Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2602—Signal structure
  • H04L27/261—Details of reference signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/05—Electric or magnetic storage of signals before transmitting or retransmitting for changing the transmission rate
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
  • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
  • H04L27/38—Demodulator circuits; Receiver circuits
  • H04L27/3845—Demodulator circuits; Receiver circuits using non - coherent demodulation, i.e. not using a phase synchronous carrier
  • H04L27/3854—Demodulator circuits; Receiver circuits using non - coherent demodulation, i.e. not using a phase synchronous carrier using a non - coherent carrier, including systems with baseband correction for phase or frequency offset
  • H04L27/3863—Compensation for quadrature error in the received signal
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/0014—Carrier regulation
  • H04L2027/0016—Stabilisation of local oscillators
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0224—Channel estimation using sounding signals
  • H04L25/0226—Channel estimation using sounding signals sounding signals per se

Definitions

• This invention relates generally to the modulation of communications and, more particularly, to systems and methods for generating a quadrature modulation rotating training signal for use in the training of receiver channel estimates.
• FIG. 1 is a schematic block diagram of a conventional receiver front end (prior art).
• a conventional wireless communications receiver includes an antenna that converts a radiated signal into a conducted signal. After some initial filtering, the conducted signal is amplified. Given a sufficient power level, the carrier frequency of the signal may be converted by mixing the signal (down-converting) with a local oscillator signal. Since the received signal is quadrature modulated, the signal is demodulated through separate I and Q paths before being combined. After frequency conversion, the analog signal may be converted to a digital signal, using an analog-to- digital converter (ADC), for baseband processing. The processing may include a fast Fourier transform (FFT).
• FFT fast Fourier transform
• a unique direction in the constellation (e.g., the I path) is stimulated, while the other direction (e.g., the Q path) is not.
• the same type of unidirectional training may also be used with pilot tones. Note: scrambling a single modulation channel with ⁇ 1 does not rotate the constellation point, and provides no stimulation for the quadrature channel.
• the above-mentioned power-saving training sequence results in a biased channel estimate.
• a biased channel estimate may align the IQ constellation well in one direction (i.e., the I path), but provide quadrature imbalance in the orthogonal direction. It is preferable that any imbalance be equally distributed among the two channels.
• 2 is a schematic diagram illustrating quadrature imbalance at the receiver side (prior art). Although not shown, transmitter side imbalance is analogous.
• the Q path is the reference.
• the impinging waveform is cos(wt + ⁇ ), where ⁇ is the phase of the channel.
• the Q path is down-converted with — sin(wt).
• the I path is down-converted with (l+2 ⁇ )cos(wt+ 2 ⁇ ).
• 2 ⁇ and 2 ⁇ are hardware imbalances, respectively a phase error and an amplitude error.
• the low pass filters Hi and HQ are different for each path. The filters introduce additional amplitude and phase distortion. However, these additional distortions are lumped inside 2 ⁇ and 2 ⁇ . Note: these two filters are real and affect both +w and —w in an identical manner. [0007] Assuming the errors are small:
• the first component on the right hand side, cos(wt), is the ideal I path slightly scaled.
• the second component, - 2 ⁇ .sin(wt), is a small leakage from the Q path.
• Wireless communication receivers are prone to errors caused by a lack of tolerance in the hardware components associated with mixers, amplifiers, and filters. In quadrature demodulators, these errors can also lead to imbalance between the I and Q paths.
• a training signal can be used to calibrate receiver channel errors. However, a training signal that does not stimulate both the I and Q paths does not address the issue of imbalance between the two paths. [0011] Accordingly, a method is provided for transmitting a quadrature modulated rotating training sequence.
• a rotating training signal is generated by a quadrature modulation transmitter.
• the rotating training signal includes training information sent via an in-phase (I) modulation path, as well as training information sent via a quadrature (Q) modulation path.
• Quadrature modulated communication data is generated either simultaneous with, or subsequent to the training signal.
• the rotating training signal and quadrature modulated communication data are transmitted.
