Wireless device and method for interference and channel adaptation in an OFDM communication system

TECHNICAL FIELD

[0001] The present invention pertains to wireless communications, and in particular to orthogonal frequency division multiplexed communications.

BACKGROUND

[0002] Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique that uses orthogonal subcarriers to transmit information within an available spectrum. Because the subcarriers are orthogonal to one another, they may be spaced much more closely together within the available spectrum than, for example, the individual channels in a conventional frequency division multiplexing (FDM) system. An OFDM system may achieve orthogonality by using subcarriers that have a null at the center frequency of the other subcarriers. The orthogonality of the subcarriers may help prevent inter-subcarrier interference within the system. Before transmission, the subcarriers may be modulated with a low rate data stream. The transmitted symbol rate of the OFDM system is low, and thus the transmitted OFDM signal may be highly tolerant to multipath delay spread within the channel. For this reason, many modem digital communication systems are turning to OFDM as a modulation scheme for signals that need to survive in environments having multipath reflections and/or strong interference. Many wireless communication standards have already adopted OFDM including, for example, the IEEE 802.11a standard, the Digital Video Broadcasting Terrestrial (DVB-T) broadcasting standard, and the High performance radio Local Area Network (HiperLAN) standard. In addition, several industry consortia, including the Broadband Wireless Internet Forum and the OFDM Forum, are proposing OFDM for fixed wireless access systems.

[0003] One problem with conventional OFDM systems is that it is difficult to make efficient use of the channel due to in-band interference and channel effects (e.g., multipath reflections/frequency selective fading). These dynamically changing channel characteristics, for example, reduce the number of bits per symbol that can be effectively communicated. Conventional OFDM systems use equalization schemes to compensate for channel effects by applying equalization coefficients to the received signal to improve the likelihood of accurate detection. Although conventional equalization schemes may allow an OFDM channel to operate at a higher data rate, they do not allow an OFDM channel to reach its “water-filling capacity” because they do not take into account channel conditions on a per subcarrier basis. Furthermore, conventional OFDM equalization schemes do not take into account channel conditions, such as in-band interference, in only portions of the channel bandwidth.

[0004] Thus there is a general need for an improved OFDM communication system and method that allows an OFDM channel to approach its “water-filling capacity”.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The appended claims are directed to some of the various embodiments of the present invention. However, the detailed description presents a more complete understanding of the present invention when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures and:

[0006]FIG. 1 is a wireless communication environment illustrating the operation of an embodiment of the present invention;

[0007]FIG. 2 is a highly simplified functional block diagram of a wireless communication device in accordance with an embodiment of the present invention;

[0008]FIG. 3 is a functional block diagram of a wireless communication device in accordance with an embodiment of the present invention;

[0009]FIG. 4A is a simplified timing diagram suitable for use by a point-to-multipoint communication system in accordance with an embodiment of the present invention;

[0010]FIG. 4B is a simplified timing diagram suitable for use by a point-to-point communication system in accordance with another embodiment of the present invention;

[0011]FIG. 5 is an example of channel response and interference in accordance with an embodiment of the present invention; and

[0012]FIGS. 6A and 6B are a flow chart of an interference and channel adaptation procedure in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0013] The following description and the drawings illustrate specific embodiments of the invention sufficiently to enable those skilled in the art to practice it. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Examples merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in or substituted for those of others. The scope of the invention encompasses the full ambit of the claims and all available equivalents.

[0014]FIG. 1 is a wireless communication environment illustrating the operation of an embodiment of the present invention. Communication environment <b>100</b> includes one or more wireless communication devices (WCD) <b>102</b> which may communicate with access point (AP) <b>104</b> over communication bi-directional OFDM links <b>110</b>. WCDs <b>102</b> may include, for example, personal digital assistants (PDAs), laptop and portable commuters with wireless communication capability, web tablets, wireless telephones, wireless headsets, pagers, instant messaging devices, MP3 players, digital cameras, and other devices that may receive and/or transmit information wirelessly. WCDs <b>102</b> may communicate with AP <b>104</b> using a multi-carrier transmission technique, such as an orthogonal frequency division multiplexing (OFDM) technique, that uses orthogonal subcarriers to transmit information within an assigned spectrum. WCDs <b>102</b> and AP <b>104</b> may also implement one or more communication standards, such as the IEEE 802.11a standard, the Digital Video Broadcasting Terrestrial (DVB-T) broadcasting standard, and the High performance radio Local Area Network (HiperLAN) standard.

