Method and apparatus to select an adaptation technique in a wireless network

An adaptation technique is selected for use in a wireless network based on estimated throughput. In at least one embodiment, throughput is estimated for a wireless channel for both prefix adaptation and postfix adaptation. An adaptation technique is then selected based on the estimates. In some embodiments, an adaptation validity duration is determined to gauge the potential effectiveness of adaptation information associated with a wireless channel.

FIELD OF THE INVENTION

The invention relates generally to wireless communications and, more particularly, to wireless networking.

BACKGROUND OF THE INVENTION

Adaptation may be used in a wireless network link to compensate for, among other things, changes in channel conditions over time. Different techniques are available for implementing adaptation in a wireless link, each technique having its own advantages and disadvantages. Methods and structures are needed for effectively selecting an adaptation technique for use in a wireless network link.

DETAILED DESCRIPTION

A wireless network may use adaptation techniques on the wireless links thereof to allow wireless devices in the network to adapt their transmissions based on, for example, current conditions in the channel. In a wireless channel, the rate of useful data transfer may be dependent upon how well the modulation scheme that is being used matches the current conditions in the channel. If the conditions in the channel are changing over time, as is common in a wireless link, the modulation scheme that is being used may become improper for the channel. Adaptation techniques may be used, for example, to allow the modulation scheme to adapt over time based on the changing channel conditions. Adaptation techniques typically involve the delivery of adaptation information to a wireless transmitting device for use in varying the modulation used to transmit data. Adaptation information may include, for example, signal to noise ratio (SNR), multiple input multiple output (MIMO) channel state and mode selection, transmit power (globally or, in the case of a multicarrier communication schemes such as orthogonal frequency division multiplexing (OFDM), per subcarrier), and/or other types of adaptation information.

In one adaptation technique, known as prefix adaptation, adaptation information is delivered to a transmitting device immediately before a data unit is transmitted and is used to appropriately modulate the data unit for transmission.FIG. 1is a timing diagram illustrating one form of prefix adaptation that is based upon an extension of the IEEE 802.11 wireless networking standard and that will be referred to herein as an RTS/TCTS exchange. As shown, a transmitting device (Device1) first transmits a request-to-send (RTS) frame10to a receiving device (Device2) to request a channel. If available, the receiving device sends a training clear-to-send frame12(TCTS) back to the transmitting device that includes adaptation information to be used in transmitting the corresponding data. The transmitting device then transmits one or more contiguous data frames14to the receiving device. The data frame14is modulated using the adaptation information from the TCTS12. After the data frame14has been received, an individual or selective acknowledgement (ACK) frame16is delivered to the transmitting device. As illustrated inFIG. 1, a short interframe space or “SIFS” interval18may occur between each successive pair of frames. The SIFS interval18may have the same duration from frame to frame.

In another adaptation technique, known as postfix adaptation, adaptation information is received by a transmitting device after a data unit has been transmitted. This postfix adaptation information is then used to modulate the next data unit to be transmitted, which may occur immediately or some arbitrary amount of time later.FIG. 2is a timing diagram illustrating one form of postfix adaptation that is derived from the acknowledgement function described in the IEEE 802.11 wireless networking standard and which will be referred to herein as “training acknowledgement” or TACK exchange. As shown, a transmitting device (Device1) first transmits one or more contiguous data frames20to a receiving device (Device2). The receiving device then returns an individual or selective training acknowledgement frame (TACK)22to the transmitting device that includes adaptation information measured over the data frame(s)20. A SIFS interval18may occur between the data frame(s)20and the TACK22. The adaptation information is then used to modulate one or more subsequently transmitted data frames24. As shown, the subsequently transmitted data frames24may be transmitted an arbitrary amount of time26after the preceding TACK frame22is received.

Prefix adaptation typically involves a greater level of overhead than postfix adaptation. However, when postfix adaptation is used, if a subsequent transmission of data occurs a significant amount of time after the adaptation information was received, changes in the channel may have caused the adaptation information to have become “old” and less effective. That is, the adaptation information may cause a modulation scheme to be used that is not in line with the present condition of the channel. In such a case, repeat transmissions may be necessary, thus extending the overall amount of time required to successfully transmit the data. In at least one aspect of the present invention, techniques and structures are provided for dynamically selecting an appropriate adaptation technique to use to transmit data in a wireless network environment.

