AU2011203042B2 - Managing spectra of modulated signals in a communication network - Google Patents

Abstract

A method for operating a communicating station in order to transmit information through a communication medium using a set of channels, wherein the channels have 5 respective carrier frequencies, the method comprising: determining a list of one or more of the carrier frequencies over which transmissions are to be attenuated; encoding information bits according to an error correction code in order to obtain coded bits, wherein the coded bits represent the received information bits with redundancy; 10 interleaving the coded bits to obtain interleaved bits; mapping blocks of the interleaved bits to corresponding sets of complex numbers, wherein each set of complex numbers includes one complex number for each of the carrier frequencies; modifying each set of complex numbers by attenuating one or more of the complex 15 numbers in the set that correspond to the one or more carrier frequencies on the list, wherein said attenuating includes scaling the one or more complex numbers so that their amplitudes are less than a predetermined amplitude level; computing an inverse Fourier transform of each modified set of complex numbers to obtain a sequence of time-domain samples; and 20 transmitting a time-domain signal over the communication medium based on the sequence of time-domain samples.

Description

Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT (ORIGINAL) Name of Applicant: Atheros Communications, Inc., of 5480 Great America Parkway, Santa Clara, California 95054, United States of America Actual Inventor: YONGE, Lawrence W. Address for Service: DAVIES COLLISON CAVE, Patent Attorneys, of I Nicholson Street, Melbourne 3000, Victoria, Australia Invention Title: Managing spectra of modulated signals in a communication network The following statement is a full description of this invention, including the best method of performing it known to us: C:\NRPortbl\DCC\DER\3707986_1.DOC - 21/6/11 C:RVoftbDCC DER3709293_jOOC2 kV6120 II -1 MANAGING SPECTRA OF MODULATED SIGNALS IN A COMMUNICATION NETWORK CROSS-REFERENCE TO RELATED APPLICATIONS 5 This application claims the benefit of U.S. Application Serial No. 60/702,717, filed on July 27, 2005, U.S. Application Serial No. 60/705,720, filed on August 2, 2005, and U.S. Application Serial No. 11/493,382, filed on July 26, 2006, each of which is incorporated herein by reference. 10 TECHNICAL FIELD The invention relates to managing spectra of modulated signals in a communication network. BACKGROUND 15 Various types of communication systems transmit signals that may radiate in a portion of the electromagnetic spectrum and cause interference with devices that operate in that portion of the electromagnetic spectrum (e.g., radio frequency (RP) spectral bands). In some cases regulatory requirements for certain geographical regions (e.g., imposed by governments) place constraints on power that may be radiated in certain spectral regions, 20 such as amateur radio bands. Some systems are wireless systems that communicate between stations using radio waves modulated with information. Other systems are wired systems that communicate using signals transmitted over a wired medium, but the wired medium may radiate enough power in restricted spectral bands to potentially cause interference. 25 Communication stations can be configured to avoid using or limit the amount of power that is radiated in certain restricted spectral bands. Alternatively, communication stations can be configured to adjust the spectral regions used for communication, based on whether the station is operating in an environment in which interference may occur. For example, orthogonal frequency division multiplexing (OFDM), also known as Discrete 30 Multi Tone (DMT), is a spread spectrum signal modulation technique in which the available bandwidth is subdivided into a number of narrowband, low data rate channels or C:\NRfnbl\CC\DERU08293_1.DOC-21fM1 1 -2 "carriers." To obtain high spectral efficiency, the spectra of the carriers are overlapping and orthogonal to each other. Data are transmitted in the form of symbols that have a predetermined duration and encompass some number of carriers. The data transmitted on these carriers can be modulated in amplitude and/or phase, using modulation schemes such 5 as Binary Phase Shift Key (BPSK), Quadrature Phase Shift Key (QPSK), or m-bit Quadrature Amplitude Modulation (m-QAM). An example of a system in which carriers can be disabled to avoid potential interference is described in more detail in U.S. Patent No. 6,278,685, incorporated herein by reference. In this system, after one or more carriers are disabled, the modulation functions (e.g., an interleaver shift mechanism) are adjusted 10 for a