[0015] In addition to facilitating communications between WCDs <b>102</b>, in one embodiment AP <b>104</b> may be coupled with one or more networks, such as an intranet or the Internet, allowing WCDs <b>102</b> to access such networks. For convenience, the term downstream is used herein to designate communications in the direction from AP <b>104</b> to WCDs <b>102</b> while the term upstream is used herein to designate communications in the direction from WCDs <b>102</b> to AP <b>104</b>, however, the terms downstream and upstream may be interchanged. In one embodiment, upstream and downstream communications may be time division multiplexed (TDM), although this is not a requirement. In another embodiment, downstream communications may be broadcast to more than one of WCDs <b>102</b> and may be frequency division multiplexed (FDM). WCDs <b>102</b> may support duplex communications utilizing different spectrum for upstream and downstream communications, although this is not a requirement. In one embodiment, upstream and downstream communications may share the same spectrum for communicating in both the upstream and downstream directions. Although FIG. 1 illustrates point-to-multipoint communications, embodiments of the present invention are suitable to both point-to-multipoint and point-to-point communications.

[0016] Communication environment <b>100</b> may also include one or more reflecting objects (RO) <b>108</b> which may cause multipath reflections and frequency selective fading within the spectrum utilized by AP <b>104</b> and WCDs <b>102</b>. Communication environment <b>100</b> may also include one or more in-band interfering devices (ID) <b>106</b> which generate interference within the spectrum utilized by AP <b>104</b> and WCDs <b>102</b>. Due to reflecting objects <b>108</b> and interfering devices <b>106</b>, WCD <b>102</b> and AP <b>104</b> may experience channel fading, multipath components, and interference conditions unique to the particular WCD. WCDs <b>102</b> and AP <b>104</b> may adapt to the local channel conditions to achieve improved communication rates. For example, WCD <b>116</b> may compensate, at least in part, for in-band interference caused by interfering devices <b>106</b> to achieve an improved communication rate. WCD <b>118</b>, for example, may compensate, at least in part, for multipath components caused by reflecting object <b>108</b> to achieve an improved communication rate. WCD <b>120</b>, for example, may compensate, at least in part, for multipath components caused by reflecting object <b>108</b> and for in-band interference caused by interfering device <b>122</b> to achieve an improved communication rate. AP <b>104</b>, for example, may adapt its communications with WCDs <b>102</b> to compensate for the conditions unique to the particular WCD to achieve an improved communication rate with WCDs <b>102</b>.

[0017] In accordance with one embodiment, background noise, in-band interference and channel effects may be measured for portions of the assigned spectrum and a modulation order is selected on a per subcarrier basis to compensate for channel effects and in-band interference. Accordingly, the subcarriers may operate at different communication rates allowing the channel to approach its “water-filling capacity”. In one embodiment, AP <b>104</b> may select modulation orders on a per subcarrier basis for upstream communications received from WCDs <b>102</b>. In another embodiment, WCDs <b>102</b> may select modulation orders on a per subcarrier basis for downstream communications received from AP <b>104</b>. In one embodiment, forward error correction (FEC) code rates may be adjusted based on the per subcarrier modulation selections. In another embodiment, the FEC code rates may be adjusted and applied to all subcarriers in a group of OFDM symbols. The FEC code rate may be adapted, for example, by puncturing, shortening or selectively erasing the code. In accordance with yet another embodiment, an interleaving scheme may also be adjusted based on the per subcarrier modulation selections to match OFDM symbol boundaries. In another embodiment, the interleaving scheme may be adjusted and applied to all subcarriers in a group of OFDM symbols.

[0018]FIG. 2 is a highly simplified functional block diagram of a wireless communication device in accordance with an embodiment of the present invention. WCD <b>200</b> may be suitable for use as WCD <b>102</b> (FIG. 1) although other devices are also suitable. With the addition of a network communication interface, among other things, WCD <b>200</b> may also be suitable for use as access point <b>104</b> (FIG. 1) although other devices are also suitable. WCD <b>200</b> includes OFDM transmitter subsystem <b>202</b>, OFDM receiver subsystem <b>204</b> and controller subsystem <b>206</b>. WCD <b>200</b> may include other functional elements that are not illustrated that allow it to serve a primary purpose, such as operating as a PDA, a laptop or portable computer with wireless communication capability, a web tablet, a wireless telephone, a wireless headset, a pager, an instant messaging device, an MP3 player, a digital camera, or other device that may receive and/or transmit information wirelessly.