• the rotating training signal may be generated by initially sending training information via the I modulation path, and subsequently sending training information via the Q modulation path.
• the training information sent via the I modulation path may include a first symbol having a reference phase (e.g., 0 degrees or 180 degrees).
• the training information sent via the Q modulation path would include a second symbol having a phase that is ⁇ 90 from the reference phase.
• FIG. 1 is a schematic block diagram of a conventional receiver front end (prior art).
• FIG. 2 is a schematic diagram illustrating quadrature imbalance at the receiver side (prior art).
• FIG. 3 is a schematic block diagram of a wireless communications device, with a system for transmitting a rotating training sequence.
• FIGS. 4A through 4D are diagrams depicting a training signal with quadrature modulated communication data.
• FIG. 5A and 5B are diagrams of the rotating training symbols as represented in a quadrature constellation.
• FIG. 6 is a diagram depicting an exemplary framework for carrying a message with a rotating training signal.
• FIG. 7 is a schematic block diagram depicting a processing device for transmitting a quadrature modulation rotating training sequence.
• FIG. 8 is a diagram depicting ideal and imbalanced constellations for 2 different phases ⁇ of the impinging waveform of FIG 2.
• FIG. 9 is a graph depicting phase imbalance as a function of the phase on the impinging waveform.
• Fig. 10 is a flowchart illustrating a method for transmitting a communications training sequence.
• a component may be, but is not limited to being, a process running on a processor, a processor, an object, an executable, a thread of execution, a program, and/or a computer.
• an application running on a computing device and the computing device can be a component.
• One or more components can reside within a process and/or thread of execution and a component may be localized on one computer and/or distributed between two or more computers.
• these components can execute from various computer readable media having various data structures stored thereon.
• the components may communicate by way of local and/or remote processes such as in accordance with a signal having one or more data packets (e.g., data from one component interacting with another component in a local system, distributed system, and/or across a network such as the Internet with other systems by way of the signal).
• a signal having one or more data packets (e.g., data from one component interacting with another component in a local system, distributed system, and/or across a network such as the Internet with other systems by way of the signal).
• FIG. 3 is a schematic block diagram of a wireless communications device 300, with a system for transmitting a rotating training sequence.
• the system 302 comprises a radio frequency (RF) transmitter 304 having an input on lines 306a and 306b to accept information, an in-phase (I) modulation path 308, a quadrature (Q) modulation path 310, and a combiner 312 for combining signals from the I and Q modulation paths, 308 and 310, respectively.
• RF transmitter is used as an example to illustrate the invention, it should be understood that the invention is applicable to any communication medium (e.g., wireless, wired, optical) capable of carrying quadrature modulated information.
• the I and Q paths may alternately be referred to as I and Q channels.
• the combined signals are supplied on line 318 to amplifier 320, and finally to antenna 322, where the signals are radiated.
• the transmitter 304 can be enabled to send a message with a rotating training signal.
• a rotating training signal which may also be referred to as an quadrature balanced training signal, balanced training signal, balanced training sequence, or unbiased training signal includes training information sent via the I modulation path 308 and training information sent via the Q modulation path 310.
• the transmitter 304 also sends quadrature modulated (non-predetermined) communication data.
• the quadrature modulated communication data is sent subsequent to sending the rotating training signal.
• the training signal is sent concurrently with the communication data in the form of pilot signals.
• the system is not limited to any particular temporal relationship between the training signal and the quadrature modulated communication data.
• FIGS. 4A through 4D are diagrams depicting a training signal with quadrature modulated communication data.
• the transmitter 304 sends the rotating training signal by initially sending training information via the I modulation path 308 and subsequently sending training information via the Q modulation path 310. That is, the training signal includes information, such as a symbol or a repeated series of symbols sent only via the I modulation path 308, followed by the transmission of a symbol or repeated series of symbols sent only via the Q modulation path 310. Alternately but not shown, training information may be sent initially via the Q modulation path 310, and subsequently via the I modulation 308.
