Multi-antenna station with distributed antennas

[0001] I. Field

[0002] The present disclosure relates generally to communication, and more specifically to a multi-antenna station.

[0003] II. Background

[0004] A wireless local area network (WLAN) has one or more access points that serve one or more user terminals. The number of access points and the number of user terminals are dependent on the size the WLAN. For example, a single access point may serve multiple user terminals distributed throughout a WLAN deployment area, which may be an entire building, a floor of a building, and so on. If the access point is stationary, which is often the case, then the performance achieved by each user terminal is typically dependent on the location of that user terminal relative to the access point. It is well known that a radio frequency (RF) signal is degraded by obstructions (e.g., walls) and artifacts (e.g., noise and interference) in the signal paths between a transmitter and a receiver. Thus, a nearby user terminal that is located close to, and in sight of, the access point can achieve better performance than a remote user terminal that is located far away from, and not in visible range of, the access point. Consequently, different levels of performance (e.g., different data rates) are typically achievable for different user terminals located in different parts of the WLAN deployment area.

[0005] It is desirable to provide similar levels of performance to all or as many user terminals as possible in the WLAN deployment area. There is therefore a need in the art for an access point capable of providing such performance to the user terminals.

[0006] A multi-antenna station with distributed antennas and capable of providing good performance for terminals distributed throughout the coverage area of the multi-antenna station is described herein. According to an embodiment of the invention, a multi-antenna station is described which includes multiple antennas, a controller, and at least one transmitter unit. The multiple antennas couple to the multi-antenna station and comprise at least one remote antenna that is located away from the multi-antenna station. The controller selects a set of one or more antennas from among the multiple antennas for data transmission to a terminal. The at least one transmitter unit transmits data via the set of one or more antennas to the terminal.

[0007] According to another embodiment, a method is provided in which a set of one or more antennas is selected from among multiple antennas for data transmission from a multi-antenna station to a terminal. The multiple antennas comprise at least one remote antenna that is located away from the multi-antenna station. Data is transmitted to the terminal via the set of one or more antennas.

[0008] According to yet another embodiment, an apparatus is described which includes means for selecting a set of one or more antennas from among multiple antennas for data transmission to a terminal, where the multiple antennas comprise at least one remote antenna that is located away from the apparatus, and means for transmitting data to the terminal via the set of one or more antennas.

[0009] Various aspects and embodiments of the invention are described in further detail below.

[0015] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments.

[0016] FIG. 1 shows an exemplary WLAN 100 with a single access point (AP) 110 that serves multiple user terminals (UT) 120. Access point 110 is a multi-antenna station that is equipped with multiple antennas, which may be used for data transmission and reception. An access point may also be called a base station, a Node B, or some other terminology. Each user terminal 120 may be equipped with a single antenna or multiple antennas. A user terminal may also be called a mobile station, a user equipment, a wireless device, or some other terminology.

[0017] Access point 110 may send a multiple-input multiple-output (MIMO) transmission to a multi-antenna user terminal or a multiple-input single-output (MISO) transmission to a single-antenna user terminal. Access point 110 may also receive a MIMO transmission from a multi-antenna user terminal or a single-input multiple-output (SIMO) transmission from a single-antenna user terminal. A MIMO channel formed by multiple (N_T) transmit antennas at a transmitter and multiple (N_R) receive antennas at a receiver may be decomposed into N_S spatial channels, where N_S≦min {N_T, N_R}. The N_S spatial channels may be used to transmit data in parallel to achieve higher throughput and/or redundantly to achieve greater reliability. A MISO channel formed by the multiple transmit antennas and the single receive antenna is composed of a single spatial channel. The multiple transmit antennas may be used to transmit data in a manner to improve reliability. A SIMO channel formed by the single transmit antenna and the multiple receive antennas is also composed of a single spatial channel. The multiple receive antennas may be used to receive data in a manner to improve reliability.

[0018] Access point 110 may be a fixed station and may be placed at any location within the WLAN deployment area, which may be a building, a floor of a building, a home, an office complex, a store, and so on. User terminals 120 may be distributed throughout the WLAN deployment area. Some user terminals (e.g., user terminals 120c and 120d) may be located close to access point 110 while some other user terminals (e.g., user terminals 120a and 120b) may be located far away from access point 110. Each user terminal 120 may be fixed or mobile. There are typically obstructions (e.g., walls) within the WLAN deployment area that degrade the RF signals transmitted between access point 110 and user terminals 120. There may also be other artifacts (e.g., interference) that can also degrade the RF signals. These obstructions and artifacts are not shown in FIG. 1 for simplicity.

[0019] To provide good performance to all or many user terminals 120, the multiple antennas of access point 110 are distributed throughout the WLAN deployment area. Access point 110 has L local antennas 112a through 112l that are mounted on or close to the access point. In general, L may be zero, one, or more than one, i.e., L≧0. Access point 110 further has M remote antennas 114a through 114m that are placed at different locations throughout the WLAN deployment area. Remote antennas 114 may be placed at different distances from access point 110, different angular positions with respect to access point 110, and so on. In general, M is one or greater, i.e., M≧1. The total number of local and remote antennas available for use by access point 110 is N_ap, which is N_ap=L+M≧2.