[0019] In accordance with one embodiment, controller subsystem <b>206</b> measures background noise, in-band interference and/or channel effects for portions of the spectrum and selects a modulation order on a per subcarrier basis to compensate for the channel effects and/or in-band interference. WCD <b>200</b> may transmit the selected modulation orders to another WCD, such as AP <b>104</b> (FIG. 1). The modulation orders may be used by the other WCD in transmitting downstream signals to WCD <b>200</b>. Receiver subsystem <b>204</b> may use the selected modulation orders to demodulate the subcarriers for receiving downstream communications from the other WCD.

[0020] In one embodiment, WCD <b>200</b> may transmit a channel sounding preamble to allow the other WCD to measure the channel. The other WCD, such as AP <b>104</b> (FIG. 1), may select modulation orders for each subcarrier based on the channel measurements and/or in-band interference and background noise, and may transmit the selected per subcarrier modulation orders to WCD <b>200</b>. Transmitter subsystem <b>202</b> may use the received modulation orders to modulate the subcarriers for upstream communications transmitted to the other WCD.

[0021] In one embodiment, the selection of modulation orders may be performed as often as the channel conditions change, depending on the coherence time of the channel. The channel conditions may be continually monitored and modulation orders may be selected when channel conditions change. Modulation orders may also be selected on a regular basis that may be less than the channel's coherence time.

[0022] In one embodiment, modulation orders may be selected based on a signal to interference and noise ratio (SINR). Higher modulation orders may be selected for subcarriers having better SINRs. Modulation orders define a number of bits per symbol that may be communicated using a particular subcarrier. Modulation orders may include binary phase shift keying (BPSK), which communicates one bit per symbol, quadrature phase shift keying (QPSK), which communicates two bits per symbol, 8PSK, which communicates three bits per symbol, 16-quadrature amplitude modulation (16-QAM), which communicates four bits per symbol, 32-QAM, which communicates five bits per symbol, and 64-QAM, which communicates six bits per symbol. Modulation orders may also include differentially coded star QAM (DSQAM). Modulation orders with lower and even higher communication rates per subcarrier may also be selected.

[0023] Accordingly, in a system that utilizes a channel comprised of a plurality of subcarriers, one or more of the subcarriers may utilize, for example, BPSK where there is a low SINR. One or more of the subcarriers may utilize a higher modulation order, such as 16-QAM for higher SINRs, and one or more of the subcarriers may utilize modulation orders of 64-QAM for even higher SINRs. Operating the subcarriers at different communication rates allows the channel to approach its “water-filling capacity” and allows the WCDs may adapt to in-band interference as well as channel fading.

[0024]FIG. 3 is a functional block diagram of a wireless communication device in accordance with an embodiment of the present invention. WCD <b>300</b> may be suitable for use as WCD <b>102</b> (FIG. 1) and WCD <b>200</b> (FIG. 2) although other devices are also suitable. With the addition of a network communication interface, among other things, WCD <b>300</b> may also be suitable for use as access point <b>104</b> (FIG. 1) although other devices are also suitable. WCD <b>300</b> includes OFDM transmitter subsystem <b>302</b> which may correspond with OFDM transmitter subsystem <b>202</b> (FIG. 2), OFDM receiver subsystem <b>304</b> with may correspond with OFDM receiver subsystem <b>204</b> (FIG. 1), and controller subsystem <b>306</b> which may correspond with controller subsystem <b>206</b> (FIG. 2). WCD <b>300</b> may also include antenna <b>308</b>, which may be, for example, a dipole antenna, monopole antenna loop antenna, microstrip antenna or other type of antenna. WCD <b>300</b> may include other functional elements that are not illustrated to allow WCD <b>300</b> to serve a primary purpose.

