WO2008128200A1

WO2008128200A1 - Achieving deep transmit notches despite transmitters i/q mismatch

Info

Publication number: WO2008128200A1
Application number: PCT/US2008/060263
Authority: WO
Inventors: Turgut Aytur; Stephan Ten Brink; Ravishankar H. Mahadevappa
Original assignee: WIONICS RESEARCH
Current assignee: WIONICS RESEARCH
Priority date: 2007-04-12
Filing date: 2008-04-14
Publication date: 2008-10-23
Anticipated expiration: 2009-10-12
Also published as: TW200904084A; TWI369876B

Abstract

A transmitter with interference avoidance circuitry is provided. The transmitter attenuates signals at image frequencies to reduce signal strength at frequencies to avoid by reducing effects of in-phase and quadrature signal mismatch. In addition signals at frequencies to avoid may be nulled or filtered.

Description

ACHIEVING DEEP TRANSMIT NOTCHES DESPITE TRANSMITTERS I/Q MISMATCH

BACKGROUND

[0001] The present invention relates generally to wideband communication systems, and more particularly to interference avoidance in wideband communication systems. [0002] Digital communication using a wideband system can provide for robust communication at high data rates. Communications using a wide band of frequencies may allow for reduction of communication errors due, for example, to signal reflection or muti- path signal particular to a single frequency. Ultrawideband (UWB) communication may be advantageous in that a range of frequencies utilized by a communication system may be further increased, further reducing muti-path or other adverse effects. Orthogonal frequency division multiplexing (OFDM) may be utilized in UWB communicating and is generally an effective method to spread communication over the frequency band of a UWB system. OFDM divides the bandwidth into a large number of frequency bins that can be individually optimized.

[0003] A complication of ultrawideband communication systems is that some frequencies within the transmission band may be reserved for other services or in use by other services. Moreover, the other services may have priority to use those frequencies or narrowband frequency bands, and UWB communication systems may be required, by regulatory authorities for example, to minimize interference with those other services. [0004] Simply entirely avoiding use of such frequencies may unduly reduce data rates provided by UWB communication systems. Moreover, frequencies to avoid may change from country-to-country or geographic region-to-geographic region or even from time-to- time. Thus, avoiding interference with other services is further complicated because the frequencies to avoid may not be static, and may vary with location and with time. Further, nulling transmissions at particular frequencies may be difficult, particularly if a UWB transmission system inadvertently emits unintended radiation at various frequencies.

BRIEF SUMMARY OF THE INVENTION

[0005] The invention provides for reduced interference with narrowband communication systems by a wide band or ultrawideband transmitter. In one aspect the invention provides a method of reducing interference with narrowband transmitters, comprising receiving an indication of a frequency to avoid, the frequency to avoid being a frequency at which transmissions are not desired; attenuating signal components of a symbol at the frequency to avoid; attenuating signal components of the symbol at an image frequency, the image frequency symmetric to the frequency to avoid about a center frequency of a band of frequencies used for transmission of the symbol; and transmitting the symbol with attenuated signal components. In another aspect the invention provides a method for use by an ultrawideband transmitter of reducing interference with other communication systems, the method comprising receiving an indication of frequencies to avoid; nulling frequency domain signal components of symbols at the frequencies to avoid; attenuating frequency domain signal components of the symbols at image frequencies to the frequencies to avoid, the image frequencies being those frequencies at which signals in the transmitter cause noise at the frequencies to avoid due to I/Q mismatch of the transmitter; converting the symbols to time domain symbols; filtering the time domain symbols; and transmitting the time domain symbols. In another aspect the invention provides a transmitter for a frequency division multiplexed communication, comprising a tone miller for receiving streams of frequency- domain symbols and nulling symbols at a notch frequency; a mask for attenuating symbols from the tone miller which may interfere at the notch frequency; a transform block for converting frequency-domain symbols from the mask to time-domain samples; and an upconverter for modulating the time-domain samples for radio frequency transmission. [0006] These and other aspects of the invention are more fully comprehended on review of this disclosure.