• the transmitter sends a rotating training signal with predetermined training information via the I and Q modulation paths.
• the first symbol may always be (1,0) and the second symbol may always be (0,1).
• the above-mentioned rotating training signal which initially sends rotating training signal via Oust) the I modulation path, may be accomplished by energizing the I modulation path 308, but not energizing the Q modulation path 310. Then, the transmitter sends the rotating training signal via the Q modulation path by energizing the Q modulation path 310, subsequent to sending training information via the I modulation path.
• FIGS. 5A and 5B are diagrams of the rotating training symbols as represented in a quadrature constellation.
• the transmitter 304 generates the rotating training signal by sending a first symbol having a reference phase via the I modulation path 308, and sending a second symbol having a phase that is either (reference phase + 90 degrees) or the (reference phase - 90 degrees), via the Q modulation path 310.
• the reference phase of the first symbol may be O degrees, in which case the phase of the second symbol might be 90 degrees (as shown) or -90 degrees (not shown).
• the first symbol may be sent through (just) the I (or Q) modulation path, and the transmitter may send training information simultaneously through both the I and Q modulation paths, and combine I and Q modulated signals to supply the second symbol.
• the transmitter may send the training information simultaneously through both the I and Q modulation paths, and combine I and Q modulated signals to supply the first symbol, while the second symbol is obtained by using just the Q (or I) modulation path.
• the training symbols can also be rotated by supplying symbols, each with both I and Q components, as is conventionally associated with quadrature modulation, see FIG. 4B. That is, the transmitter 304 may send training information simultaneously through both the I and Q modulation paths 308/310, and combine I and Q modulated signals to supply the first symbol on line 318. For example, the first symbol may occupy a position at 45 degrees in the constellation, see FIG. 5B. Likewise, the transmitter would send training information simultaneously through both the I and Q modulation paths 308/310, and combine I and Q modulated signals to supply the second symbol. For example, the second symbol may be rotated to a position of -45 degrees, which is orthogonal to the first symbol (45 degrees).
• a rotating training symbol minimally includes a sequence of two symbols with a phase difference of 90 degrees.
• the system is not limited to a system that uses only two symbols. Generally, an even number of symbols is preferred so that half the symbols may be generated by using the I modulation path, and the other half generated using the Q modulation path.
• a 90 degree rotation need not be performed between every symbol. That is, there is no particular order of phase between symbols.
• half the symbols are different from the other half by 90 degrees, on average.
• UWB Ultra Wideband
• FIG. 6 is a diagram depicting an exemplary framework for carrying a message with a rotating training signal.
• the transmitter 304 is operated in accordance with the OSI model. In this typically 7-layer model, the transmitter is associated with the physical (PHY) layer. As shown, the transmitter 304 sends a physical layer (PHY) signal 600 including a preamble 602, header 604, and payload 606. The transmitter sends the rotating training signal in the PHY header 604, and sends the quadrature modulated communication data in the PHY payload 606.
• PHY physical layer
• beacon information may be considered as a special case of training signals.
• pilot tones are transmitted together with the quadrature modulated communication data on a subset of (reserved) frequencies.
• this reserved set is comprised of pilot tones. That is, the pilot tones are associated with P frequencies, and the data is associated with the remaining N-P frequencies.
• Training signals and pilot signals are similar in that the information content of transmitted data is typically predetermined or "known" data that permits the receiver to calibrate and make channel measurements.
• receiving communication (non-predetermined) data there are 3 unknowns: the data itself, the channel, and noise.
• the receiver is unable to calibrate for noise, since noise changes randomly.
• Channel is a measurement commonly associated with delay and multipath. For relatively short periods of time, the errors resulting from multipath can be measured if predetermined data is used, such as training or pilot signals. Once the channel is known, this measurement can be used to remove errors in received communication (non-predetermined) data. Therefore, some systems supply a training signal to measure a channel before data decoding begins.