[0025] Transmitter subsystem <b>302</b> may include transmit media access controller (TX MAC) <b>310</b>, FEC encoder <b>312</b>, adaptive interleaver <b>314</b>, serial to parallel (S/P) converter <b>316</b>, subcarrier modulator <b>318</b>, OFDM physical layer element (PHY) <b>320</b> and radio frequency transmitter (RF TX) <b>322</b>. TX MAC <b>310</b> receives data in the form of a stream comprised of bits to be transmitted, FEC encoder <b>312</b> applies forward error correcting codes to the stream and adaptive interleaver <b>314</b> applies an interleaving scheme to the stream. S/P converter <b>316</b> converts the stream to parallel symbols <b>324</b>. In one embodiment, S/P converter <b>316</b> may select a constellation point from the modulation order identified for that subcarrier, given the proper number of bits for that subcarrier. In an alternate embodiment, modulator <b>318</b> may perform this function. Subcarrier modulator <b>318</b> modulates parallel input symbols <b>324</b> to generate symbol-modulated subcarriers <b>326</b> for transmission. Subcarrier modulator <b>318</b> may use input symbols <b>324</b> to modulate a corresponding one of the subcarriers to generate symbol-modulated subcarriers <b>326</b>. Subcarriers <b>326</b> of the OFDM system may be substantially orthogonal to each other to reduce inter-subcarrier interference. Subcarrier modulator <b>318</b> may use a modulation order selected from any of a plurality of modulation orders to modulate the subcarriers using any one of the modulation orders. In one embodiment, controller subsystem <b>306</b> may provide the selected modulation orders to subcarrier modulator <b>318</b> and/or S/P <b>316</b>.

[0026] Symbol modulated subcarriers <b>326</b> form a frequency domain representation of the OFDM symbol. Symbol-modulated subcarriers <b>326</b> are applied to an Inverse Fast Fourier transform (IFFT) element, which may be part of OFDM PHY <b>320</b>, to generate a time domain representation of the OFDM symbol. The time domain representation of the OFDM symbol is comprised of a plurality of time domain samples. Any form of inverse discrete Fourier transform (IDFT) may be used to perform the inverse transform operation, however an IFFT operation may be more computationally efficient. The number of time domain samples generated by the IFFT element may be equal to the number of frequency components input thereto.

[0027] OFDM PHY <b>320</b> may convert the time domain samples generated by the IFFT operation, which may be in a parallel form, to a serial sample stream representing the OFDM symbol. OFDM PHY <b>320</b> may also add a cyclic extension (or guard interval) to reduce inter-symbol interference in the channel, which may be caused by the channel's memory (i.e., multipath reflections). OFDM PHY <b>320</b> may provide the serial OFDM symbols, including its corresponding cyclic extension, in a continuous symbol stream to RF TX <b>322</b>.

[0028] RF TX <b>322</b> converts the OFDM symbol stream into a radio frequency signal for transmission into the wireless channel. To perform this function, RF TX <b>322</b> may include, for example, a digital to analog converter, a frequency conversion unit (e.g., an up converter), a power amplifier, and/or other equipments to generate an RF transmit signal. Antenna <b>308</b> transmits the RF transmit signal into the channel. It should be appreciated that other processing functionality, such as error coding circuitry, may also be included within OFDM transmitter subsystem <b>302</b>.

[0029] OFDM receiver subsystem <b>304</b> includes receive media access controller (RX MAC) <b>330</b>, FEC decoder <b>332</b>, adaptive de-interleaver <b>334</b>, parallel to serial (P/S) converter <b>336</b>, subcarrier demodulator <b>338</b>, OFDM physical element (PHY) <b>350</b> and radio frequency receiver (RF RX) <b>342</b>. Antenna <b>308</b> receives an RF communication signal from the channel. RF RX <b>342</b> converts the received RF signal to a format for subsequent processing. RF RX <b>342</b> may include, for example, a low noise amplifier, one or more frequency conversion units (e.g., a down converter), an analog to digital converter, and/or other functionality to achieve a desired signal format. RF RX <b>342</b> provides the signal to OFDM PHY <b>340</b>. OFDM PHY <b>340</b> may include a synchronization element to synchronize the signal in a manner that allows the individual OFDM symbols within the signal to be recognized and the cyclic extensions to be discarded. The OFDM symbols, in a serial format, are converted into a parallel group of time domain samples. The samples are input into a Fast Fourier transform (FFT) element that may be part of OFDM PHY <b>340</b>, to generate frequency domain symbol modulated subcarriers <b>346</b>.

[0030] Subcarrier demodulator <b>338</b> demodulates symbol-modulated subcarriers <b>346</b> to produce symbols <b>344</b>. In one embodiment, subcarrier demodulator <b>338</b> may demodulate symbol-modulated subcarriers <b>346</b> in accordance with a modulation order provided by controller subsystem <b>306</b>. The modulation orders may have been selected by WCD <b>300</b> and provided to the WCD transmitting the received RF communication signal. WCD <b>300</b> may have selected the modulation orders based on channel conditions, such as background noise, in-band interference and/or channel response.