BRIEF DESCRIPTION OF THE FIGURES

[0007] FIG. 1 is a block diagram of a transmitter in accordance with aspects of the invention;

[0008] FIG. 2 is a diagram showing OFDM symbols with a null post-fix;

[0009] FIG. 3 is a diagram showing effect of I/Q imbalance;

[0010] FIG. 4 is a diagram showing an example of frequency imaging caused by I/Q imbalance;

[0011] FIG. 5 is an example diagram of transmitter power spectral density with an interference avoidance notch limited by transmitter I/Q imbalance;

[0012] FIG. 6 is an example diagram of transmitter power spectral density with an interference avoidance notch and image or mirror frequency attenuation I/Q imbalance in accordance with aspects of the invention;

[0013] FIG. 7 is a flow diagram of a process for reducing interfering tones in accordance with aspects of the invention; and

[0014] FIG. 8 is a flow diagram of a further process for reducing interfering tones in accordance with aspects of the invention.

DETAILED DESCRIPTION

[0015] FIG. 1 is a block diagram of a transmitter in accordance with aspects of the invention. The transmitter may be implemented using integrated circuitry, for example as a PHY chip, other chip, or a multiple chip system. Preferably, the transmitter receives an input bit stream for transmission using frequency division multiplexing. Accordingly, the transmitter codes the input bit stream, maps the coded bits into frequency-domain symbols, transforms the symbols to time-domain samples, converts the time-domain samples to an analog representation, upconverts the analog representation to a radio frequency signal, and transmits the radio frequency signal from an antenna. The transmitter may be used in an ultra- wideband (UWB) system with orthogonal frequency division multiplexing (OFDM). [0016] As shown in FIG. 1, the transmitter includes a channel coder 111. The channel coder receives an input bit stream from a source. The source of the input bit stream may be a media access controller (MAC) or another protocol source. The channel coder codes the received bit stream. The channel coding may include convolutional coding or another forward error correction or detection scheme. The channel coding may additionally include interleaving. Interleaving distributes coded bits so that a burst of errors is less likely to cause an uncorrected or undetected bit error or packet error at a receiver.

[0017] The coded bits from the channel coder are received by a mapper 113. The mapper maps the coded bits into frequency-domain symbols. The mapping may use quadrature phase shift keying (QPSK), dual carrier modulation (DCM), quadrature amplitude modulation (QAM), or other mapping or modulation schemes. The mapped symbols may be considered to include real and imaginary parts, with the real parts provided to an in-phase circuit path and imaginary parts provided to a quadrature circuit path, as is understood by those of skill in the art. For convenience, the in-phase and quadrature paths are shown in FIG. 1 in combined format.

[0018] Also shown in FIG. 1, a tone miller 115 receives the mapped symbols, which are frequency-domain symbols, from the mapper. The tone nuller also receives a signal indicating frequencies at which no energy should be transmitted. The signal may be provided by a media access controller, other device, or otherwise determined or generated by circuitry of or associated with the transmitter. The tone nuller zeros or nulls symbols that correspond to frequencies, sometimes also referred to as tones, at which no energy should be transmitted, for example, to avoid interfering with other communication systems utilizing these frequencies.

[0019] The symbols with nulling are received by a pre-emphasis/image attenuation mask block 116. The mask block, which is generally implemented in circuitry, although a processor executing program instructions may be used in some embodiments, scales the frequency- domain symbols by a gain factor generally to achieve a desired power spectrum from the transmitter, hi most embodiments the mask block multiplies each sample of a frequency domain symbol, or selected ones of those samples, by the gain factor, which may be different for different samples, hi many embodiments the gain factor for each sample, corresponding to a frequency, is determined based at least in part on the passband frequency response of the notch filter, later discussed. [0020] In accordance with aspects of the invention, the mask also attenuates image frequencies of the frequencies at which no energy should be transmitted. The attenuation is generally in the range of 2 to 5 dB. The image frequencies are generally those frequencies for which in-phase and quadrature phase and/or amplitude mismatch causes signal components to be generated at the frequencies at which no energy should be transmitted. Usually the image frequencies are, for a baseband signal representation, symmetric about DC. For example, in a baseband representation, a frequency f_n generally has an image frequency -f_n. Similarly, for transmission frequencies about a carrier frequency f_c, frequency f_c + f_n generally has an image frequency of f_c-f_n. Accordingly, the pre-emphasis/image attenuation mask block also receives the signal indicating frequencies at which no energy should be transmitted, although in some embodiments the block instead receives a signal indicating image frequencies to attenuate.