[0020] Various types of antennas may be used for local antennas 112 and remote antennas 114, which are also called the access point (AP) antennas. For example, each AP antenna may be a cross-pole antenna, a dipole antenna, a patch antenna (or chip), and so on. An antenna may also be called an antenna element, a radiation element, and so on. Each AP antenna is designed for the operating frequency band used for WLAN 100. Each AP antenna may also have any radiation pattern. The antenna type and the radiation pattern for each AP antenna may be selected based on the intended coverage area for that AP antenna. For example, an antenna with an omni-directional radiation pattern may be used for remote antenna 114d located near the middle of the WLAN deployment area in FIG. 1, antennas with directional radiation patterns may be used for remote antennas 114a, 114c, 114e and 114n located at the corners of the WLAN deployment area.

[0021] For the embodiment shown in FIG. 1, each remote antenna 114 couples to a respective remote front-end (RFE) 116 that performs signal conditioning (e.g., amplification, filtering, and so on) for the RF signals transmitted and received via that remote antenna. Several embodiments of remote front-end 116 are described below. Remote front-end 116 for each remote antenna 114 couples to access point 110 via a cable 118. Cable 118 may be a co-axial cable that is commonly used for cable television or may be some other type of cable that supports the operating frequency band for WLAN 100. Cable 118 may also be replaced with other types of communication link (e.g., RF, infrared, and so on). Remote front-end 116 can reduce signal loss due to cable 118 and can thus improve performance. In general, remote front-end 116 may or may not be used for each remote antenna 114.

[0022] The local and remote antennas for access point 110 may be arranged and selected in various manners. Several exemplary configurations of the local and remote antennas are described below. Several exemplary embodiments for selecting antennas for data transmission are also described below.

[0023] FIG. 2A shows an access point 110a with no local antenna and multiple (M>1) remote antennas 114a through 114m. Access point 110a is one embodiment of access point 110 in FIG. 1 and has N_ap=M antennas that are available for data transmission and reception. Each remote antenna 114 couples to access point 110a via an associated remote front-end 116 and cable 118. For the embodiment shown in FIG. 2A, each remote antenna 114 couples to a respective power detector (Power Det) 290 within access point 110a. Each power detector 290 measures the power of the RF signal received by the associated antenna and provides power measurements to a controller 220. Controller 220 uses the power measurements from all power detectors 290 to select antennas for data transmission and reception.

[0024] FIG. 2B shows an access point 110b with a single (L=1) local antenna 112 and multiple (M>1) remote antennas 114a through 114m. Access point 110b is another embodiment of access point 110 in FIG. 1 and has N_ap=M+1 antennas that are available for data transmission and reception. For the embodiment shown in FIG. 2B, each of the M+1 antennas couples to a respective power detector 290 within access point 110. Controller 220 receives the power measurements from all power detectors 290 and selects antennas for data transmission and reception.

[0025] FIG. 2C shows an access point 110c with multiple (L>1) local antennas 112a through 112l and multiple (M>1) remote antennas 114a through 114m. Access point 110c is yet another embodiment of access point 110 in FIG. 1 and has N_ap=L+M antennas that are available for data transmission and reception. In general, L may or may not be equal to M.

[0026] For the embodiments shown in FIGS. 2A through 2C, each of the N_ap antennas available at the access point may be individually selected for data transmission and/or reception. Controller 220 may select one antenna, a subset of the available antennas, or all of the available antennas for data transmission to and/or data reception from a given user terminal. Controller 220 may select antennas based on the power measurements for the available antennas, as described below.

[0027] FIG. 2D shows an access point 110d with multiple (M>1) local antennas 112a through 112m and multiple (M) remote antennas 114a through 114m. Access point 110d is yet another embodiment of access point 110 in FIG. 1 and has N_ap=2M antennas that are available for data transmission and reception. For the embodiment shown in FIG. 2D, each local antenna 112 is associated with one remote antenna 114. M pairs of antennas are formed with the M local antennas 112 and the M remote antennas 114. In an embodiment, controller 220 selects one or more antenna pairs for data transmission to a given user terminal and further selects one antenna in each selected pair, e.g., based on the power measurements for the two antennas in that pair.

[0028] WLAN 100 may impose a constraint that at most N antennas may be used for data transmission, where N may be equal to two, four, or some other value. WLAN 100 may also support multiple transmission modes. Each transmission mode may require a certain minimum number of antennas or a specific number of antennas for data transmission using that transmission mode, as described below. For simplicity, the following description assumes that access point 110 can select up to N antennas for data transmission to a given user terminal. Access point 110 also selects a transmission mode to use for data transmission based on the number of selected antennas. Access point 110 may select antennas for data transmission to the user terminal in various manners.

[0029] In an embodiment, access point 110 selects N antennas with the highest power measurements for data transmission to the user terminal, where N<N_ap. For the embodiments shown in FIGS. 2A through 2C, access point 110 receives power measurements for the N_ap available antennas, sorts the power measurements (e.g., from highest to lowest), and selects the N antennas with the N highest power measurements. For the embodiment shown in FIG. 2D, access point 110 selects the N best pairs of antennas based on the power measurements and selects the better of the two antennas in each pair.

[0030] In another embodiment, access point 110 selects up to N antennas with power measurements exceeding a predetermined power threshold, P_th. Access point 110 may compare the power measurement for each antenna against the power threshold, retain the antenna if its power measurement exceeds the power threshold, and discard the antenna otherwise. Access point 110 selects the N best antennas if more than N antennas have power measurements that exceed the power threshold. Access point 110 may select fewer than N antennas if the power measurements for only these antennas exceed the power threshold. Access point 110 may select the best available antenna or a predetermined number of best antennas if no antennas have power measurements that exceed the power threshold.

