relatively high power among received fixed beams or linearly combines the received fixed beams. This common eigen space transmit diversity scheme improves the link performance between the transmitter and the receiver. <CIT> provides a method and apparatus that measures received radio communications signals to determine whether or what degree to use transmit diversity and beam forming.

It is the object of the present invention to enable an improved performance in signal transmission.

The object is solved by the subject matter of the independent claims.

To address the above-discussed deficiencies of the prior art, it is a primary object to provide, for use in a wireless network capable of communicating with a plurality of subscriber stations, a base station capable of transmitting in a downlink channel to a first one of the plurality of subscriber stations using a plurality of transmit antennas. In an advantageous embodiment, the base station transmits to the first subscriber station using a selected one of a transmit diversity scheme and a beamforming scheme according to an amount of correlation observed at the first subscriber station in downlink signals transmitted by the plurality of transmit antennas. The base station transmits to the first subscriber station using the transmit diversity scheme if the amount of correlation observed at the first subscriber station is relatively low and transmits to the first subscriber station using the beamforming scheme if the amount of correlation observed at the first subscriber station is relatively high.

In an advantageous embodiment, the base station transmits to the first subscriber station using a selected one of a first cyclic delay diversity scheme having a non-zero delay and a second cyclic delay diversity scheme having a zero delay according to an amount of correlation observed at the first subscriber station in downlink signals transmitted by the plurality of transmit antennas. The base station is further capable of transmitting to the first subscriber station from a first antenna and a second antenna, wherein the signal transmitted from the second antenna is phase-shifted relative to the signal transmitted from the first antenna.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms "include" and "comprise," as well as derivatives thereof, mean inclusion without limitation; the term "or," is inclusive, meaning and/or; the phrases "associated with" and "associated therewith," as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term "controller" means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same.

Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless network.

The present disclosure discloses a new technique wherein the decision between transmit diversity schemes and beamforming schemes is made based on the antenna correlations. When the antennas are uncorrelated, a transmit diversity scheme is selected to exploit the channel diversity. On the other hand, when the channel or antennas are correlated, a beamforming approach is selected to exploit the beamforming gains.

In an advantageous embodiment, the cyclic delays in adaptive cyclic delay diversity (ACDD) are adapted based on channel and antenna correlations. When the antennas are uncorrelated, the ACDD operation delays transmissions from multiple transmit antennas to exploit the diversity. However, when the antennas are correlated, ACDD operation introduces no delays when transmitting the same information from multiple transmit antennas. ACDD operation provides additional beamforming gains when the same information is transmitted from multiple antennas in the case of correlated transmit antennas.

<FIG> illustrates exemplary orthogonal frequency division multiple access (OFDMA) wireless network <NUM>, which selects between a transmit diversity scheme and a beamforming scheme based on the antenna correlations according to the principles of the present disclosure. In the illustrated embodiment, wireless network <NUM> includes base station (BS) <NUM>, base station (BS) <NUM>, base station (BS) <NUM>, and other similar base stations (not shown). Base station <NUM> is in communication with base station <NUM> and base station <NUM>. Base station <NUM> is also in communication with Internet <NUM> or a similar IP-based network (not shown).

Base station <NUM> provides wireless broadband access (via base station <NUM>) to Internet <NUM> to a first plurality of subscriber stations within coverage area <NUM> of base station <NUM>. The first plurality of subscriber stations includes subscriber station <NUM>, which may be located in a small business (SB), subscriber station <NUM>, which may be located in an enterprise (E), subscriber station <NUM>, which may be located in a WiFi hotspot (HS), subscriber station <NUM>, which may be located in a first residence (R), subscriber station <NUM>, which may be located in a second residence (R), and subscriber station <NUM>, which may be a mobile device (M), such as a cell phone, a wireless laptop, a wireless PDA, or the like.

Base station <NUM> provides wireless broadband access (via base station <NUM>) to Internet <NUM> to a second plurality of subscriber stations within coverage area <NUM> of base station <NUM>. The second plurality of subscriber stations includes subscriber station <NUM> and subscriber station <NUM>. In an exemplary embodiment, base stations <NUM>-<NUM> may communicate with each other and with subscriber stations <NUM>-<NUM> using OFDM or OFDMA techniques.