[0031] Parallel to serial converter <b>336</b> converts symbols <b>344</b> from a parallel form to a serial stream based on the selected modulation orders, adaptive de-interleaver <b>334</b> may perform a deinterleaving operation on the serial stream, and FEC decoder <b>332</b> may decode the serial stream. Receive media access controller (RX MAC) <b>330</b> receives the decoded serial bit stream and provides it to another portion of WCD <b>300</b>, such as a system processor, for subsequent use.

[0032] Controller subsystem <b>306</b> may include controller <b>350</b>, subcarrier modulation order selector <b>352</b>, signal to interference and noise (SINR) calculator <b>354</b>, channel estimator <b>356</b> and interference measuring element <b>358</b>. In one embodiment, channel estimator <b>356</b> may generate a channel estimate of a downstream OFDM communication channel. The channel estimate may comprise a channel response across the channel bandwidth, and may be measured based on a channel sounding preamble transmitted by another WCD, such as AP <b>104</b> (FIG. 1). The channel sounding preamble may substantially occupy the entire downstream channel bandwidth. The channel estimate generated by channel estimator <b>356</b> may include a channel estimate for each subcarrier frequency.

[0033] Interference measuring element <b>358</b> measures interference within the downstream channel. The interference may include an interference level measured for each subcarrier frequency. In one embodiment, interference measuring element <b>358</b> may measure in-band interference during a period when communication devices of the system are instructed to refrain from transmitting (i.e., during a pre-designated dead time) allowing element <b>358</b> to measure in-band interference produced by non-system devices and noise levels.

[0034] SINR calculator <b>354</b> may calculate one or more parameters for use by subcarrier modulation selector <b>352</b> in selecting modulation orders. For example, SINR calculator <b>354</b> may use the channel estimate generated by channel estimator <b>356</b> and the interference measured by element <b>358</b> to calculate a SINR. In one embodiment, SINR calculator <b>354</b> may calculate a SINR for each subcarrier frequency of the downstream channel. In other embodiments, SINR calculator <b>354</b> may calculate other parameters based on background noise, in-band interference and/or channel effects for one or more subcarriers. The parameters may be used by subcarrier modulation selector <b>352</b> to select modulation orders for one or more of the subcarriers for use in demodulating received communications.

[0035] Controller <b>350</b>, among other things, may receive the selected modulation orders from modulation selector <b>352</b> and may encode the selected modulation orders in a data message for TX MAC <b>310</b> for transmission to the other WCD. The other WCD may decode the data message, and use the selected modulation orders in transmitting downstream data to WCD <b>300</b> on the OFDM downstream channel. Controller <b>350</b> may also instruct subcarrier demodulator <b>338</b> to demodulate subcarriers in accordance with the selected modulation orders, which may be used by the other WCD for transmission.

[0036] In one embodiment, the decoded serial bit stream received by WCD <b>300</b> may include modulation orders selected by another WCD, such as AP <b>104</b> (FIG. 1) for use by WCD <b>300</b> in transmitting upstream communication signals. In this embodiment, controller <b>350</b> may interpret the decoded serial bit stream received from RX MAC <b>330</b> and provide the modulation orders to subcarrier modulator <b>318</b> for use in modulating subcarriers for transmission to the other WCD. Controller <b>350</b> may provide modulation orders for use in individually modulating one or more of the subcarriers of the upstream OFDM communication channel.