• pilot signals are a subset of a more general class of training signals. That is, as used herein, training signals refer to both an initial training sequence, as well as the tracking training sequence referred to pilot tones in a UWB or 802.11 system. Alternately stated, the terms “initial training” and “tracking training” or “pilot tones”, are all types of training signals.
• the transmitter 304 sends a message where the quadrature modulated communication data is a beacon signal, sent at a beacon data rate, following the rotating training signal. That is, the beacon signals used by many communication systems can be transmitted with a rotating training signal. Further, the transmitter 304 may alternately, or in addition, send a message with quadrature modulated communication data at a communication data rate, greater than the beacon data rate, following a rotating training signal.
• the transmitter may send a combination of messages with rotating and non-rotating training signals.
• the transmitter 304 may send multi-burst messages that include a balanced message, following an unbalanced message.
• the phase "balanced message" is used to describe a message that includes both a rotating training signal and quadrature modulated communication data.
• An unbalanced message is a message comprising a non-rotating training signal where training information is sent via the I modulation path, for example, but not sent via the Q modulation path.
• the unbalanced message also includes a message format signal, embedded in the header for example, indicating that a balanced message (with a rotating training signal) is sent subsequent to the unbalanced message.
• the unbalanced message includes quadrature modulated communication data, which may be sent subsequent to the message format signal, in the payload.
• the system is not limited to any particular temporal relationship between training signal, message format signal, and quadrature modulated data.
• the unbalanced message may be a beacon signal or initial training message. Alternately, the unbalanced message may be sent subsequent to the balanced message, or unbalanced messages may be interspersed with balanced messages.
• the rotating training signal may be enabled in the form of pilot signals.
• P rotating pilot symbols may be generated with (N — P) quadrature modulated communication data symbols.
• Each rotating pilot symbol includes training information that changes by 90 degrees every symbol.
• a balanced message, with a rotating training signal is sent by simultaneously transmitting N symbols.
• less than P rotating pilot symbols are used, as some of the pilot symbols are non-rotating symbols.
• the rotating training signal includes symbols simultaneously generated for a plurality of subcarriers using training information sent via the I modulation path, but not the Q modulation path, for i subcarriers. Further, the training signal uses training information sent via the Q modulation path, but not the I modulation path, forj subcarriers. Then, IQ modulated communication data is generated for the i and j subcarriers subsequent to the generation of the training information.
• the subset of i subcarriers includes "paired subcarriers" or "paired tones", which is a pair of tones at frequency -/ and frequency +/. Likewise, tones in the subset j can be paired.
• the pairing of tones at — / and +/ aids in the achievement of I channel training, Q channel training, and rotation training.
• this system may still be considered as generating a rotating training signal, since a channel estimation averaging technique may be used at the receiver to average adjacent subcarriers. Then, the overall effect of using adjacent non-rotating I and Q training symbols is a rotating training signal.
• the training signal is designed so that the odd-numbered subcarriers use non-rotating training symbols sent through the I modulation path (channel X), and the even-numbered subcarriers use the Q modulation path (channel X + 90 degrees).
• the wireless communications device 300 of FIG. 3 can be considered as comprising a means 308/310 for rotating a training signal using the I and Q modulation paths, and a means 308/310 for generating quadrature modulated communication data.
• the training signal may be pilot symbols sent simultaneously with communication data, or the communication data may be sent subsequent to the rotating training signal.
• the device 300 includes a means 320/322 for transmitting as an RF communication.
• FIG. 7 is a schematic block diagram depicting a processing device for transmitting a quadrature modulation rotating training sequence.
• the processing device 700 comprises an I path modulation module 702 having an input on line 704 to accept information and an input on line 706 to accept I control signals.
• the I path modulation module 702 has an output on line 708 to supply I modulated information.
• a Q path modulation module 710 has an input on line 712 to accept information and an input on line 714 to accept Q control signals.
• the Q path modulation module 710 has an output on line 716 to supply Q modulated information.
• a combiner module 718 has inputs on lines 708 and 716 to accept the I and Q modulated information, respectively, and an output on line 720 to supply a quadrature modulated RF signal.