FIG. 3is a flowchart illustrating an example method30for transmitting data in a wireless network in accordance with an embodiment of the present invention. Before data is transmitted through a wireless channel, the throughput of the data transfer using prefix adaptation is estimated (block32). The throughput of the data transfer using postfix adaptation is also estimated (block34). An adaptation technique is then selected for the data transfer based on estimated throughput (block36). For example, in one approach, the adaptation technique having the highest estimated throughput is selected. The data is then transferred through the channel using the selected adaptation technique (block38). As used herein, the phrase “data transfer” may refer to the entire information exchange required to effect data transmission through a channel. For example, the entire frame exchange illustrated inFIG. 1may be considered a data transfer.

Any of a number of different throughput metrics may be used to estimate the throughput of a data transfer (e.g., a frame exchange). In at least one embodiment of the invention, the throughput is estimated as the expected “goodput” of the data transfer divided by the expected total duration of the data transfer. The word “expected” may include the effect of estimated packet error rate and collision probabilities. The expected goodput is defined as the amount of data expected to be successfully received by the exchange. For a single packet of data, the expected goodput may be defined as the probability that the data is transmitted multiplied by the probability that it is received correctly multiplied by the amount of data. For multiple data packets, the estimated goodput may be defined as the probability that the burst is transmitted multiplied by the sum, over all packets, of the packet size multiplied by the probability of correct reception. The probability of correct reception may be determined from previously known adaptation parameters and the length(s) of the data packet(s). The expected total duration of an exchange may be calculated, for example, as a function of the estimated collision probability, the expected data rate, and the amount of data to be sent. In one approach, the expected duration is calculated as a weighted sum of the duration when a collision occurs and the duration when no collision occurs. A wireless device may keep track of collision rate, either globally or per destination. A device may also monitor the average number of slots between channel accesses in order to infer a collision rate. The expected data rate may be based upon, for example, the rate observed in a previous exchange.

In at least one embodiment of the present invention, for the case of prefix adaptation, the estimated throughput may be calculated as follows:

Tpostfix=(1-Pcollision)⁢∑Li·(1-PER⁡(Li))DDATA/TACK
where DDATA/TACKis the duration of a channel access/DATA/TACK sequence. The above equations assume that the data is being transmitted in multiple sub-packets. If only a single data packet is being transmitted, the summations in the equations would vanish. It should be appreciated that the above equations are merely examples of one type of metric that may be used to estimate the throughput of a frame exchange in accordance with an embodiment of the present invention. Alternative techniques also exist.

The above equations may be modified to take into account other parameters such as, for example, fragmentation threshold, modulation type, collision penalty mitigation schemes, effects of virtual carrier sense, and/or others. In at least one embodiment, a number of different combinations of parameters are considered and a combination that results in a highest estimated throughput is selected for the subsequent data transmission. For example, a number of combinations of fragmentation threshold, modulation type, and prefix adaptation may be evaluated and a number of combinations of fragmentation threshold, modulation type, and postfix adaptation may be evaluated and the combination generating the highest estimated throughput may be selected. Other different parameter combinations may alternatively be used.

In at least one embodiment of the invention, a wireless device may determine an “adaptation validity duration” parameter that may be used to gauge the potential effectiveness of adaptation information. In one approach, for example, a device may monitor the variation of adaptation parameters present in TCTS and TACK frames as a function of time. An adaptation validity duration may then be calculated as a time beyond which adaptation information may be considered old and invalid. The adaptation validity duration may be calculated either globally or for a particular destination. In addition, the adaptation validity duration may be a one time calculation or the value may be periodically or continually updated over time. Once an adaptation validity duration has been determined, the device may compare the time since it last received adaptation information to the adaptation validity duration as a technique to control its behavior. For example, in one possible scenario, a device may be programmed to always perform a prefix exchange, rather than a postfix exchange, if the adaptation information is older than the adaptation validity duration.FIG. 4is flowchart illustrating an example method40for transmitting data in a wireless network that makes use of such a technique. First, an adaptation validity duration DAVis calculated based on observed adaptation parameter variations over time (block42). When data is to be transmitted, a time T is determined since adaptation information was last obtained (block44). If T exceeds DAV(block46), it is determined that prefix adaptation will be used to transmit the data (block48). The data is then transmitted using prefix adaptation (block52). If T does not exceed DAV(block46), a determination is then made as to whether prefix or postfix adaptation should be used (block50). In at least one embodiment, the method30ofFIG. 3is utilized to determine whether prefix or postfix adaptation should be used. Other methods may alternatively be used. After an adaptation technique has been selected, the data is transmitted using the selected technique (block52).

