Abstract:
A network device includes a first transceiver configured to wirelessly receive a first packet from a second transceiver via a channel. The second transceiver is in a wireless device separate from the network device. A circuit is configured to determine a condition of the channel based on the first packet. The circuit has an inactive state and an active state. Elements of the circuit are powered while the circuit is in the active state. The elements of the circuit are not powered while the circuit is in the inactive state. A controller is configured to transition the circuit from the inactive state to the active state in response to the first transceiver wirelessly receiving the first packet, and return the circuit to the inactive state responsive to the circuit having completed determining the condition of the channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/154,031 (now U.S. Pat. No. 8,045,535) filed on May 20, 2008, which is a continuation of U.S. patent application Ser. No. 11/068,441 (now U.S. Pat. No. 7,385,959), filed Feb. 28, 2005, which claims the benefit of U.S. Provisional Application No. 60/652,423, filed on Feb. 11, 2005. The disclosures of the above applications are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates generally to wireless data communications. More particularly, the present disclosure relates to providing feedback describing the condition of an Orthogonal Frequency Division Modulation (OFDM) channel. 
     BACKGROUND 
     Conventional OFDM-based Multiple Input, Multiple Output (MIMO) wireless data communication systems employ a feedback mechanism that provides information describing the condition of the OFDM channel so that transmitters on the channel can maximize the bandwidth of the channel. Each packet transmitted over the channel includes a preamble, a signal field, and a payload. Receivers use the preamble to synchronize with the transmitter, perform channel estimation, and adjust gain settings. The signal field specifies the data rate of the data packet that follows, the number of antennas used, and additional information to assist the receiver in decoding the data packet. 
     Each receiver on the channel measures the channel while receiving the preamble of each packet. The signal field includes a channel condition request bit that, when set, requests a channel condition report from the receiver. When the receiver receives a packet with the channel condition request bit set, the receiver transmits a channel condition report based on the most recent channel condition measurement taken by the receiver. 
     The receiver does not know in advance when it will receive a channel condition request. And by the time the receiver processes the signal field in a packet to determine whether the channel condition request bit in the signal field is set, the preamble portion of the packet has already passed through the receiver, and so cannot be measured. Therefore this scheme requires that the measurement circuit that measures the channel condition be constantly active (that is, powered). Therefore the measurement circuit constantly consumes power, which constitutes a significant battery drain for a mobile receiver. 
     SUMMARY 
     A network device is provided and includes a first transceiver configured to wirelessly receive a first packet from a second transceiver via a channel. The second transceiver is in a wireless device separate from the network device. A circuit is configured to determine a condition of the channel based on the first packet. The circuit has an inactive state and an active state. Elements of the circuit are powered while the circuit is in the active state. The elements of the circuit are not powered while the circuit is in the inactive state. A controller is configured to transition the circuit from the inactive state to the active state in response to the first transceiver wirelessly receiving the first packet, and return the circuit to the inactive state responsive to the circuit having completed determining the condition of the channel. 
     In other features, a method is provided and includes receiving a first packet via a first transceiver. The first packet is transmitted from a second transceiver to the first transceiver via a channel. A condition of the channel is determined based on the first packet via a circuit. The circuit has an inactive state and an active state. Elements of the circuit are powered while the circuit is in the active state. The elements of the circuit are not powered while the circuit is in the inactive state. The circuit is transitioned from the inactive state to the active state in response to the first packet being received via the first transceiver. The circuit is returned to the inactive state in response to completing the determining of the condition of the channel. 
     In other features, a wireless data communications device is provided and includes: a Multiple Input, Multiple Output (MIMO) transceiver to communicate on an Orthogonal Frequency Division Modulation (OFDM) channel; and a measurement circuit having an active state and an inactive state. The measurement circuit measures a channel condition of the OFDM channel in the active state. A controller places the measurement circuit in the active state after a request for the channel condition is received on the OFDM channel, and places the measurement circuit in the inactive state after the measurement circuit measures the channel condition of the OFDM channel and until a further request for the channel condition is received on the OFDM channel. 