[0031] In yet another embodiment, access point 110 initially selects the N best antennas with the highest power measurements and then discards all antennas having small contributions to the data transmission to the user terminal. This antenna pruning may be achieved as follows. Access point 110 orders the N best antennas based on their power measurement, e.g., from the highest power measurement P₁ to the lowest power measurement P_N for the N antennas. Access point 110 then computes a power gap ΔP_i between each two adjacent sorted antennas i and i+1 as the difference in their power measurements, or ΔP_i=P_i−P_i+1 for i=1, . . . , N-1. Access point 110 next determines whether the power gap ΔP_i for any antenna pair exceeds a predetermined amount ΔP_th, or whether ΔP_i>ΔP_th for i=1, . . . , N-1. If the power gap ΔP_j for a given antenna j exceeds the predetermined amount, then access point 110 discards all antennas with power measurements of P_j+1 or lower. This embodiment removes antennas having small contributions to the data transmission sent to the user terminal, which reduces crosstalk among the antennas.

[0032] The embodiments described above are for antenna selection based on power measurements available at access point 110. Access point 110 may select antennas based on other parameters instead of or in additional to received power. For example, access point 110 may select antennas based on (1) received signal-to-noise ratio (SNR), which is the ratio of the desired signal (e.g., pilot) over the total noise and interference, (2) received signal strength, which is the ratio of the desired signal over the total received power, or (3) some other indication of received signal quality. In the following description, an antenna measurement can refer to any type of measurement (e.g., power, SNR, signal strength, and so on) that is suitable for use to select antennas.

[0033] Access point 110 may also select antennas based on other information available for the antennas. In an embodiment, access point 110 selects antennas based on location information for the local and remote antennas. Each antenna may be associated with a set of one or more nearby antennas. This location information may be stored in a database. If a high power measurement is obtained for a given antenna, then access point 110 may select one or more other antennas that are known to be located nearby this antenna. For example, if a high power measurement is obtained for remote antenna 114a in FIG. 1, then access point 110 may select antenna 114b and/or antenna 114d for data transmission to the user terminal.

[0034] In another embodiment, access point 110 selects antennas based on location information for the local and remote antennas. If the locations of the local and remote antennas and the location of a user terminal are known, then access point 110 can select one or more antennas that are located nearby the user terminal. The locations of the AP antennas may be ascertained and provided during deployment. The location of the user terminal may be approximated via measurements and/or ascertained in some other manner.

[0035] In yet another embodiment, access point 110 selects antennas based on correlation information for the local and remote antennas. Some antennas may have high correlation, which results in excessive crosstalk and poor spatial separation between these antennas. Consequently, antennas with high correlation should not be selected together. The correlation information for the available antennas may be ascertained based on the placement, type and/or measurements for the antennas. For example, local antennas 112 may have high correlation because of their close spacing, and it may be preferable to select only one or a few local antennas for data transmission.

[0036] Access point 110 may also select antennas based on information obtained from higher layers that sit above a physical layer in a protocol stack. Access point 110 typically transmits data in packets to a user terminal. This user terminal may send back an acknowledgment (ACK) for each packet that is decoded correctly and a negative acknowledgment (NAK) for each packet that is decoded in error. Access point 110 may initially select a set of antennas for data transmission to the user terminal, e.g., based on the power measurements. If a large percentage of packets is decoded in error, then access point 110 may select a different set of antennas for data transmission to the user terminal.

[0037] Access point 110 may select antennas for data transmission to the user terminal in other manners, and this is within the scope of the invention. Access point 110 may select antennas based on any criterion or any combination of criteria.

[0038] Access point 110 may select different sets of antennas for data transmission to different user terminals, e.g., based on the measurements for these user terminals. Access point 110 may obtain measurements for each user terminal prior to data transmission to the user terminal and may select a set of antennas for the user terminal based on these measurements. This allows access point 110 to use the best set of antennas for each data transmission.

[0039] Access point 110 may store the antenna set selected for each user terminal in a look-up table. This antenna set may be indexed by an identifier for the user terminal. This identifier may be a Medium Access Control identifier (MAC ID) that access point 110 assigns to the user terminal at the start of a communication session or may be some other type of identifier. Table 1 shows an exemplary look-up table for user terminals 120a through 120d in the example shown in FIG. 1.

[0040]

	TABLE 1
	User terminal	Antenna Set
	User terminal 120a	Antenna 114a
	User terminal 120b	Antennas 114a, 114d, 114e and 114f
	User terminal 120c	Antennas 112a, 112l and 114m
	User terminal 120d	Antennas 112l and 114c

[0041] Access point 110 may not have any measurements for a given user terminal at the start of a data transmission to the user terminal. Access point 110 may then access the look-up table with the user terminal's MAC ID and retrieve the set of antennas previously selected for the user terminal. Access point 110 may transmit data to the user terminal using this previously selected set of antennas until the set is updated, e.g., with new measurements.

[0042] Access point 110 selects a first set of T antennas for downlink data transmission to a user terminal and selects a second set of R antennas for reception of an uplink data transmission from the user terminal. In general, N≧T≧1, N≧R≧1, and T may or may not be equal to R. R may also be greater than N if supported by the spatial processing at access point 110, but this possibility is not described below for simplicity. The number of transmit antennas (T) is dependent on the number of good antennas available for downlink data transmission, the transmission mode used by access point 110 for the downlink data transmission, and possibly other factors. An antenna may be considered as good if it passes one or more selection criteria, e.g., if its power measurement exceeds the power threshold. The number of receive antennas (R) is dependent on the number of good antennas available for uplink data reception, the transmission mode used by the user terminal for the uplink data transmission, and possibly other factors.