Base station <NUM> may be in communication with either a greater number or a lesser number of base stations. Furthermore, while only six subscriber stations are depicted in <FIG>, it is understood that wireless network <NUM> may provide wireless broadband access to additional subscriber stations. It is noted that subscriber station <NUM> and subscriber station <NUM> are located on the edges of both coverage area <NUM> and coverage area <NUM>. Subscriber station <NUM> and subscriber station <NUM> each communicate with both base station <NUM> and base station <NUM> and may be said to be operating in handoff mode, as known to those of skill in the art.

Subscriber stations <NUM>-<NUM> may access voice, data, video, video conferencing, and/or other broadband services via Internet <NUM>. In an exemplary embodiment, one or more of subscriber stations <NUM>-<NUM> may be associated with an access point (AP) of a WiFi WLAN. Subscriber station <NUM> may be any of a number of mobile devices, including a wireless-enabled laptop computer, personal data assistant, notebook, handheld device, or other wireless-enabled device. Subscriber stations <NUM> and <NUM> may be, for example, a wireless-enabled personal computer (PC), a laptop computer, a gateway, or another device.

<FIG> illustrates selected portions of exemplary base station (BS) <NUM> according to one embodiment of the present disclosure. BS <NUM> comprises P+<NUM> transmit antennas, labeled ANT0 through ANTP. BS <NUM> also comprises P cyclic delay blocks <NUM>, including exemplary cyclic delay blocks 210b, 210c, and 210d, P+<NUM> gain control blocks <NUM>, including exemplary gain control block 220a, 220b, 220c and 220d, and P+<NUM> add cyclic prefix (CP) blocks <NUM>, including exemplary cyclic prefix (CP) blocks 230a, 230b, 230c and 230d.

In a first transmit path, an undelayed copy of a first N-sample OFDM symbol is amplified by complex gain factor g0 by gain control block 220a. Add CP block 230a receives the scaled output of gain control block 220a, copies the last G samples of the N-sample block, and appends the last G samples to the start of the N-sample block, thereby generating N+G samples that are sent to antenna ANT0 (other parts of the transit path, such as an IFFT block are omitted for clarity).

The OFDM symbol is also applied to the remaining P transmit paths in BS <NUM>, except that a delay is applied to the other P copies of the OFDM symbol. By way of example, cyclic delay block 210b receives the OFDM symbol and delays the sample by a cyclic delay value Dm1. Gain control block 220b amplifies the delayed OFDM symbol by complex gain factor g1. Add CP block 230b receives the delayed, scaled output of gain control block 220b, copies the last G samples of the N-sample block, and appends the last G samples to the start of the N-sample block, thereby generating N+G samples that are sent to antenna ANT1. Thus, P+<NUM> copies of the OFDM symbol are transmitted from the P+<NUM> antennas of BS <NUM>.

<FIG> depicts flow diagram <NUM>, which illustrates the selection between transmit diversity and beamforming based on antenna correlation according to one embodiment of the present disclosure. Initially, BS <NUM> receives feedback message from a subscriber station that contains antenna (or channel) correlation information (process step <NUM>). Next, BS <NUM> determines from the feedback whether there is a high amount (or degree) of correlation or a low amount (or degree) of antenna/channel correlation (process step <NUM>). By way of example, BS <NUM> may make this determination by comparing the measured level of correlation to a known threshold value. If there is a low degree of correlation for the subscriber station, BS <NUM> selects a transmit diversity scheme for downlink transmission to the subscriber station (process step <NUM>). BS <NUM> then transmits according to the selected scheme (process step <NUM>). If there is a high degree of correlation for the subscriber station, BS <NUM> selects a beamforming scheme for downlink transmission to the subscriber station (process step <NUM>). BS <NUM> then transmits according to the selected scheme (process step <NUM>).