In at least one embodiment of the present invention, the transmission rate for a postfix adaptation exchange may be reduced as the age of the adaptation information increases. The reduced rate may then be taken into consideration when selecting whether to use prefix or postfix adaptation for a subsequent data transmission.FIG. 5is a flowchart illustrating an example method60for transmitting data in a wireless network that makes use of such a technique. As shown inFIG. 5, when data is to be transmitted, a time T is determined since adaptation information was last obtained (block62). A transmission rate is then determined, based on the value of T, that will be used if postfix adaptation is selected for the subsequent data transmission (block64). In one possible approach, a full data rate may be used if the time T does not exceed an adaptation validity duration and a reduced data rate may be used if the time T exceeds the adaptation validity duration. In another possible approach, an equation or lookup table may be used to determine a postfix data transmission rate based on T. Other data rate selection techniques may alternatively be used. The throughput of the subsequent data transfer using prefix adaptation is estimated (block66). The throughput of the data transfer using postfix adaptation and the data rate determined above is also estimated (block68). An adaptation technique is then selected for the subsequent transfer based on estimated throughput (block70). The data is then transmitted using the selected adaptation technique (block72).

In at least one implementation, a wireless network device is able to combine both prefix and postfix adaptation techniques. For example, a receiving device may always provide postfix adaptation and a transmitting device may decide to also use prefix adaptation if the time that has elapsed since a previous exchange exceeds a threshold value (e.g., an adaptation validity duration or similar value). A wireless device may also request varying amounts of adaptation information in a request-to-send (RTS) frame based on, for example, its observation of the stability of the channel and/or the age of the adaptation information known in the device. For example, a device may request MIMO channel state per subcarrier information infrequently, but request overall received signal power frequently. In at least one embodiment, a device may periodically calculate per destination or per signal to noise ratio (SNR) threshold values for total data length versus modulation type to avoid having to frequently perform estimated throughput calculations. The values may be stored within, for example, a lookup table from which they may be retrieved in a timely fashion when needed.

FIG. 6is a block diagram illustrating an example wireless device80that may be used in a wireless network in accordance with an embodiment of the present invention. As illustrated, the wireless device80may include one or more of: a wireless transceiver82, a controller84, a selector86, a throughput estimator88, a user interface90, and an antenna92. The wireless device80may be programmed for operation in accordance with one or more wireless networking standards including, for example, IEEE 802.11 a, b, and g, HiperLAN 1 and 2, HomeRF, Ultra Wideband, Bluetooth, one or more cellular network standards, and/or others. The wireless transceiver82is operative for communicating with one or more remote wireless entities within the network, via antenna92. The antenna92may be any type of antenna including, for example, a patch, a dipole, a helix, an array, and/or others. In at least one embodiment, multiple antennas92are used. The controller84is operative for controlling the operation of the device80. The controller may include, for example, one or more digital processing devices that are capable of executing programs. Such digital processing devices may include, for example, a general purpose microprocessor, a digital signal processor (DSP), a reduced instruction set computer (RISC), a complex instruction set computer (CISC), a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), and/or others. The user interface90provides an interface between the device80and a user thereof.

When the controller84determines that data is to be transmitted by the wireless transceiver82, it may signal the throughput estimator88to estimate a throughput for the data transmission using prefix adaptation and also using postfix adaptation. The selector86will then select an adaptation technique for use in transmitting the data based on the throughput estimates. The controller84may then cause the data to be transmitted from the wireless transceiver82using the selected adaptation technique. The throughput estimator88and the selector86may be separate units or one or both may be integral with the controller84(e.g., software instructions or routines executed within a common processor or processor complex, etc.). As will be appreciated, the wireless device80ofFIG. 6is merely an example of one type of device architecture that may be used in accordance with the present invention. Many alternative architectures also exist.

In the foregoing detailed description, various features of the invention are grouped together in one or more individual embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects may lie in less than all features of each disclosed embodiment.