[0043] Access point 110 may select the first set of T transmit antennas as described above and may also select the second set of R receive antennas in similar manner. The first set of antennas may be the same as, or different from, the second set of antennas. The processing at access point 110 may be simplified by using a single set of antennas for both data transmission and reception. In this case, each selected antenna is used to both transmit and receive RF signals to/from the user terminal.

[0044] FIG. 3 shows a process 300 performed by access point 110 for transmitting data to and receiving data from a user terminal. Measurements are obtained for the local and remote antennas at access point 110, e.g., based on a pilot transmitted by the user terminal (block 310). The measurements may be for received power and/or some other parameters. A first set of one or more (T) transmit antennas is selected from among the N_ap antennas available at access point 110 based on the measurements and/or other information, where N_ap>N≧T≧1 (block 312). The antenna selection may be performed in various manners, as described above. If no measurements are available, then the set of antennas last used for data transmission to the user terminal may be retrieved from the look-up table and used for the current downlink data transmission. A transmission mode is selected for data transmission to the user terminal based on the number of transmit antennas selected.

[0045] A second set of one or more (R) receive antennas is also selected from among the N_ap antennas available at access point 110 based on the measurements and/or other information, where N_ap>N≧R≧1 (block 314). The first and second sets may have the same or different numbers of antennas, e.g., depending on the transmission modes used for downlink and uplink data transmissions. Even if R=T, the second set may contain the same or different antennas than the antennas in the first set.

[0046] Access point 110 processes data in accordance with the selected transmission mode for the downlink (block 316) and then transmits the processed data from the first set of T antennas to the user terminal (block 318). Access point 110 receives the uplink data transmission from the user terminal via the second set of R antennas (block 320).

[0047] Referring back to FIG. 1, access point 110 may serve multiple user terminals 120 within WLAN 100. Each user terminal 120 may require a specific set of transmit antennas for good downlink performance and a specific set of receive antennas for good uplink performance. Access point 110 may dynamically switch between different sets of antennas used for different user terminals so that each user terminal is served with the sets of transmit/receive antennas that provide good performance for that user terminal. The electronics at access point 110 may be designed with the capability to quickly switch (e.g., on a per data packet basis, or a per frame basis) to different sets of antennas for different user terminals.

[0048] FIG. 4 shows an embodiment of access point 110. For this embodiment, access point 110 includes a digital section 210 that performs digital processing, N transceivers 230a through 230n that perform signal conditioning for RF and baseband signals for N antennas, and an RF switch 280 that couples the N transceivers 230 to the N antennas selected from among the N_ap available antennas at access point 110.

[0049] Each transceiver 230 includes a transmitter unit (TMTR) 240 and a receiver unit (RCVR) 260. The transmitter and receiver units may be implemented with a super-heterodyne architecture or a direct-conversion architecture. For the super-heterodyne architecture, frequency conversion between RF and baseband is performed in multiple stages, e.g., from RF to an intermediate frequency (IF) in one stage, and from IF to baseband in another stage. For the direct-conversion architecture, frequency conversion is performed in a single stage, e.g., from RF directly to baseband. For simplicity, FIG. 4 shows an embodiment of transmitter unit 240 and receiver unit 260 implemented with the direct-conversion architecture.

[0050] Within transmitter unit 240, a digital-to-analog converter (DAC) 242 receives a stream of digital chips from digital section 210, converts the chips to analog, and provides an analog baseband signal. A filter 244 then filters the analog baseband signal to remove undesired images generated by the digital-to-analog conversion and provides a filtered baseband signal. An amplifier (Amp) 246 amplifies and buffers the filtered baseband signal and provides an amplified baseband signal. A mixer 248 modulates a TX_LO carrier signal from a voltage controlled oscillator (VCO) (not shown in FIG. 4) with the amplified baseband signal and provides an upconverted signal. A power amplifier (PA) 250 amplifies the upconverted signal and provides an RF modulated signal to RF switch 280.

[0051] For the transmit path, RF switch 280 receives up to N RF modulated signals from transmitter units 240 in N transceivers 230a through 230n. RF switch 280 also receives an Ant_Sel control signal that indicates which transceiver is to be coupled to which AP antenna. RF switch 280 routes each received RF modulated signal to either a selected local antenna 112 or a remote front-end 116 for a selected remote antenna 114. For the receive path, RF switch unit 280 receives an RF input signal from each local antenna 112 and each remote antenna 114 selected for data reception. RF switch unit 280 routes each received RF input signal to receiver unit 260 within the designated transceiver 230. RF switch unit 280 may be implemented with transmit/receive (T/R) switches, duplexers, and so on, as is known in the art. RF switch unit 280 avoids the need to have a separate transceiver 230 for each of the N_ap antennas at access point 110.

[0052] Within receiver unit 260, a low noise amplifier (LNA) 262 receives an RF input signal from RF switch 280 for a selected AP antenna. LNA 262 amplifies the received RF signal and provides a conditioned RF signal having the desired signal level. A mixer 264 demodulates the conditioned RF signal with an RX_LO signal from the VCO and provides a downconverted signal. A filter 266 filters the downconverted signal to pass the desired signal components and remove noise and undesired signals that may be generated by the frequency downconversion process. An amplifier 268 amplifies and buffers the filtered signal and provides an analog baseband signal. An analog-to-digital converter (ADC) 270 digitizes the analog baseband signal and provides a stream of samples to digital section 210.