different number of usable carriers. SUMMARY According to the present invention there is provided a method for operating a communicating station in order to transmit information through a communication medium 15 using a set of channels, wherein the channels have respective carrier frequencies, the method comprising: determining a list of one or more of the carrier frequencies over which transmissions are to be attenuated; encoding information bits according to an error correction code in order to obtain 20 coded bits, wherein the coded bits represent the received information bits with redundancy; interleaving the coded bits to obtain interleaved bits; mapping blocks of the interleaved bits to corresponding sets of complex numbers, wherein each set of complex numbers includes one complex number for each of the carrier frequencies; 25 modifying each set of complex numbers by attenuating one or more of the complex numbers in the set that correspond to the one or more carrier frequencies on the list, wherein said attenuating includes scaling the one or more complex numbers so that their amplitudes are less than a predetermined amplitude level; computing an inverse Fourier transform of each modified set of complex numbers 30 to obtain a sequence of time-domain samples; and transmitting a time-domain signal over the communication medium based on the CANRonbl\DCCDER\37I293. DOC -2 I62 11 -3 sequence of time-domain samples. The invention also provides a communicating station for transmitting information through a communication medium using a set of channels, wherein the channels have respective carrier frequencies, the system comprising: 5 memory for storing a list of one or more of the carrier frequencies over which transmissions are to be attenuated; an encoding unit configured to encode information bits according to an error correction code in order to obtain coded bits, wherein the coded bits represent the received information bits with redundancy; 10 an interleaver configured to interleave the coded bits to obtain interleaved bits; a modulation module configured to: map blocks of the interleaved bits to corresponding sets of complex numbers, wherein each set of complex numbers includes one complex number for each of the carrier frequencies; 15 modify each set of complex numbers by attenuating one or more of the complex numbers in the set that correspond to the one or more carrier frequencies on the list, wherein said attenuating includes scaling the one or more complex numbers so that their amplitudes are less than a predetermined amplitude level; and compute an inverse Fourier transform of each modified set of complex 20 numbers to obtain a sequence of time-domain samples. Among the many advantages of the invention (some of which may be achieved only in some of its various aspects and implementations) are the following. The amplitude mask technique can be used to preserve interoperability between a user's local network (e.g., a home powerline network of devices such as computer, Ethernet 25 bridge, TV, DVR, etc.) and an access network of a service provider, for example. The service provider may need to limit power radiated in a given spectral band due a constraint such as a prohibition from interfering with a licensed entity. The Federal Communications Commission (FCC) may require that the service provider be able to have a way to stop transmitting power in a given spectral band if they interfere with a licensed entity such as 30 an amateur radio device or a radio station, for example. The amplitude mask technique enables the service provider to adjust the transmitted spectrum while preserving C:\NRhotblDCCDER\370B2931.DOC-21i6/201 I -4 communication without the need to negotiate a change in modulation scheme with receiving stations. For example, if a service provider is already communicating with a user's device using a given set of carriers, and the service provider needs to turn off one or more of the 5 carriers, the amplitude mask technique enables the service provider to stop radiating power on an interfering carrier while still using that carrier in a modulation scheme agreed upon with the user station. Since the amplitude mask changes the amplitude of selected carriers but does not eliminate those carriers from the modulation scheme, the amplitude mask technique avoids the communication overhead of updating modulation parameters (e.g., 10 the tone mask) before adjusting the transmitted spectrum. Other features and advantages of the invention will be found in the detailed description, drawings, and claims. DESCRIPTION OF DRAWINGS 15 FIG. 1 is a schematic diagram of a network configuration. FIG. 2 is a block diagram of a communication system. FIG. 3 is a block diagram of an encoder module. FIG. 4 is a block diagram of a modulation module.