[0037] One consequence of the dynamic adaptation of the modulation density in each portion of the spectrum is the difficulty in matching constant-length FEC blocks to OFDM symbol boundaries. Since the number of coded bits transmitted in an OFDM symbol or group of symbols may change between frames due to modulation adaptation, the FEC and interleaving may change as well. In one embodiment of the present invention, controller <b>350</b> may adjust the FEC code rate applied by FEC encoder <b>312</b> and/or modify the interleaving applied by adaptive interleaver <b>314</b>. In this embodiment, parameters for adjustment of the FEC code rate and/or modification of the interleaving for upstream communications may be determined by the other WCD and may be based on the modulation orders for the subcarriers selected by the other WCD for upstream communications. In one embodiment of the present invention, controller <b>350</b> may adjust the FEC code rate applied by FEC decoder <b>332</b> and/or modify the deinterleaving applied by adaptive de-interleaver <b>334</b>. In this embodiment, parameters for adjustment of the FEC code rate and/or modification of the interleaving for downstream communications may be determined by controller <b>350</b> and may be based on the selected modulation orders for the subcarriers. The parameters for adjustment of the FEC code rate and/or modification of the interleaving may be transmitted to the other WCD for use in transmitting downstream communications to WCD <b>300</b>. In these embodiments, the adjustment of the FEC code rate may include the use of puncturing, shortening and/or erasing the code. Also in these embodiments, the interleaving scheme may be adapted to match the OFDM symbol boundaries considering the selected per-subcarrier modulation orders. For example, continuous interleaving methods, such as helical interleaving, may be adjusted by zero padding up to the nearest symbol boundary. Random interleaving may be accomplished by storing multiple random interleaver configurations and using a best fit interleaver with small adjustments by zero padding to the nearest symbol boundary. Analytically generated interleaving patterns specific to each configuration may also be generated.

[0038] Although WCD <b>300</b> is illustrated in FIG. 3 as having separate functional transmitter, receiver and controller subsystems, one or more of the functional elements of these subsystems may be combined and may be implemented by combinations of software configured elements and/or hardware. For example, TX MAC <b>310</b> and RX MAC <b>330</b> may be implemented by one functional element, and a software-configured processor may implement one or more of the functional elements of controller subsystem <b>306</b>. Although WCD <b>300</b> is illustrated with interleaver <b>314</b> and de-interleaver <b>334</b>, these functional elements, among others, are optional and several of the embodiments of the present invention may be implemented without requiring interleaving.

[0039]FIG. 4A is a simplified timing diagram suitable for use by a point-to-multipoint communication system in accordance with an embodiment of the present invention. Timing diagram <b>400</b> illustrates access point transmission <b>402</b> and terminal transmissions <b>404</b>. Access point (AP) transmissions <b>402</b> may be transmitted by a WCD such as AP <b>104</b> (FIG. 1) and terminal transmissions <b>404</b> may be transmitted by WCDs <b>102</b> (FIG. 1) although other devices may also be suitable. Timing diagram <b>400</b> is an example of one embodiment in which AP point may provide point-to-multipoint communications with one or more WCDs. The AP transmits channel sounding preamble <b>406</b> which may be used by one or more WCDs to measure the channel. Subsequent to channel sounding preamble <b>406</b> may be dead time <b>408</b> wherein WCDs may refrain from transmitting. The WCDs may refrain from transmitting during dead time <b>408</b> in response to channel sounding preamble <b>406</b> received from the AP. The WCDs, including the AP, may measure in-band interference and noise levels at their location during dead time <b>408</b>. The WCDs may use the in-band interference and channel measurements to select communication parameters including modulation orders, FEC codes and/or an interleaving scheme, for subsequent use by the AP. Following dead time <b>408</b>, the WCDs may sequentially transmit channel sounding preambles <b>410</b> to the AP.

[0040] In one embodiment, the WCDs may transmit its selected communication parameters prior to, as part of, or subsequent to the transmission of channel sounding preambles <b>410</b>. The AP may use the channel sounding preambles <b>410</b> to measure the upstream channel conditions, which may differ for each WCD. The AP may select communication parameters for the WCDs based on the in-band interference and noise levels measured during dead time <b>408</b> and channel conditions, which may be measured for each WCD. The AP may transmit downstream data <b>412</b> to the WCDs. In one embodiment, AP may use the communication parameters, which may be selected by a WCD in transmitting downstream data <b>412</b> to the WCD. Downstream data <b>412</b> may include modulation orders, FEC codes and/or interleaving selected by AP for use by a WCD. A WCD may utilize the communication parameters selected by AP to communicate upstream data <b>414</b>. Accordingly, the parameters for communicating downstream data <b>412</b> may be configured for the downstream channel conditions, and the parameters for communicating upstream data <b>414</b> may be configured for the upstream channel conditions.

[0041] The framing duration illustrated in FIG. 4A may be shorter than the coherence time of the channel so that the AP and the WCDs are able to adapt to the environmental dynamics of the channel. In one embodiment, the AP may determine the channel coherence time to determine a frame rate at which to update the preamble handshakes (preambles <b>406</b>, <b>410</b>) illustrated in FIG. 4A.