• a controller module 722 has outputs on lines 706 and 714 to supply the I and Q control signals, respectively.
• the controller module 722 uses the I and Q control signals to generate a message with a rotating training signal including training information sent via the I modulation path and training information sent via the Q modulation path, as well as quadrature modulated communication data.
• the functions performed by the above-mentioned modules are similar to those performed by the device of FIG. 3, and will not be repeated here in the interest of brevity.
• the present invention rotating training signal may be used to modify conventional systems that use only the I modulation path for training in an effort to save power.
• Such a system can be modified by momentarily enabling the Q modulation path during the second part of the training sequence.
• This solution uses only slightly more power, while stimulating both I and Q channels during the training sequence.
• the unbalanced message with the non-rotating training signal can be used for a beacon, while balanced messages, with rotating training signals are used for high data rates.
• This solution may require that a receiver be programmed to associate rotating training signal messages with high data rates and unbalanced messages with beacons. To eliminate the need for a receiver to "guess" the type of training signal to be received, information can be embedded in the preamble to inform the receiver of the type of training sequence that is to follow.
• a conventional unbalanced message can be used as the first burst in a multi-burst transmission.
• the receiver can easily be informed, in each burst, of the type of training sequence that is to appear in the following burst.
• the first burst can be an unbalanced message, with all the subsequent bursts being balanced messages.
• These messages may be optionally enabled, used only for example, if they are supported by both the transmitter and receiver. In this manner, the invention can be made backward compatible with existing devices.
• Another solution which is not backward compatible, is to modify all training sequences, including the beacon's training sequence, such that training sequences are always balanced. In this variation the receiver does not have to operate on two different types of training signals.
• the training sequence is a repeated OFDM symbol. This means that the same constellation point is transmitted repeatedly for each subcarrier. A unique direction in the constellation (e.g., I path) is stimulated while the other direction (e.g., Q path) is not.
• the errors associated with such a system have been presented above in the BACKGROUND Section, above.
• FIG. 8 is a diagram depicting ideal and imbalanced constellations for 2 different phases ⁇ of the impinging waveform of FIG 2.
• the angle of the impinging waveform depends on the both the data and the channel and can take any value between 0 and 360 degrees.
• AWGN Gaussian noise
• FIG. 9 is a graph depicting phase imbalance as a function of the phase on the impinging waveform.
• the solid line on the figure below shows the phase imbalance in the case of a repeated training sequence.
• the dotted line shows the case of the rotating training sequence.
• the loss is between 0 dB and 1.5 dB for an imbalance varying between 0 and 10 degrees (depending on the phase of the impinging waveform).
• Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA) in AWGN.
• TDMA Time Division Multiple Access
• CDMA Code Division Multiple Access
• a training sequence is assumed with all the symbols lying on the I axis (I channel). After transmission through an AWGN channel, the axis can rotate to a direction X in the quadrature 2D plane (depending on the channel phase).
• direction X is properly estimated and any data symbol in that direction lies on the proper axis (after rotation).
• the symbols in the orthogonal direction Y will be off by 2 ⁇ degrees from the ideal position. They will incur significantly more errors.
• the dotted line curve in the figure shows the phase imbalance in each direction.
• the dotted line curve is essentially 0.5 times the solid line curve.
• Each direction X and Y now shares half of the quadrature imbalance burden.
• the loss is 0 to 0.5 dB corresponding to 5 degrees maximum imbalance on each axis.
• the gain varies between 0 and 1 dB.
• AWGN LOS channel
• most carriers can be aligned at the same phase and degraded by 1.5 dB for the repeated training sequence case.
• the degradation is only 0.5 dB for the rotating training sequence, which is a 1 dB gain.
• phase imbalance varies between 0 and 10 degrees.
• the error is partly smoothed. But for high data rates, diversity may not be enough to compensate for the excessive error that regularly hits the subcarriers. The effect on high data rates is more important.
• An implementation of a rotating training sequence does not necessarily imply any greater hardware complexity in a receiver or transmitter.