     In other features, the MIMO transceiver includes: a MIMO receiver to receive a signal on the OFDM channel. The signal includes a first packet including a first preamble and a first signal field including a channel condition request, and a second packet following the first packet. The second packet includes a second preamble. The measurement circuit measures the channel condition of the OFDM channel during reception of the second preamble. In other features, the MIMO transceiver includes: a MIMO transmitter to transmit a packet on the OFDM channel. The packet represents the channel condition of the OFDM channel measured by the measurement circuit. In other features, the packet includes channel coefficients representing the channel condition of the OFDM channel. In other features, the channel coefficients include an amplitude gain and frequency offset for each point in a Quadrature Amplitude Modulation (QAM) constellation representing the OFDM channel. In other features, the MIMO transmitter transmits the packet according to a predetermined schedule. In other features, the wireless data communications device is compliant with IEEE standard 802.11n. 
     In other features, a method for a Multiple Input, Multiple Output (MIMO) wireless data communications device communicating on an Orthogonal Frequency Division Modulation (OFDM) channel is provided. The method includes: receiving, on the OFDM channel, a request for the channel condition of the OFDM channel; and placing a measurement circuit of the wireless communications device in an active state after the request for the channel condition is received. The measurement circuit measures the channel condition of the OFDM channel in the active state; and places the measurement circuit in an inactive state after the measurement circuit measures the channel condition of the OFDM channel and until a further request for the channel condition of the OFDM channel is received on the OFDM channel. 
     In other features, the receiving of the request for the channel condition of the OFDM channel includes: receiving a signal on the OFDM channel. The signal includes a packet including a preamble and a signal field including a channel condition request. Some other features further include receiving a second packet following the packet, the second packet including a second preamble. The measurement circuit measures the channel condition of the OFDM channel during reception of the second preamble. Some other features further include transmitting a packet on the OFDM channel, the packet representing the channel condition of the OFDM channel measured by the measurement circuit. In some other features, the packet includes channel coefficients representing the channel condition of the OFDM channel. In some other features, the channel coefficients include an amplitude gain and frequency offset for each point in a Quadrature Amplitude Modulation (QAM) constellation representing the OFDM channel. In some other features, the packet is transmitted according to a predetermined schedule. In some other features, the wireless data communications device is compliant with IEEE standard 802.11n. 
     In other features, a wireless data communications device is provided and includes: Multiple Input, Multiple Output (MIMO) transceiver means for communicating on an Orthogonal Frequency Division Modulation (OFDM) channel; and means for measuring having an active state and an inactive state. The means for measuring measures a channel condition of the OFDM channel in the active state. The wireless data communications device further includes controller means for placing the means for measuring in the active state after a request for the channel condition is received on the OFDM channel, and for placing the means for measuring in the inactive state after the means for measuring measures the channel condition of the OFDM channel and until a further request for the channel condition is received on the OFDM channel. 
     In some other features, the MIMO transceiver means includes MIMO receiver means for receiving a signal on the OFDM channel. The signal includes a first packet including a first preamble and a first signal field including a channel condition request, and a second packet following the first packet. The second packet includes a second preamble. The means for measuring measures the channel condition of the OFDM channel during reception of the second preamble. In some other features, the MIMO transceiver means includes MIMO transmitter means for transmitting a packet on the OFDM channel. The packet represents the channel condition of the OFDM channel measured by the means for measuring. In some other features, the packet includes channel coefficients representing the channel condition of the OFDM channel. In some other features, the channel coefficients include an amplitude gain and frequency offset for each point in a Quadrature Amplitude Modulation (QAM) constellation representing the OFDM channel. In some other features, the MIMO transmitter means transmits the packet according to a predetermined schedule. In some other features, the wireless data communications device is compliant with IEEE standard 802.11n. 
     In other features a computer program for a Multiple Input, Multiple Output (MIMO) wireless data communications device communicating on an Orthogonal Frequency Division Modulation (OFDM) channel is provided. The computer program includes instructions for placing a measurement circuit of the wireless communications device in an active state after a request for a channel condition of the OFDM channel is received on the OFDM channel. The measurement circuit measures the channel condition of the OFDM channel in the active state; and places the measurement circuit in an inactive state after the measurement circuit measures the channel condition of the OFDM channel and until a further request for the channel condition of the OFDM channel is received on the OFDM channel. 
     In some other features, the request for the channel condition of the OFDM channel includes receiving a packet including a preamble and a signal field including a channel condition request. In some other features, the measurement circuit measures the channel condition of the OFDM channel during reception of a preamble of a second packet received after the packet. Some features further include causing the MIMO wireless data communications device to transmit a packet on the OFDM channel, the packet representing the channel condition of the OFDM channel measured by the measurement circuit. In some other features, the packet includes channel coefficients representing the channel condition of the OFDM channel. In some other features, the channel coefficients include an amplitude gain and frequency offset for each point in a Quadrature Amplitude Modulation (QAM) constellation representing the OFDM channel. In some other features, the packet is transmitted according to a predetermined schedule. In some other features, the wireless data communications device is compliant with IEEE standard 802.11n. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  shows an OFDM MIMO wireless data communications system including two wireless data communications devices communicating over a OFDM channel according to the present disclosure. 