[0042]FIG. 4B is a simplified timing diagram suitable for use by a point-to-point communication system in accordance with another embodiment of the present invention. Timing diagram <b>420</b> illustrates terminal transmissions <b>422</b> of a first WCD and terminal transmissions <b>424</b> of a second WCD. Terminal transmissions <b>422</b> and <b>424</b> may be transmitted by WCDs <b>102</b> (FIG. 1) although other devices may also be suitable. Timing diagram <b>420</b> is an example of one embodiment providing point-to-point communications between two or more WCDs. In accordance with this embodiment, in-band interference and noise levels may be measured during dead time <b>426</b> until the first WCD transmits channel sounding preamble <b>428</b>. The second WCD may respond (i.e., handshaking) to channel sounding preamble <b>428</b> from the first WCD by transmitting channel sounding preamble <b>430</b>. A WCD may use the channel sounding preamble received from the other WCD to measure the channel, and may select communication parameters for the subcarriers for use by the other WCD in transmitting data. The second WCD may include the communication parameters selected for the subcarriers with channel sounding preamble <b>430</b>, while the first WCD may include the selected communication parameters with data transmissions <b>432</b>. Accordingly, data <b>432</b> may be communicated through the channel in accordance with the communication parameters selected by the second WCD, and data <b>434</b> may be communicated through the channel in accordance with the communication parameters selected by the first WCD.

[0043] In the point-to-point embodiment, a WCD may transmit a preamble to initiate a data transfer. Other WCDs may respond to the preamble allowing the WCDs to determine communication parameters for transmitting to the other WCDs. In this embodiment, interference measurements may be made continuously or may be made periodically, such as during dead times when the channel is not being used for transmissions.

[0044]FIG. 5 is an example of channel response and interference of an OFDM channel in accordance with an embodiment of the present invention. OFDM channel <b>500</b> may be comprised of a plurality of subcarriers <b>506</b> and may have channel response <b>502</b> across the channel bandwidth as well as noise level <b>504</b>. A WCD, such one of WCDs <b>102</b> (FIG. 1) or AP <b>104</b> (FIG. 1), may measure channel response <b>502</b> during a channel sounding preamble received from another WCD. Channel response <b>502</b> around point <b>510</b> illustrates particular frequencies within the channel having a low signal to noise ratio, while the channel response around point <b>512</b> illustrates particular frequencies within the channel having a high signal to noise ratio. As WCDs change location and as channel conditions change, channel response <b>502</b> may change. In-band interference <b>508</b> along with noise level <b>504</b> may be measured by WCDs during dead times. In-band interference <b>508</b> may come and go as in-band interfering devices transmit. In accordance with one embodiment, WCDs may calculate a signal to interference and noise level (SINR) at a subcarrier frequency. In the illustration of FIG. 5, the SINR is the difference between channel response <b>502</b> and either noise level <b>504</b> or the level of in-band interference <b>508</b>. Subcarriers <b>506</b> are illustrated as having their communication parameters (e.g., modulation orders, FEC codes and/or an interleaving scheme) selected to approach the water-filled capacity of the channel. In other words, subcarrier <b>506</b> may be configured to communicate at a maximum rate based on the SINR at the subcarrier frequency. Conventional OFDM systems, on the other hand, use the same communication parameters for each subcarrier. Conventional OFDM systems also select the same communication parameters for all subcarriers based on the point having the worst channel response (i.e., point <b>510</b>) and/or highest noise/interference level in the channel (interference <b>508</b>). As can be appreciated, this conventional approach results in a much lower communication rate.

[0045]FIGS. 6A and 6B are a flow chart of an interference and channel adaptation procedure in accordance with an embodiment of the present invention. Procedure <b>600</b> may be performed by one or more WCDs such as WCDs <b>102</b> (FIG. 1) and AP <b>104</b> (FIG. 1) although other devices are also suitable. Through the performance of procedure <b>600</b>, WCDs dynamically determine communication parameters for communication through an OFDM channel based on channel conditions that include in-band interference and channel response. Although the individual operations of procedure <b>600</b> are illustrated and described as separate operations, one or more of the individual operations may be performed concurrently and nothing requires that the operations be performed in the order illustrated. Although procedure <b>600</b> is described for point-to-multipoint embodiments, it may be equally applicable to point-to-point embodiments of the present invention.