[0021] The scaled symbols are received by an inverse fast Fourier transform (iFFT) block 117. The iFFT block transforms the frequency-domain symbols into time-domain samples. The transform may be, for example, a 128-point transform. The time-domain samples from the iFFT block are received by a null inserter 119. The null inserter adds zero- valued samples to a block of samples from the iFFT. For example, each block of 128 samples received by the null inserter may be appended with 37 zero-value samples. Alternatively, zero-value samples may be inserted ahead of the samples from the iFFT, or a combination of prefixing and appending may be used. The zero-valued samples provide, for example, a guard period between symbols, allowing for changes in frequency bands between symbols or to reduce inter-symbol interference.

[0022] Also shown in FIG. 1, a notch filter 121 receives the samples after null insertion. Null insertion generally creates energy at various frequencies, including previously nulled frequencies. The notch filter substantially removes signal energy from frequencies that correspond to frequencies at which no energy should be transmitted. The notch filter may be implemented, for example, as a finite impulse response (FIR) filter, with frequency shifters as appropriate to effectively implement the notch filter at selectable frequencies. In an embodiment a FIR with 31 taps may be used. Thus, as with the tone miller and pre- emphasis/image attenuator, the notch filter also receives a signal indicative of frequencies at which no power should be transmitted. Practical implementations of the notch filter may have ripples in the frequency response of the filter passband. In some embodiments, therefore, the pre-emphasis mask compensates for the ripples by scaling frequencies in a complementary manner.

[0023] The filtered samples are received by a digital-to-analog converter (DAC) 123. The DAC converts the samples into analog form. The analog signal is received by an upconverter 125. The upconverter modulates the analog signal to a radio frequency. The radio frequency signal from the upconverter is received by an amplifier 127. The amplifier increases the power of the radio frequency signal and then transmits the signal via radio waves.

[0024] Returning to discussion relating to the image attenuation mask, imperfections, such as mismatch between in-phase and quadrature paths (I/Q imbalance), may be present in the upconverter and result in intermodulation, which may image signal power from desired frequencies into the nulled and notch filtered frequency band.

[0025] The frequency of an image frequency may be translated in the various transmitter stages. I/Q intermodulation generally creates an image at a frequency that is mirrored about a carrier frequency. For example, for a desired notch at frequency f_c + /, , where f_c is carrier frequency, a signal at frequency -/, may create an image in the desired notch. The frequency of the desired notch will generally vary with the operating environment. It may depend on the regulations of a particular geographic region and on what other communication services are operating at a particular time. The image frequency to be attenuated will generally be scaled by null insertion by the ratio of OFDM symbol length before and after null insertion. The transformation between frequency-domain and time-domain representations provides identification of the frequency bins to be attenuated among the frequency-domain symbols. The full transformation of frequencies provides identification of which points in the image attenuation mask will reduce interference power in the radio frequency signal.

[0026] In addition to which frequencies to attenuate, the magnitude of attenuation may vary with the embodiment and with operating conditions. A desired notch depth may depend on local regulations, which may vary with notch frequency. The magnitude of an interfering image varies with the amount of I/Q imbalance present in an embodiment. The amount of IQ imbalance may be determined by measurement during operation. Alternatively, the amount of imbalance could be determined in a previous calibration or manufacturing step. Another alternative is to use a worst case value for imbalance based on a particular embodiment. A notch frequency may contain power from noise sources, such as quantization noise from digital-to-analog conversion. Mathematical combination of the preceding factors will produce a desired attenuation. By limiting attenuation to that required to achieve a desired notch depth, the attenuated frequency bin may still communicate useful information. [0027] After the image attenuation mask reduces the signal at imaged frequencies, signal power at interfering frequencies is generally sufficiently attenuated so that other systems may communicate successfully.

[0028] In many embodiments the transmitter of FIG. 1 is for an ultrawideband communication system, for example in accordance with the "MultiBand OFDM Physical Layer Specification", WiMedia alliance specification document, ver. 1.1 May 26, 2005, and/or "High Rate Ultra Wideband PHY and MAC Standard", ECMA-368. In most instances the signal indicative of frequencies at which no or little power should be transmitted is determined as part of a detect-and-avoid process, or by circuitry implementing part of such a process. For the detect function of detect and avoid, a receiver may analyze incoming signals to identify possible victim systems to protect from interference and provide an indication to a transmitter of frequencies to avoid.