• rotation by 90 degrees before accumulation is performed by swapping the I and Q channels, and sign-inverting one of them. This operation can be done either in the time domain (if all frequencies are rotated the same way) or in the Fourier domain, which is the more general case.
• x is the complex transmitted signal
• x * its complex conjugate y the complex received signal
• a ⁇ 1 and ⁇ ⁇ 0 are complex quantities that characterize the quadrature imbalance distortion. They are given by
• the received signal is identical to the transmitted signal.
• the channel (or nearly) is obtained, but on the right hand side a noise or bias occurs.
• This noise does not vanish as more and more training symbols are averaged: it remains as only the white noise vanishes.
• the estimate of the channel is biased if a training sequence is transmitted that is exclusively aligned with the symbol u.
• the metric that goes into a Viterbi decoder for example is obtained by multiplying the complex conjugate of the channel (channel's match filter) to the received signal.
• Metric ⁇ a ⁇ 2 ⁇ c ⁇ 2 x + a ⁇ ⁇ c ⁇ 2 x * + a ⁇ k c 2 u 2 x
• the first component in the metric formula above is ideally a positive real scalar, proportional to the channel energy, which multiplies the original constellation point. But the second and third components of that formula are undesired noises created by the bias. Their noise variance is identical and equal to
• This noise does not have the distribution of white Gaussian noise, but if various symbols are arriving from different independent channels a (multi-paths in CDMA, or interleaving, etc), after the symbols are combined, a slow convergence to white Gaussian noise is obtained.
• This SNR can be of the order of 10 to 20 dB. For data rates running at low SNRs this additional noise may not be an issue. But for high data rates running at high SNR, this additional noise has a significant impact.
• Metric ⁇ a ⁇ 2 ⁇ c ⁇ 2 x + a ⁇ ⁇ c ⁇ 2 x *
• index m denotes the vector mirrored over the sub-carriers.
• the only contributors to the received symbol at frequency +/ are the channels and symbols at the symmetric frequencies +/ and — /.
• the two symmetric sub- carriers, +/and -/ can be isolated and the received symbol for sub-carrier +/ written as
• Metric(+/) I a 1 2 1 c 1 2 X + a * ⁇ c * c m * Xm * + a ⁇ * c u Cm u m ⁇ + I ⁇ 1 2 1 c m 1 2 u m u ⁇ m *
• the 4 th (noisy) term in the formula above can no longer be neglected, since the channel
• the noisy terms now depend on the strength of the channel at frequency — / and can be significant.
• the frequency — / acts as an interferer that can confuse the Viterbi decoder, which may sometimes interpret a weak metric with plenty of interference as a good metric.
• the channel estimate is a C and the 2 noisy terms are eliminated from the equation to obtain
• Quadrature imbalance is also present at the transmitter side and adds to the distortion. If ⁇ ' and ⁇ are denoted as the imbalance coefficients at the transmitter side, then the output of the transmitter can be written as
• the receiver obtains after the channel c and the distortion a, ⁇ ,
• the noise from distortion is increased.
• Using the unbiased training sequence helps eliminate some of the terms contributing to the noise on the metrics, as explained above.
• Transmitting an unbiased training sequence can be achieved in a conventional UWB system by transmitting the first part of the training sequence using the I path and the second part on the Q path. Even if an unbiased (non-rotating training signal) is used for beaconing, to save power by turning off the Q channel, a special signal embedded in the preamble can inform the receiver of the type of training sequence. Alternately, the receiver can automatically detect the training sequence that is transmitted. This is not a difficult task, as it is enough to look at a few strong sub- carriers to decide if the transmission was identical or rotated by 90 degrees.