[0046] In operation <b>602</b>, one or more WCDs receive a channel sounding preamble from an AP. The channel sounding preamble may by transmitted so as to evenly occupy the entire channel bandwidth. The channel sounding preamble may be considered a burst. In operation <b>604</b>, the one or more WCDs estimate a channel response from the received channel sounding preamble. In operation <b>604</b>, channel response may be measured for each subcarrier frequency. In operation <b>606</b>, the WCDs and AP may measure the noise level including in-band interference during a dead time. In operation <b>606</b>, the noise level and in-band interference may be measured for each subcarrier frequency. The dead time may follow the channel sounding preamble, may be at predesignated time, or may be at a time when no transmissions are occurring. In operation <b>608</b>, parameters, such as a SINR, may be calculated for each subcarrier frequency from the channel response measurements of operation <b>604</b> and measurements from operation <b>606</b>.

[0047] In operation <b>610</b>, the one or more WCDs may calculate channel communication parameters for each subcarrier. The channel communication parameters may include modulation orders, FEC codes and/or an interleaving scheme for each subcarrier. In operation <b>612</b>, the one or more WCDs may transmit a channel sounding preamble to the AP. In the point-to-multipoint embodiments the channel sounding preambles may be transmitted sequentially by the WCDs. In response to channel sounding preambles, the AP may determine the channel response of the channel with each WCD. Due to the different locations of the WCDs, the channel response may be different for each WCD. In operation <b>614</b>, the WCDs may transmit the communication parameters selected in operation <b>610</b> to the AP. Operation <b>614</b> may be performed as part of operation <b>612</b>. In transmitting communication parameters to the AP, a predetermined modulation order, FEC coding rate and interleaving scheme may be used. The predetermined modulation order may be a lower modulation order, such as BPSK.

[0048] In operation <b>616</b>, the AP may calculate parameters, such as SINRs, for each subcarrier frequency based on the interference measured in operation <b>606</b> and based on the channel conditions measured for each WCD in operation <b>612</b>. In operation <b>618</b>, the AP may select channel communication parameters for each subcarrier of each of the channels. In a point-to-multipoint system where the AP communicates with several WCDs, the AP may determine communication parameters for receiving communications from each of the WCDs. The channel communication parameters may include modulation orders, FEC codes and/or an interleaving scheme for each subcarrier. In operation <b>620</b>, the WCDs receive the selected communication parameters from the AP. In transmitting the communication parameters to the WCDs, a predetermined modulation order, FEC coding rate and interleaving scheme may be used by the AP. The predetermined modulation order may be a lower modulation order, such as BPSK.

[0049] In operation <b>622</b>, the AP may communicate downstream data to the WCDs using the communication parameters received in operation <b>614</b>. In operation <b>624</b>, the WCDs may communicate upstream data to the AP using the communication parameters received in operation <b>620</b>. In operation <b>626</b>, operations <b>602</b> through <b>624</b> may be repeated. Operation <b>626</b> may be performed on a regular basis, which may be less than the coherence time of the channel so as to respond to changes in channel conditions. In one embodiment, the coherence time of the channel may be dynamically determined based on channel measurements. Operation <b>626</b> may be performed when channel conditions change.

[0050] Thus, a wireless communication device and method have been described that provide a more efficient use of an OFDM communication channel. In-band interference and channel effects may be measured for each portion of the spectrum and a modulation order may selected on a per subcarrier basis to compensate, at least in part for channel effects and in-band interference. Accordingly, the subcarriers may operate at different communication rates allowing the channel to approach its “water-filling capacity”. In one embodiment, an access point may select modulation orders on a per subcarrier basis for upstream communications received from each of a plurality of wireless communication devices. In another embodiment, the wireless communication devices may select modulation orders on a per subcarrier basis for downstream communications received from the access point. In other embodiments, forward error correction (FEC) code rates and interleaving may be adjusted to the per subcarrier modulation selections.

[0051] The foregoing description of specific embodiments reveals the general nature of the invention sufficiently that others can, by applying current knowledge, readily modify and/or adapt it for various applications without departing from the generic concept. Therefore such adaptations and modifications are within the meaning and range of equivalents of the disclosed embodiments. The phraseology or terminology employed herein is for the purpose of description and not of limitation. Accordingly, the invention embraces all such alternatives, modifications, equivalents and variations as fall within the spirit and scope of the appended claims.