[0053] For the embodiment shown in FIG. 4, power detector 290 within receiver unit 260 receives the conditioned RF signal from LNA 262, measures the received power in the conditioned RF signal, and provides power measurements to main controller 220 within digital section 210. Power detector 290 may also measure the received power based on a baseband signal (e.g., after filter 266 or amplifier 268). Power detector 290 may be implemented in various manners, as is known in the art.

[0054] FIG. 4 shows an exemplary design for the transmitter and receiver units. In general, the transmitter and receiver units may each include one or more stages of amplifier, filter, mixer, and so on, which may be arranged differently from the configuration shown in FIG. 4. The transmitter and receiver units may also include different and/or additional elements not shown in FIG. 4.

[0055] FIG. 4 also shows an embodiment of digital section 210, which includes various processing units that perform digital processing for data transmission and reception. Within digital section 210, a data processor 212 performs encoding, interleaving, and symbol mapping for data transmission and symbol demapping, deinterleaving, and decoding for data reception. A spatial processor 214 performs transmitter spatial processing (e.g., for beamforming, eigensteering, and so on) for data transmission and receiver spatial processing (e.g., spatial matched filtering) for data reception, as described below. A modulator 216 performs modulation (e.g., for orthogonal frequency division multiplexing (OFDM)) for data transmission. A demodulator 218 performs demodulation (e.g., for OFDM) for data reception. A detection/acquisition unit 224 performs processing to detect and acquire signals from the user terminals. Main controller 220 controls the operation of various processing units within access point 110 and generates controls for transceivers 230 and remote front-ends 116. For example, main controller 220 may generate the T_i control signals used to enable and disable each transmitter unit 230 and the R_i control signals used to enable and disable each receiver unit 260. A power controller 226 performs power management for access point 110. For example, power controller 226 may determine whether or not to send DC power to remote front-ends 116. A random access memory (RAM) and a read only memory (ROM) 222 store data and program codes used by various processing units within digital section 210. For example, memory 222 may store the antenna set selected for each user terminal.

[0056] FIG. 5A shows an embodiment of a remote front-end 116x, which may be used for each remote front-end 116 shown in FIG. 1. Remote front-end 116x may be used for a time division duplexed (TDD) communication system that transmits data on the downlink and uplink on the same frequency band at different times. For the embodiment shown in FIG. 5A, remote front-end 116x includes switches 510 and 540, a power amplifier 520, a low noise amplifier 530, and a bandpass filter 550. Switches 510 and 540 receive a transmit/receive (T/R) control signal that indicates whether RF signals are being transmitted or received by access point 110. Each switch couples its input to a “T” output during the transmit portion and to an “R” output during the receive portion, as indicated by the T/R control signal. Main controller 220 may generate the T/R control signal and provide this signal to each remote front-end 116 via the associated transceiver 230 (not shown in FIG. 4).

[0057] For the transmit path, an RF modulated signal from an associated transmitter unit 240 is received via a first port, routed via switch 510, and amplified by power amplifier 520 with a fixed or variable gain to obtain the desired output signal level. The amplified signal from power amplifier 520 is routed via switch 540, filtered by filter 550 to remove out-of-band noise and undesired signal components, and provided via a second port to an associated remote antenna 114. For the receive path, an RF input signal from the associated remote antenna 114 is received via the second port, filtered by filter 550 to remove out-of-band noise and undesired signal components, routed via switch 540, and amplified by LNA 530 with a fixed or variable gain. The amplified signal from LNA 530 is routed via switch 510 and provided via the first port to an associated receiver unit 260.

[0058] Power amplifier 520 and/or LNA 530 may be powered down whenever possible to reduce power consumption. For example, the T/R control signal may power down power amplifier 520 during the receive portion and power down LNA 530 during the transmit portion. The RF signals, T/R control signal, and DC power may be provided by access point 110 to remote front-end 116x via cable 118 or by some other means.

[0059] FIG. 5B shows an embodiment of a remote front-end 116y, which may also be used for each remote front-end 116 shown in FIG. 1. Remote front-end 116y may be used for a frequency division duplexed (FDD) communication system that can transmit data on the downlink and uplink simultaneously on different frequency bands. For the embodiment shown in FIG. 5B, remote front-end 116y includes duplexers 512 and 542, power amplifier 520, and LNA 530.

[0060] For the transmit path, an RF modulated signal from an associated transmitter unit 240 is received via the first port, filtered by duplexer 512, routed to power amplifier 520, amplified with a gain to obtain the desired output signal level, filtered by duplexer 542, and provided via the second port to an associated remote antenna 114. For the receive path, an RF input signal from the associated antenna 114 is received via the second port, filtered by duplexer 542, routed to LNA 530, amplified with a gain, filtered by duplexer 512, and provided via the first port to an associated receiver unit 260. The T/R control signal is not needed for remote front-end 116y.

[0061] FIGS. 5A and 5B show specific embodiments of remote front-ends 116x and 116y, respectively. In general, the transmit and receive paths may each include one or more stages of amplifier, filter, and so on. The transmit and receive paths may also include fewer, different, and/or additional circuit blocks not shown in FIGS. 5A and 5B.