         FIG. 2  shows a process for the wireless data communications system of  FIG. 1  according to the present disclosure. 
         FIG. 3  shows a signal transmitted by the MIMO transmitter of  FIG. 1  with respect to time t according to the present disclosure. 
         FIG. 4  shows a WLAN including a wireless access point communicating with wireless clients over an OFDM channel according to the present disclosure. 
         FIG. 5  shows a process for the WLAN of  FIG. 4  according to the present disclosure. 
         FIG. 6  shows an example schedule for transmission of the channel report packets. 
     
    
    
     The leading digit(s) of each reference numeral used in this specification indicates the number of the drawing in which the reference numeral first appears. 
     DESCRIPTION 
     Techniques of the present disclosure provide OFDM channel condition feedback mechanisms that significantly reduce power consumption compared to conventional mechanisms. In particular, receivers according to these techniques measure the OFDM channel condition only when requested by a transmitter on the channel. Therefore the measurement circuits in these receivers can be inactivated (that is, powered down) when channel measurements are not required, thereby achieving significant power savings compared to conventional receivers that measure the channel condition for each packet received, and therefore must power their measurement circuits continuously. While these mechanisms are especially useful in an N×M MIMO OFDM channel where the number of transmit antennas N&gt;1 and/or the number of receive antennas M&gt;1, they are also applicable to the case where N=M=1. 
       FIG. 1  shows a OFDM MIMO wireless data communications system  100  including two wireless data communications devices  102  and  104  communicating over a OFDM channel  106  that can be a direct wireless link between devices  102  and  104  or a wireless network such as a wireless local-area network (WLAN) that can operate in ad-hoc mode, infrastructure mode, or any other network mode. Wireless data communications devices  102  and  104  are compliant with IEEE standard 802.11n in techniques where the number of transmit antennas N&gt;1 and/or the number of receive antennas M&gt;1, and with IEEE standards 802.11a and 802.11g where N=M=1. 
     Wireless data communications device  102  includes a controller  108  and a MIMO transceiver  110  that includes a MIMO receiver  112  and a MIMO transmitter  114 . Of course, controller  108  can be implemented as part of MIMO transceiver  110  rather than separately. 
     Wireless data communications device  104  includes a controller  118  and a MIMO transceiver  120  that includes a MIMO receiver  122  and a MIMO transmitter  124 . Wireless data communications device  104  also includes a measurement circuit  116  to measure a channel condition of OFDM channel  106 . Of course, controller  118  and/or measurement circuit  116  can be implemented as part of MIMO transceiver  120  rather than separately. 
     Measurement circuit  116  has an active state (that is, a state in which the active elements of measurement circuit  116  are powered) and an inactive state (that is, a state in which the some or all of the active elements of measurement circuit  116  are not powered). Measurement circuit  116  measures a channel condition of OFDM channel  106  in the active state according to well-known techniques. Measurement circuit  116  changes between active and inactive states under the control of controller  118 . 
       FIG. 2  shows a process  200  for wireless data communications system  100  of  FIG. 1 . Controller  108  of wireless data communications device  102  causes MIMO transmitter  114  to transmit a packet that includes a request for a channel condition of OFDM channel  106  (step  202 ). 
       FIG. 3  shows a signal  300  transmitted by MIMO transmitter  114  with respect to time t. Each packet  302  transmitted by transmitter  114  includes a preamble  304  followed first by a signal field  306 , and then by a payload  308 . The payload  308  may include a header  312  and a data field  314 . Signal field  306  includes a channel condition request bit  310  that, when set, indicates a request for the channel condition of the OFDM channel. When channel condition request bit  310  is set, the length of data field  314  may be set to zero. However, in some implementations data field  314  is populated. When the channel condition request is directed to one or more specific wireless data communication devices  104 , header  312  in payload  308  includes the addresses of those devices  104 . Otherwise header  312  can be omitted as well. 
     Receiver  122  of wireless data communications device  104  receives the packet including the channel condition request (step  204 ), which is shown as packet  302 A in  FIG. 3 . Controller  118  places measurement circuit  116  in the active state after the request for the channel condition is received (step  206 ). 