[0029] In various embodiments the detect function may be performed local to a transmitter, or local to a receiver receiving signals from the transmitter. Some UWB systems may perform detect analysis by circuitry associated with a transceiver of which the transmitter is part. Since transmitted signals are strongest at the point of transmission, this method is advantageous in detecting possible victim services that are near the transmitter and more susceptible to interference. Other UWB systems may detect frequencies to avoid at a receiver remote from the transmitter, with the remote receiver communicating detection results to the transmitter. This approach may be advantageous to increase the spatial scope of the detect analysis and may reduce required signal sensitivity.

[0030] The detect function generally involves analysis of a received signal. Detect may be performed by scanning the available frequency band, detecting signal power at each frequency bin, and comparing that power to a noise floor, hi an orthogonal frequency division multiplexed receiver, spectral analysis may advantageously exploit Fourier transform capability present for decoding received communication data. A detect function may include additional analysis, for example, decoding of preamble packets, of a signal at a frequency with detected power to identify its source. By identifying the type of signal, a UWB system may refine detection to discriminate victims that should be protected from other sources of signal power.

[0031] In many embodiments a transmitter in accordance with aspects of the invention transmits OFDM symbols with nulls applied to the symbols. FIG. 2 is timing diagram of OFDM symbols with null post-fix. Addition of a null guard period to a symbol may facilitate decoding the symbols in a receiver. Each set of time-domain samples from the iFFT block is followed in time by a set of zero-valued samples. FIG. 2 shows a first OFDM symbol 211, a second OFDM symbol 213, and a third OFDM symbol 215, for an example case of Fourier transform length is 128 and post- fix length of 37. As shown in FIG. 2, an OFDM symbols begins, in time, with a set of 128 signal samples followed by 37 null samples. As shown in FIG. 2, the null samples of an OFDM symbols are followed by the signal samples of a subsequent symbol. An alternative embodiment may insert some, or all, of the null samples ahead of the signal samples.

[0032] Many signals in a frequency division multiplexed communication system are represented with complex arithmetic, that is, a signal includes an in-phase component and a quadrature component. Mismatch between the two components is a common impairment in digital communication systems. The mismatch may include both a difference in gain and a rotation in phase. A desired complex signal can be represented as x(t) = x, (t) + jx_Q (t) , where x, (t) is the in-phase component and x_Q (t) is the quadrature component.

[0033] FIG. 3 shows the effect of the I/Q mismatch on the signal represented in the complex plane. In-phase components are represented in FIG. 3 by a horizontal axis with values increasing to the right. FIG. 3 represents quadrature components by a vertical axis with values increasing in the up direction. A vector 311 in FIG. 3 shows the effect of I/Q imbalance on quadrature signal components. In the absence of I/Q imbalance, the vector would align with the vertical axis and have unity length. A gain mismatch of ε , which has value one in the absence of mismatch, has the effect of scaling the quadrature component from its proper magnitude. The vector of FIG. 3 is of length ε to show to show I/Q gain mismatch. A phase mismatch of φ , which is zero in the absence of mismatch, has the effect of rotating the quadrature component away from alignment with the imaginary axis. The vector of FIG. 3 is rotated from the vertical axis by angle φ to show I/Q phase mismatch, hi effect, the signal x(t) has been replaced with x_[QM (t) = x, (t) - εsm{φ)x_Q (t) + jεcos{φ)x_Q (t) .

[0034] When the complex signal x(t) is upconverted, as in the upconverter 125 of FIG. 1, a radio frequency signal y(t) = x_I(t)cos(ω_ct) - x_Q(t)cos(ω_ct) is produced, where ω_c is the carrier frequency. In the presence of I/Q mismatch, upconversion results in intermodulation between in-phase and quadrature signal components. In particular, a tone desired at one frequency will have an image across from the carrier frequency after upconversion. For a carrier frequency of f_c and a baseband tone at frequency - /, , I/Q intermodulation creates an image tone at frequency f_c + /J in addition to the desired tone at frequency /_c - /, . FIG.

4 is a spectral diagram showing the effect of I/Q intermodulation on the spectrum of a signal tone. As may be seen in FIG. 4, a signal with a tone at frequency produces a potentially interfering image tone. This may be seen in FIG. 4 as a signal 411 tone at frequency f_c -/, and a tone 413 of reduced magnitude at frequency /_c +/, that is mirrored about the carrier frequency f_c.