In addition to selecting between transmit diversity and beamforming based on channel/antenna correlation, the present disclosure provides that wireless network <NUM> may select between zero delay and non-zero delay in cyclic delay diversity mode based on channel/antenna correlation. <FIG> depicts flow diagram <NUM>, which illustrates the selection between non-zero cyclic delay diversity (CDD) and zero cyclic delay diversity (zero-CDD) in wireless network <NUM> according to one embodiment of the present disclosure. Initially, BS <NUM> transmits a first pilot signal (or reference signal) from antenna ANT1 (process step <NUM>) and transmit a second pilot signal (or reference signal) from antenna ANT2 (process step <NUM>) to subscriber station (SS) <NUM>. Next, SS <NUM> computes the channel correlations on the pilot or reference signals (process step <NUM>) and reports back the antenna correlation and phase information to BS <NUM> (process step <NUM>). BS <NUM> then processes the channel correlation information (process step <NUM>) and decides, based on the channel correlations, between <NUM>) cyclic delay diversity (CDD) mode with a delay of one or more samples; and <NUM>) zero cyclic delay diversity (zero-CDD) (process step <NUM>). Finally, BS <NUM> then transmits to SS <NUM> in the downlink according to the selected mode (process step <NUM>).

<FIG> depicts flow diagram <NUM>, which illustrates the selection between non-zero cyclic delay diversity (CDD) and zero cyclic delay diversity (zero-CDD) in BS <NUM> according to one embodiment of the present disclosure. Initially, BS <NUM> receives feedback message from SS <NUM> that contains antenna (or channel) correlation information (process step <NUM>). Next, BS <NUM> determines from the feedback message whether there is a high degree of correlation or a low degree of correlation (process step <NUM>). Again, by way of example, BS <NUM> may make this determination by comparing the amount of correlation observed by SS <NUM> to a known threshold value. If there is a low amount (or degree) of correlation for the subscriber station, BS <NUM> selects a non-zero delay value in cyclic delay diversity (CDD) mode scheme for downlink transmission to SS <NUM> (process step <NUM>). BS <NUM> then transmits according to the selected scheme (process step <NUM>).

If there is a high amount (or degree) of correlation for SS <NUM>, BS <NUM> selects a zero-delay value in zero-CDD mode for downlink transmission to SS <NUM> (process step <NUM>). BS <NUM> applies a fixed phase shift to the signals transmitted from the second antenna (ANT2) to compensate for the phase difference between signals transmitted to SS <NUM> at an angle, φ, from bore sight of the transmitter in BS <NUM> (process step <NUM>). BS <NUM> then transmits according to the selected scheme (process step <NUM>).

<FIG> illustrates transmission from base station <NUM> to subscriber station <NUM> using a phase adjustment to co-phase the signals transmitted from antennas ANT1 and ANT2 according to one embodiment of the disclosure. The phase adjustment depends upon the Angle-of-Arrival (AoA) or Angle-of-Departure (AoD), φ, of the transmitted signals as shown in <FIG>. When φ=<NUM>, no phase adjustment is needed because the signals arrive in co-phase at SS <NUM>. In general, a phase shift of <MAT> is applied for signals transmitted from antenna ANT2 relative to signals transmitted from antenna ANT1.

<FIG> illustrates zero cyclic delay diversity (Zero-CDD) using a phase adjustment applied to subcarriers on antenna ANT2 relative to antenna ANT1 according to one embodiment of the disclosure. In <FIG>, wireless network <NUM> transmits in OFDM using, by way of example, <NUM> subcarriers that are divided into <NUM> subbands (SB1 through SB8), where each subband contains <NUM> subcarriers.

<FIG> illustrates zero cyclic delay diversity (Zero-CDD) using a phase adjustment to co-phase the transmitted signals from each of the subbands and each of the antennas according to one embodiment of the disclosure. It is noted that the phase adjustment is a function of frequency (i.e., wavelength) and therefore different subcarriers or different subbands require different phase adjustments to co-phase the transmitted signals. In general, a phase shift that is inversely proportional to the subband frequency is applied, with <MAT> phase shift applied to the ith subband.