     Transmitter  114  of wireless data communications device  102  transmits a second packet (step  208 ), which is shown as packet  302 B in  FIG. 3 . Receiver  122  of wireless data communications device  104  receives packet  302 B (step  210 ). Measurement circuit  116  measures the channel condition of OFDM channel  106  during reception of preamble  304 B of packet  302 B (step  212 ). After measurement circuit  116  measures the channel condition of OFDM channel  106 , controller  118  places measurement circuit  116  in the inactive state (step  214 ), thereby conserving power when no measurement is needed. 
     MIMO transmitter  124  of wireless data communications device  104  then transmits a channel report packet on OFDM channel  106  that represents the channel condition of OFDM channel  106  as measured by measurement circuit  116  (step  216 ). The channel report packet may include channel coefficients representing the channel condition of OFDM channel  106 . The channel coefficients represent an amplitude gain and frequency offset for each point in a Quadrature Amplitude Modulation (QAM) constellation representing OFDM channel  106 . 
     The coefficients are complex numbers that are a function of the number Nc of subcarriers, the number Nt of antennas at the transmitter, and the number Nr of antennas at the receiver. Each complex coefficient is typically represented by 4 bytes of binary data. For IEEE 802.11a, Nc=52 requiring 52×4=208 bytes. For an IEEE 802.11n MIMO system with Nc=52 and Nt=Nr=2, the coefficients require 52×2×2×4=832 bytes. 
     Wireless data communications device  102  receives the channel condition report (step  218 ). Controller  108  modifies the transmission parameters of MIMO transmitter  114  in accordance with the channel coefficients in the channel condition report (step  220 ). 
     In some techniques, a single wireless communications device, such as an access point, collects channel condition measurements from a plurality of other wireless communications devices, such as wireless clients in an IEEE 802.11 infrastructure-mode WLAN.  FIG. 4  shows a WLAN  400  including a wireless access point  402  communicating with wireless clients  404 A,  404 B, through  404 N over an OFDM channel  406 . 
       FIG. 5  shows a process  500  for WLAN  400  of  FIG. 4 . Wireless access point  402  transmits a packet  302 A including a channel condition request (step  502 ). Packet  302  can be a broadcast packet or a multicast packet including the addresses of wireless clients  404 A,  404 B, through  404 N. 
     Wireless clients  404  receive the packet (step  504 ), and place their measurement circuits  116  in the active state (step  506 ). Wireless access point  402  then transmits a second packet  302 B (step  508 ). The measurement circuits  116  in wireless clients  404  receive the packet (step  510 ) and measure the condition of OFDM channel  406  during reception of the preamble  304 B of packet  302 B (step  512 ). 
     After measurement circuits  116  measure the channel condition of OFDM channel  406 , wireless clients  404  place measurement circuits  116  in the inactive state (step  514 ), thereby conserving power when no measurement is needed. 
     Each wireless client  404  then transmits a channel report packet on OFDM channel  406  that represents the channel condition of OFDM channel  406  as measured by the measurement circuit  116  in that wireless client  404  (step  516 ). The wireless clients  404  may transmit the channel report packets according to a predetermined schedule that is communicated by wireless access point  402  to wireless clients  404  before the measurements take place. In other techniques, the wireless clients  404  transmit the channel report packets according to conventional channel access methods. 
       FIG. 6  shows an example schedule  600  for transmission of the channel report packets. Wireless client  404 A transmits its channel report packet during a first predetermined time slot  602 A. Wireless client  404 B transmits its channel report packet during a second predetermined time slot  602 B, and so on. Finally, wireless client  404 N transmits its channel report packet during a final predetermined time slot  602 N. Of course, other scheduling techniques can be used. 
     Wireless access point  402  receives the channel condition reports (step  518 ). Controller  108  in wireless access point  402  modifies the transmission parameters of MIMO transmitter  114  in wireless access point  402  in accordance with the channel coefficients in the channel condition reports (step  520 ). 
     Aspects of the present disclosure can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. Apparatus of the present disclosure can be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and method tasks of the present disclosure can be performed by a programmable processor executing a program of instructions to perform functions disclosed herein by operating on input data and generating output. 
     The techniques disclosed herein can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language. 
     Suitable processors include, by way of example, both general and special purpose microprocessors. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer will include one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. Any of the foregoing can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits). 
     A number of implementations of the present disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, other implementations are within the scope of the following claims.