[0035] If image tones caused by I/Q intermodulation fall at frequencies that are desired to be notched in a transmitter output, it may cause the notched frequencies to exceed required power level limits. For example, a phase mismatch of six degrees will create an image tone attenuated by 25 dB. A gain mismatch of 1 dB will produce an image tone of the same magnitude. By contrast, a regulatory standard may require a notch depth of -29 dB to protect other communication services. In such a situation, I/Q intermodulation may effectively fill, or provide a floor to, the notch to, or at, an unacceptable level. [0036] An example of the power spectral density from a transmitter in the presence of I/Q imbalance is shown in FIG. 5. A tone miller and notch filter, as described above, operated to reduce signal power around a frequency of f_c + f_λ . This may be seen in FIG. 5 by a notch

511 in signal power at the notched frequencies. However, I/Q intermodulation has partially filled the notched frequencies. This may be seen in FIG. 5 by the presence of still significant power present at the notched frequencies. For example, with 1 dB of I/Q imbalance, the notch depth is limited to approximately -25 dB. In many instances, this level of attenuation of interfering tones may be inadequate to avoid interfering with other communication systems. [0037] The notch depth at interfering frequencies can be increased by attenuating those frequencies which I/Q intermodulation images to the notch frequencies. For example, if image tones are attenuated by 4 dB, a notch depth that would be -25 dB will increase to -29 dB. This may be sufficient in many instances to avoid interfering with other communication systems. FIG. 6 shows an example of a resulting power spectral density. Tones at frequency fc ~ f_\ ^were attenuated before upconversion, which may be seen in FIG. 6 by the reduced signal power near frequency /_c - /, . The attenuation of image tunes reduces the power caused by I/Q intermodulation at the image frequency f_c + f_x . In FIG. 6, this is shown by an increased notch depth, as compared to the case without attenuation shown in FIG. 5. The image attenuation may be performed on frequency domain symbols as described previously for the transmitter shown in FIG. 1.

[0038] FIG. 7 is a flowchart of a process for reducing transmission of radio waves at frequencies that may interfere with other communication systems. In some embodiments, the process is performed by a transmitter, for example, the transmitter of FIG. 1. In some embodiments, a PHY, implemented as a standalone chip or in a chip with other components, performs the process. In block 711, the process receives indicia of frequencies at which it should avoid transmission. In some embodiments, a transmitter receives the indicia from a remote receiver, hi some embodiments a PHY receives the indicia of frequencies to avoid from the signal at a local antenna, hi many embodiments a MAC or other circuitry may determine frequencies to avoid, based in some embodiments on locally received signals, signals detected and processed by remote receivers, or some combination thereof, hi block 713, the process attenuates signals at image frequencies of the indicated frequencies to avoid. In some embodiments, image frequencies are calculated by arithmetically mirroring indicated frequencies to avoid about a carrier frequency, hi some embodiments, attenuation is performed by a frequency mask as shown, for example, in the transmitter of FIG. 1. In some embodiments, attenuation is performed by a frequency selective filter. The process thereafter returns.

[0039] FIG. 8 is a flowchart of a process for reducing transmission of interfering radio waves at undesired frequencies. In some embodiments the process is performed by a transmitter, for example, the transmitter of FIG. 1. In some embodiments the process is performed by a PHY chip of a transmission system. In block 711, tones that would interfere with other radio-frequency signals are nulled by the process, hi some embodiments, nulling is performed by setting frequency-domain representation of a signal to zero. In block 712, the process attenuates signals at frequencies that will be imaged to interfering frequencies by implementation non-idealities in subsequent transmission. Attenuation of the signal frequency, rather that complete nulling, allows transmission of information at the attenuated frequency. In some embodiments, attenuation is performed by a pre-emphasis mask as in the transmitter of FIG. 1. In other embodiments, attenuation is performed by a filter. In block 713, individual frequencies are scaled by the process to compensate for ripples in the frequency response of subsequent processing. In some embodiments, reducing ripples is performed by a pre-emphasis mask. In some embodiments, attenuating interfering images is combined with reducing ripples. In block 715, the process transforms signals from frequency-domain representation to time-domain representation. In some embodiments, transform to time domain is performed by an inverse fast Fourier transform as shown, for example, in FIG. 1. In some embodiments, the transform is performed with a split-radix algorithm. In other embodiments, the transform is performed by a prime- factor algorithm, hi block 717, zero-valued (null) samples are inserted by the process into the time-domain representation. In some embodiments null post- fix insertion is performed as shown in FIG. 2. hi block 719 the process filters time-domain samples to remove frequency components at undesired frequencies, which were caused by the null insertion. In some embodiments, filtering is performed by a band-stop FIR filter. In some embodiments, filtering is performed by a low-pass filter combined with frequency shifting, hi block 721, the filtered signal is transmitted by the process, hi some embodiments, transmission is performed by a DAC, upconverter, amplifier, and antenna as shown, for example, in the transmitter of FIG. 1. In some embodiments, transmission is performed by direct RF synthesis. The process thereafter returns.