• pilot tones are considered to be a special case of training signals, since many conventional systems use pilots that are transmitted in a unique direction in the complex plane. As the pilot tones are tracked, a bias is constantly introduced along that direction. Better pilots are obtained by changing them every OFDM symbol by 90 degrees, or within the same OFDM symbol, rotating some paired ( ⁇ f) subcarriers by 90 degrees with respect to other paired subcarriers (on different frequencies). This change in pilot tones is simple and has almost a zero cost. As the clocks between transmitters and receivers drift, the pilot tones may have the potential of compensating for some of the bias introduced with the initial biased training sequence when an unbalanced training signal is used.
• Fig. 10 is a flowchart illustrating a method for transmitting a communications training sequence. Although the method is depicted as a sequence of numbered steps for clarity, the numbering does not necessarily dictate the order of the steps. It should be understood that some of these steps may be skipped, performed in parallel, or performed without the requirement of maintaining a strict order of sequence.
• the method starts at Step 1000.
• Step 1002 generates a rotating training signal in an quadrature modulation transmitter. Typically, predetermined or known information is sent as the training signal. Step 1002a sends training information via an I modulation path, and Step 1002b sends training information via a Q modulation path. Step 1004 generates quadrature modulated communication data. Step 1004 may be performed subsequent to Step 1002, or simultaneous with the performance of Step 1002. In one aspect, Step 1004 generates a beacon signal at a beacon data rate. Alternately, Step 1004 generates information at a communication data rate, greater than the beacon data rate. Step 1006 transmits the rotating training signal and quadrature modulated communication data. Typically, the generation and transmission of symbols or information occurs almost simultaneously.
• transmitting the rotating training signal in Step 1006 includes initially sending training information via the I modulation path, and subsequently sending training information via the Q modulation path.
• initially generating training information via the I modulation path may include energizing the I modulation path, but not energizing the Q modulation path.
• generating training information via the Q modulation path, subsequent to generating training information via the I modulation path includes energizing the Q modulation path.
• the training information may be sent in the opposite order.
• generating training information via the I modulation path in Step 1002a may include generating a first symbol having a reference phase.
• Step 1002b generates training information via the Q modulation path using the following substeps (not shown).
• Step 1002bl generates training information simultaneously through both the I and Q modulation paths, and Step 1002b2 combines I and Q modulated signals to supply the second symbol.
• generating training information via the I modulation path may include substeps (not shown).
• Step 1002a 1 generates training information simultaneously through both the I and Q modulation paths, and Step 1002a2 combines I and Q modulated signals to supply the first symbol.
• transmitting includes substeps.
• Step 1006a organizes a physical layer (PHY) signal including a preamble, header, and payload. Note, this organization typically occurs as a response to receiving the information to be transmitted in a corresponding MAC format.
• Step 1006b transmits the rotating training signal in the PHY header, and Step 1006c transmits the IQ modulated communication data in the PHY payload.
• PHY physical layer
• Step 1001a sends a multi-burst transmission with an unbalanced message (Step 1001b) followed by the rotating training signal (Step 1006).
• the unbalanced, or imbalanced message includes a non- rotating training signal with training information sent via the I modulation path (Step lOOlbl), but no training information sent via the Q modulation path (Step 100 Ib2).
• the unbalanced message includes a generated message format signal (Step 1001b3) indicating that a rotating training signal is sent subsequent to the unbalanced message.
• Quadrature modulated communication data is generated in Step 1001b4.
• generating a rotating training signal in Step 1002 includes generating P rotating pilot symbols
• generating quadrature modulated communication data in Step 1004 includes generating (N — P) communication data symbols.
• transmitting in Step 1006 includes simultaneously transmitting N symbols.
• generating a rotating training signal in Step 1002 includes simultaneously generating symbols for a plurality of subcarriers. More explicitly, Step 1002a uses training information sent via the I modulation path, but not the Q modulation path, for i subcarriers. Step 1002b uses training information sent via the Q modulation path, but not the I modulation path, forj subcarriers. Then, generating quadrature modulated communication data in Step 1004 includes generating quadrature modulated communication data for the i andj subcarriers subsequent to the generation of the training information. In one aspect, each i subcarrier is adjacent a j subcarrier. [0086] More formally, the channel estimated by subcarrier i is

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• 2008
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