[0062] For clarity, the description above shows each remote antenna 114 being coupled to an associated remote front-end 116, and each transceiver 230 processing the RF signals for one AP antenna. In general, each remote front-end 116 and/or each transceiver 230 may be associated with a set of one or more antenna elements. If a remote front-end or a transceiver is associated with multiple antenna elements, then these antenna elements may be viewed as a single (distributed) “antenna” for the remote front-end or transceiver.

[0063] In WLAN 100, “dummy” stations may be placed throughout the WLAN deployment area and used for various functions such as system configuration, calibration of the transmitter and receiver electronics, antenna selection, and so on. These dummy stations may be inexpensive stations that have basic MAC/PHY functionality and may not require all of the software needed for regular stations.

[0064] Each dummy station may transmit training/pilot/sounding packets at designated times or whenever directed by access point 110. Access point 110 may use the training packets to perform various functions. For example, access point 110 may calibrate the frequency responses of transmitter unit 240 and receiver unit 260 within transceivers 230a through 230n and the frequency responses of remote front-ends 116a through 116m based on the training packets. If the dummy stations are at known locations, then access point 110 can ascertain the channel quality observed by remote antennas 114 and may use the channel quality information for antenna selection.

[0065] An exemplary scenario for using the dummy stations may be as follows. The dummy stations may be placed strategically at entrances and exits to the coverage area, e.g., at entrances to a big office complex with many cubicles and offices. Each dummy stations can transmit training packets to the access point, which can process these training packets and construct transmit and receive eigenvectors for the dummy station. When a new station enters this office complex with an active call already in progress, handoff of the new station to the access point may be simplified by using pre-computed eigenvectors from the nearest dummy station, which may be identified based on signal strength measurements. This can make handoffs smoother and faster. Eventually, the packet by packet transmissions will enable derivation of more optimal eigenvectors for the new station, but the dummy stations will provide a reasonable starting point.

[0066] WLAN 100 may support multiple transmission modes such as, for example, no steering, beamsteering, eigensteering, space-time transmit diversity (STTD), space-frequency transmit diversity (SFTD), and so on. Table 2 lists the number of antennas used for data transmission and the number of antennas used for data reception for each transmission mode. N_ap is the total number of antennas available at the access point, and N_ut is the total number of antennas available at a user terminal scheduled for downlink and uplink data transmission. In Table 2 and in the description below, for each transmission mode, T is the number of antennas used by the access point for downlink data transmission to the user terminal using that transmission mode, N is the maximum number of antennas allowed by the WLAN for data transmission on the downlink and uplink, and S_dn is the number of data streams being transmitted simultaneously by the access point to the user terminal, where S_dn≦min {N, N_ut} and N_ap>N. For each transmission mode, R is the number of antennas used by the access point for reception of an uplink data transmission sent by the user terminal using that transmission mode, and S_up is the number of data streams being transmitted simultaneously by the user terminal to the access point.

[0067]

	TABLE 2
	Transmission Mode
				STTD &
	No Steering	Beamforming	Eigensteering	SFTD
Number of	N ≧ T = Sdn	N ≧ T > 1	N ≧ T ≧ Sdn	T = 2
transmit
antennas
(T)
Number of	N ≧ R ≧	N ≧ R ≧ 1	N ≧ R ≧ Sup	N ≧ R ≧ 1
receive	Sup
antennas
(R)

[0068] The same or different transmission modes may be used for downlink and uplink data transmission between the access point and the user terminal. The access point may use the same or different sets of antennas for downlink data transmission and uplink data reception. The spatial processing performed by access point 110 for the transmission modes listed in Table 2 is described below.

[0069] Access point 110 may perform beamsteering to steer a downlink data transmission toward a specific user terminal x. User terminal x may have a single antenna and may be user terminal 120a or 120c in FIG. 1. Access point 110 selects multiple (T) antennas from among the N_ap available antennas for data transmission to user terminal x. A MISO channel is formed between the T selected antennas at access point 110 and the single antenna at user terminal x. This MISO may be characterized by a 1×T channel response row vector h_dn,x(k)=[h_x,1(k) h_x,2(k) . . . h_x,T(k)] for each subband k, where h_x,j(k), for j=1, . . . , T, is the complex channel gain between AP antenna j and the single UT antenna for subband k. Access point 110 may perform spatial processing for beamforming, as follows:
x_dn,x(k)=h_dn,x^H(k)·s_dn,x(k), for kε{1, . . . , K},  Eq (1)
where s_dn,x(k) is a data symbol to be sent on subband k to user terminal x, x_dn,x(k) is a vector with T transmit symbols to be sent from the T selected antennas at access point 110, “^H” denotes a conjugate transpose, and K is the number of subbands used for data transmission.

[0070] Access point 110 may transmit S_dn data streams simultaneously from up to N antennas to a user terminal y. User terminal y has multiple (N_ut) antennas and may be user terminal 120b or 120d in FIG. 1. Access point 110 selects multiple (T) antennas from among the N_ap available antennas for downlink data transmission to user terminal y, where T=S_dn for no steering. Access point 110 may perform spatial processing for no steering, as follows:
x_dn,ns,y(k)=s_dn,y(k), for kε{1, . . . , K},  Eq (2)
where s_dn,y(k) is a vector with S_dn data symbols to be sent on subband k to user terminal y and x_dn,ns,y(k) is a vector with T transmit symbols to be sent from the T selected antennas on subband k to user terminal y for no steering.