[0040] The invention therefore provides for reduction of interference with other communication systems. Although the invention has been described with respect to certain embodiments, it should be recognized that the invention comprises the claims and their insubstantial variations supported by this disclosure.

Claims

CLAMS:

1. A method of reducing interference with narrowband transmitters, comprising: receiving an indication of a frequency to avoid, the frequency to avoid being a frequency at which transmissions are not desired; attenuating signal components of a symbol at the frequency to avoid; attenuating signal components of the symbol at an image frequency, the image frequency symmetric to the frequency to avoid about a center frequency of a band of frequencies used for transmission of the symbol; and transmitting the symbol with attenuated signal components.

2. The method of claim 1 wherein the center frequency of the band of frequencies used for transmission of the symbol is fc, the frequency to avoid is fc + fh, and the image frequency is fc-fh.

3. The method of claim 1 wherein the attenuation of signal component of the symbol at the image frequency is attenuation by 2 to 5 dB.

4. The method of claim 1 wherein the attenuating signal components of the symbol at the image frequency comprises multiplying inputs to a inverse Fast Fourier Transform by a gain factor.

5. The method of claim 1 further comprising attenuating signal components of the symbol at a plurality of frequencies about the image frequency.

6. The method of claim 1 wherein attenuating signal components of the symbol at the frequency to avoid comprises notch filtering the signal components of the symbol at the frequency to avoid.

7. The method of claim 6 wherein attenuating signal components of the symbol at the frequency to avoid further comprises setting signal components at the frequency to avoid to zero.

8. The method of claim 1 wherein the symbol is an orthogonal frequency division multiplexing (OFDM) symbol including null samples.

9. The method of claim 8 wherein the null samples comprise a null postfix.

10. A method for use by an ultrawideband transmitter of reducing interference with other communication systems, the method comprising: receiving an indication of frequencies to avoid; nulling frequency domain signal components of symbols at the frequencies to avoid; attenuating frequency domain signal components of the symbols at image frequencies to the frequencies to avoid, the image frequencies being those frequencies at which signals in the transmitter cause noise at the frequencies to avoid due to I/Q mismatch of the transmitter; converting the symbols to time domain symbols; filtering the time domain symbols; and transmitting the time domain symbols.

11. The method of claim 10 wherein the symbols are orthogonal frequency division multiplexing (OFDM) symbols.

12. The method of claim 11 wherein the symbols include a null postfix.

13. The method of claim 12 wherein the attenuation is 2 to 5 dB.

14. A transmitter for a frequency division multiplexed communication, comprising: a tone nuller for receiving streams of frequency-domain symbols and nulling symbols at a notch frequency; a mask for attenuating symbols from the tone nuller which may interfere at the notch frequency; a transform block for converting frequency-domain symbols from the mask to time- domain samples; and an upconverter for modulating the time-domain samples for radio frequency transmission.

15. The transmitter for frequency division multiplexed communication of claim 14, wherein the notch frequency is selected based on a received indication of frequencies to avoid.

16. The transmitter for frequency division multiplexed communication of claim 14, further comprising a null inserter for adding guard samples to the time-domain samples.

17. The transmitter for frequency division multiplexed communication of claim 16, further comprising a filter for reducing power in the time-domain samples at the notch frequency.

18. The transmitter for frequency division multiplexed communication of claim 17, wherein the mask additionally compensates for ripple in the filter.

19. The transmitter for frequency division multiplexed communication of claim 18, the upconverter comprising a radio frequency modulator, and the transmitter further comprising a digital-to-analog converter, and an amplifier.

20. The transmitter for frequency division multiplexed communication of claim 19, wherein the source of the streams of frequency-domain symbols comprises a convolutional coder and quadrature amplitude modulator.