<FIG> depicts flow diagram <NUM>, which illustrates the selection in BS <NUM> between non-zero cyclic delay diversity (CDD) and zero cyclic delay diversity (zero-CDD) based on the channel correlations measured from uplink transmissions according to one embodiment of the disclosure. Initially, BS <NUM> receives signals transmitted in the uplink by SS <NUM>, including, for example, data signals, pilot signals, and control signals (process step <NUM>). BS <NUM> then measures the channel correlations and relative phases of the received signals (process step <NUM>). BS <NUM> processes the channel correlation information (process step <NUM>) and selects either cyclic delay diversity (CDD) with a delay of one or more samples or zero cyclic delay diversity (zero-CDD) with no delay based on the channel correlations measured from uplink transmissions (process step <NUM>). BS <NUM> may also apply a phase shift on antenna ANT2 relative to antenna ANT1 according to the relative phase information measured from uplink transmissions (process step <NUM>). Finally, BS <NUM> transmits downlink signals to SS <NUM> according to the selected mode (process step <NUM>).

<FIG> depicts flow diagram <NUM>, which illustrates the selection between non-zero cyclic delay diversity (CDD) and zero cyclic delay diversity (zero-CDD) in SS <NUM> based on the channel correlations measured from downlink transmissions according to one embodiment of the disclosure. Initially, BS <NUM> transmits a first pilot signal (or reference signal) from antenna ANT1 (process step <NUM>) and transmit a second pilot signal (or reference signal) from antenna ANT2 (process step <NUM>) to subscriber station (SS) <NUM>. SS <NUM> computes or measures the channel correlations on the pilot or reference signals (process step <NUM>) and then processes the channel correlation information (process step <NUM>) to determine the amount of correlation. Based on the amount of correlation, SS <NUM> selects either non-zero CDD or zero-CDD for downlink transmissions from BS <NUM> (process step <NUM>).

Next, SS <NUM> transmits a message back to BS <NUM> that indicates whether non-zero CDD or zero-CDD has been selected and also feedbacks information on the relative phase to be applied to transmissions from transmit antenna ANT2 relative to antenna ANT1 (process step <NUM>). BS <NUM> then selects the mode indicated by SS <NUM> and applies the indicated phase shift on antenna ANT2 relative to antenna ANT1 (process step <NUM>). BS <NUM> then transmits in the downlink (process step <NUM>).

<FIG> depicts flow diagram <NUM>, which illustrates the selection between block codes-based transmit diversity and beamforming according to one embodiment of the disclosure. Initially, BS <NUM> receives feedback message from a subscriber station that contains antenna (or channel) correlation information (process step <NUM>). Next, BS <NUM> determines from the feedback information whether there is a high degree of correlation or a low degree of antenna/channel correlation (process step <NUM>). If there is a low degree of correlation for the subscriber station, BS <NUM> selects a block code-based transmit diversity scheme, such as SFBC or STBC for downlink transmission to the subscriber station (process step <NUM>). BS <NUM> then transmits according to the selected scheme (process step <NUM>). If there is a high degree of correlation for the subscriber station, BS <NUM> selects beamforming for downlink transmission to the subscriber station (process step <NUM>). BS <NUM> then transmits according to the selected scheme (process step <NUM>).

In an advantageous embodiment, BS <NUM> may switch between zero-CDD and CDD (i.e., non-zero CDD) happens based on the hybrid ARQ ACK/NACK feedback. For example, a first hybrid ARQ transmission may be performed using zero-CDD mode. However, if an error occurs, as indicated by a negative acknowledgment (NACK) message from SS <NUM>, BS <NUM> sends the subsequent hybrid ARQ transmissions using a non-Zero CDD mode.

Many of the details of the disclosure have been explained in an embodiment using two transmit antennas. However, the principles of the current invention are easily applied to the case of more than two transmit antennas in a straight forward manner.

Claim 1:
A method of a subscriber station in a wireless network, the method comprising:
receiving (<NUM>, <NUM>), from a base station, pilot signals corresponding to a plurality of antennas of the base station, respectively;
generating (<NUM>, <NUM>) correlation information based on the received pilot signals, the correlation information indicating a correlation level for downlink channels corresponding to the plurality of antennas;
determining (<NUM>) one of a first cyclic delay diversity scheme having a non-zero delay and a second cyclic delay diversity scheme having a zero delay as a downlink transmission scheme based on the generated correlation information; and
transmitting (<NUM>), to the base station, information about the determined downlink transmission scheme.