[0071] Access point 110 may perform eigensteering to transmit multiple data streams on orthogonal spatial channels (or eigenmodes) to user terminal y. A MIMO channel is formed between the T selected antennas at access point 110 and the N_ut antennas at user terminal y. This MIMO may be characterized by an N_ut×T channel response matrix H_dn,y(k) for subband k, which may be expressed as:

[0072] $\begin{array}{cc}{\underset{\_}{H}}_{\mathrm{dn},y}\left(k\right)=\left[\begin{array}{cccc}{h}_{y,1,1}\left(k\right)& h_{y,1,1}\left(k\right)& \cdots & h_{y,1,T}\left(k\right)\\ h_{y,2,1}\left(k\right)& h_{y,1,1}\left(k\right)& \cdots & h_{y,1,T}\left(k\right)\\ ⋮& ⋮& ⋰& ⋮\\ h_{y,N_{\mathrm{ut}},1}\left(k\right)& h_{y,N_{\mathrm{ut}},2}\left(k\right)& \cdots & h_{y,N_{\mathrm{ut}},T}\left(k\right)\end{array}\right],\mathrm{for}k\in \left\{1,\dots ,K\right\},& \mathrm{Eq}\left(3\right)\end{array}$
where h_y,i,j(k), for i=1, . . . , N_ut and j=1, . . . , T, is the complex channel gain between antenna j at access point 110 and antenna i at user terminal y for subband k. The channel response matrix H_dn,y(k) may be diagonalized via eigenvalue decomposition, as follows:
R_dn,y(k)=H_dn,y^H(k)·H_dn,y(k)=E_dn,y(k)·Λ_dn,y(k)·E_dn,y^H(k),  Eq (4)
where E_dn,y(k) is a unitary matrix of eigenvectors and Λ_y(k) is a diagonal matrix of eigenvalues for subband k. The diagonal elements of Λ_y(k) are eigenvalues that represent the power gains for S eigenmodes of H_dn,y(k), where S≦min {T, N_ut}. The eigenmodes may be viewed as orthogonal spatial channels. Access point 110 may use the eigenvectors (or columns) in E_dn,y(k) to transmit data on the eigenmodes of H_dn,y(k). Access point 110 may transmit up to S data streams simultaneously on the S eigenmodes of H_dn,y(k), or S_dn≦S.

[0073] Access point 110 may perform spatial processing for eigensteering, as follows:
x_dn,es,y(k)=E_dn,y(k)·s_dn,y(k),  Eq (5)
where x_dn,es,y(k) is a vector with T transmit symbols to be sent from the T selected antennas on subband k to user terminal y for eigensteering. Access point 110 may also transmit data on the best eigenmode of H_dn,y(k) by performing spatial processing for beamsteering with the eigenvector for this best eigenmode, e.g., as shown in equation (1).

[0074] Access point 110 may transmit a single data stream from two antennas to a user terminal using STTD or SFDT. For STTD, access point 110 generates two vectors x_dn,td,1=[s₁ s₂]^T and x_dn,td,2=[s*₂ −s*₁]^T for each pair of data symbols s₁ and s₂, where “*” denotes the complex conjugate and “^T” denotes the transpose. Access point 110 transmits the two coded symbols in vector x_dn,td,1 from two selected antennas on one subband in a first symbol period and then transmits the two coded symbols in vector x_dn,td,2 from the same two antennas on the same subband in a second symbol period. For SFDT, access point 110 transmits the two coded symbols in vector x_dn,td,1 from the two selected antennas on a first subband and transmits the two coded symbols in vector x_dn,td,2 on a second subband in the same symbol period.

[0075] Access point 110 may use multiple (R) antennas for reception of an uplink data transmission from a user terminal. Access point 110 selects R antennas from among the N_ap available antennas, where R is dependent on the transmission mode used by the user terminal for the uplink data transmission, as shown in Table 2. R may also be greater than N if supported by the spatial processing at the access point. For a TDD system, the downlink and uplink may be assumed to be reciprocal so that the channel response for the uplink is equal to the transpose of the channel response for the downlink, e.g., h_up,x(k)=h_dn,x^T(k) and H_up,y(k)=H_dn,y^T(k).

[0076] The received symbols at access point 110 for an uplink data transmission from single-antenna terminal x may be expressed as:
r_up,x(k)=h_up,x(k)·s_up,x(k)+w_up,x(k),  Eq (6)
where s_up,x(k) is a data symbol sent on subband k by user terminal x, r_up,x(k) is a vector with R received symbols for user terminal x, and w_up,x(k) is a received noise vector at access point 110.

[0077] Access point 110 may perform receiver matched filtering as follows:
ŝ_up,x(k)=h_up,x^H(k)·r_up,x(k)=∥h_up,x(k)∥·s_up,x(k)+w_up,x(k)  Eq (7)
where ŝ_up,x(k) is an estimate of s_up,x(k) and w_up,x(k) is the post-processed noise observed by s_up,x(k).

[0078] The received symbols at access point 110 for an uplink data transmission from multi-antenna terminal y using no steering or eigensteering may be expressed as:
r_up,y(k)=H_up,y(k)·x_up,y(k)+w_up,y(k)=H_eff,y(k)·s_up,y(k)+w_up,y(k),  Eq (8)
where s_up,y(k) is a vector of data symbols sent by user terminal y, x_up,y(k) is vector of transmit symbols for the N_ut antennas at user terminal y, H_eff,y(k) is an effective channel response matrix for the uplink, and r_up,y(k) is a vector of received symbols at access point 110 for user terminal y. H_eff,y(k) is dependent on the transmission mode used by user terminal y for the uplink data transmission, e.g., H_eff,y(k)=H_up,y(k)·E_up,y(k) if user terminal y performs eigensteering and H_eff,y(k)=H_up,y(k) if user terminal y performs no steering.

[0079] Access point 110 may perform receiver spatial processing as follows:
ŝ_up,y(k)=M_up,y(k)·r_up,y(k)=s_up,y(k)+{tilde over (w)}_up,y(k),  Eq (9)
where M_up,y(k) is a spatial filter matrix for subband k and {tilde over (w)}_up,y(k) is the post-detection noise. Access point 110 may derive the spatial filter matrix M_up,y(k) using any one of the following:
M_up,y1(k)=Λ_up,y⁻¹(k)·E_up,y^H(k)·H_up,y^H(k),  Eq (10)
M_up,y2(k)=[H_eff,y^H(k)·H_eff,y(k)]⁻¹·H_eff,y^H(k),  Eq (11)
M_up,y3(k)=D_up,y(k)·[H_eff,y^H(k)·H_eff,y(k)+σ_n²·I]⁻¹·H_eff,y^H(k),  Eq (12)
where
D_y(k)=[diag [M′_up,y3(k)·H_eff,y(k)]]⁻¹,
M′_up,y3(k)=[H_eff,y^H(k)·H_eff,y(k)+σ_n²·I]⁻¹·H_eff,y^H(k),

[0080] I is the identity matrix, and σ_n² is the variance of the noise at access point 110.

[0081] Equation (10) is for a matched filtering technique for eigensteering, equation (11) is for a zero-forcing technique, and equation (12) is for a minimum mean square error (MMSE) technique. The zero-forcing and MMSE techniques may be used for the no steering and eigensteering transmission modes.

[0082] The received symbols at access point 110 for an uplink data transmission from multi-antenna terminal y using STTD may be expressed as:
r_up,td,y1(k)=h_up,ya1(k)·s_y1(k)+h_up,ya2(k)·s_y2(k)+w_up,y1(k), and  Eq (13)
r_up,td,y2(k)=h_up,ya1(k)·s*_y2(k)−h_up,ya2(k)·s*_y1(k)+w_up,y2(k),  Eq (14)
where s_y1(k) and s_y2(k) are two data symbols sent from two UT antennas y1 and y2 in two symbol periods on subband k using STTD, h_up,ya1(k) and h_up,ya2(k) are vectors of channel gains between the two UT antennas y1 and y2, respectively, and the R selected AP antennas, r_up,td,y1(k) and r_up,td,y2(k) are vectors of received symbols for subband k in the two symbol periods, and w_up,y1(k) and w_up,y1(k) are the noise vectors for the two symbol periods. R≧1 for the STTD and SFTD transmission modes.

[0083] Access point 110 may derive estimates of the two data symbols, s_y1(k) and s_y2(k), as follows:

[0084] $\begin{array}{c}{\hat{s}}_{y1}\left(k\right)=\frac{{\underset{\_}{h}}_{\mathrm{up},\mathrm{ya}1}^H\left(k\right)·{\underset{\_}{r}}_{\mathrm{up},\mathrm{td},y1}\left(k\right)-{\underset{\_}{r}}_{\mathrm{up},\mathrm{td},y2}^H\left(k\right)·{\underset{\_}{h}}_{\mathrm{up},\mathrm{ya}2}\left(k\right)}{{{\underset{\_}{h}}_{\mathrm{up},\mathrm{ya}1}\left(k\right)}^2+{{\underset{\_}{h}}_{\mathrm{up},\mathrm{ya}2}\left(k\right)}^2}\\ =s_{y1}\left(k\right)+{\tilde{w}}_{y1}\left(k\right),\end{array}$ $\mathrm{and}$ $\begin{array}{c}{\hat{s}}_{y2}\left(k\right)=\frac{{\underset{\_}{h}}_{\mathrm{up},\mathrm{ya}2}^H\left(k\right)·{\underset{\_}{r}}_{\mathrm{up},\mathrm{td},y1}\left(k\right)+{\underset{\_}{r}}_{\mathrm{up},\mathrm{td},y2}^H\left(k\right)·{\underset{\_}{h}}_{\mathrm{up},\mathrm{ya}1}\left(k\right)}{{{\underset{\_}{h}}_{\mathrm{up},\mathrm{ya}1}\left(k\right)}^2+{{\underset{\_}{h}}_{\mathrm{up},\mathrm{ya}2}\left(k\right)}^2}\\ =s_{y2}\left(k\right)+{\tilde{w}}_{y2}\left(k\right),\end{array}$
where ŝ_y1(k) and ŝ_y2(k) are estimates of s_y1(k) and s_y2(k), respectively, and {tilde over (w)}_y1(k) and {tilde over (w)}_y2(k) are post-processed noise observed by s_y1(k) and s_y2(k), respectively.

[0085] The multi-antenna station described herein may be implemented by various means. For example, the multi-antenna station and any functions described herein may be implemented with a combination of hardware, firmware, or software. The units used to make measurements for the AP antennas, select antennas for data transmission and reception, and process data and signals may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, RF integrated circuits (RFICs), other electronic units designed to perform the functions described herein, or a combination thereof.

[0086] The antenna selection may be performed with hardware or software. For a software implementation, the antenna selection may be performed with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit (e.g., memory unit 222 in FIG. 4) and executed by a processor (e.g., controller 220). The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.

[0087] The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.