C:\R bDCC\DfERU708293_1.DOC-2|f6/20 I1 -5 DETAILED DESCRIPTION There are a great many possible implementations of the invention, too many to describe herein. Some possible implementations that are presently preferred are described 5 below. It cannot be emphasized too strongly, however, that these are descriptions of implementations of the invention, and not descriptions of the invention, which is not limited to the detailed implementations described in this section but is described in broader terms in the claims. As shown in FIG. 1, a network configuration 100 provides a shared communication 10 medium 110 for a number of communication stations 102A - 102E (e.g., computing devices, or audiovisual devices) to communicate with each other. The communication medium 110 can include one or more types of physical communication media such as coaxial cable, unshielded twisted pair, power lines, or wireless channels for example. The network configuration 100 can also include devices such as bridges or repeaters. The 15 communication stations 102A - 102E communicate with each other using predetermined physical (PHY) layer and medium access control (MAC) layer communication protocols used by network interface modules 106. The MAC layer is a sub-layer of the data link layer and provides an interface to the PHY layer, according to the Open Systems Interconnection (OSI) network architecture standard, for example. The network 20 configuration 100 can have any of a variety of network topologies (e.g., bus, tree, star, mesh).

The stations use an amplitude mask technique, described in more detail below, for managing the spectra of modulated signals without needing to exchange information among stations indicating which carriers are in use or disabled. The amplitude mask technique is used with a redundant coding scheme that spreads data 5 over multiple carriers so that the station can control the spectrum of modulated signals with a high likelihood that the modulated data can be recovered using redundant information. In some implementations, the network interface modules 106 use protocols that include features to improve performance when the network configuration 100 includes 10 a communication medium 110 that exhibits varying transmission characteristics. For example, the communication medium 110 may include AC power lines in a house, optionally coupled to other media (e.g., coaxial cable lines). Power-line communication systems use existing AC wiring to exchange information. Owing to their being designed for much lower frequency transmissions, 15 AC wiring provides varying channel characteristics at the higher frequencies used for data transmission (e.g., depending on the wiring used and the actual layout). To increase the data rate between various links, stations adjust their transmission parameters dynamically. This process is called channel adaptation. Channel adaptation results in adaptation information specifying a set of transmission parameters that can be 20 used on each link. Adaptation information includes such parameters as the frequencies used, their modulation, and the forward error correction (FEC) used. The communication channel between any two stations provided by the communication medium 110 may exhibit varying channel characteristics such as periodic variation in noise characteristics and frequency response. To improve 25 performance and QoS stability in the presence of varying channel characteristics, the stations can synchronize channel adaptation with the frequency of the AC line (e.g., 50 or 60 Hz). There are typically variations in the phase and frequency of the AC line cycle from the power generating plant and local noise and load changes. This synchronization enables the stations to use consistent channel adaptation optimized for 30 a particular phase region of the AC line cycle. An example of such synchronization is described in U.S. Patent Application No. 11/337,946, incorporated herein by reference. Another aspect of mitigating potential impairments caused by the varying channel characteristics involves using a robust signal modulation format such as 6 OFDM. An exemplary communication system that uses OFDM modulation is described below. Any of a variety of communication system architectures can be used to implement the portion of the network interface module 106 that converts data to and 5 from a signal waveform that is transmitted over the communication medium. An application running on a station provides and receives data to and from the network interface module 106 in segments. A "MAC Protocol Data Unit" (MPDU) is a segment of information including overhead and payload fields that the MAC layer has asked the PHY layer to transport. An MPDU can have any of a variety of formats based on the 10 type of data being transmitted. A "PHY Protocol Data Unit (PPDU)" refers to the modulated signal waveform representing an MPDU that is transmitted over the power line. In OFDM modulation, data are transmitted in the form of OFDM "symbols." Each symbol has a predetermined time duration or symbol time T. Each symbol is 15 generated from a superposition of N sinusoidal carrier waveforms that are orthogonal to each other and form the OFDM carriers. Each carrier has a peak frequencyfi and a phase ti measured from the beginning of the symbol. For each of these mutually orthogonal carriers, a whole number of periods of the sinusoidal waveform is contained within the symbol time T. Equivalently, each carrier frequency is an integral multiple 20 of a frequency interval Af = I/T,. The phases 4i and amplitudes Ai of the carrier waveforms can be independently selected (according to an appropriate modulation scheme) without affecting the orthogonality of the resulting modulated waveforms. The carriers occupy a frequency range between frequenciesfi andfN referred to as the OFDM bandwidth. 25 Referring to FIG. 2, a communication system 200 includes a transmitter 202 for transmitting a signal (e.g., a sequence of OFDM symbols) over a communication medium 204 to a receiver 206. The transmitter 202 and receiver 206 can both be incorporated into a network interface module 106 at each station. The communication medium 204 represents a path from one station to another over the communication 30 medium 110 of the network configuration 100. At the transmitter 202, modules implementing the PHY layer receive an MPDU from the MAC layer. The MPDU is sent to an encoder module 220 to perform processing such as scrambling, error correction coding and interleaving. Referring to 7 FIG. 3, an exemplary encoder module 220 includes a scrambler 300, a Turbo encoder 302, and an interleaver 304. The scrambler 300 gives the information represented by the MPDU a more random distribution (e.g., to reduce the probability of long strings of zeros or ones). In 5 some implementations, the data is "XOR-ed" with a repeating Pseudo Noise (PN) sequence using a generator polynomial such as: S(x)=x" +x, +1 The state bits in the scrambler 300 are initialized to a predetermined sequence (e.g., all ones) at the start of processing an MPDU. 10 Scrambled information bits from the scrambler 300 can be encoded by an encoder that uses any of a variety of coding techniques (e.g., convolutional codes). The encoder can generate a stream of data bits and in some cases auxiliary information such as one or more streams of parity bits. In this example, the Turbo encoder 302 uses a Turbo code to generate, for each block of m input information bits, a block of m "data 15 bits" (d) that represent the input information, a first block of n/2 "parity bits" (p) corresponding to the information bits, and a second block of n/2 parity bits (q) corresponding to a known permutation of the information bits. Together, the data bits and the parity bits provide redundant information that can be used to correct potential errors. This scheme yields a code with a rate of m/(m + n). 20 The interleaver 304 interleaves the bits received from the Turbo encoder 302. The interleaving can be performed, for example on blocks corresponding to predetermined portions of an MPDU. The interleaving ensures that the redundant data and parity bits for a given block of information are distributed in frequency (e.g., on different carriers) and in time (e.g., on different symbols) to provide the ability to 25 correct errors that occur due to localized signal interference (e.g., localized in time and/or frequency). The signal interference maybe due to a jammer or maybe due to spectral shaping of the spectral shaping module 400 described below. The interleaving can ensure that the redundant information for a given portion of the MPDU is modulated onto carriers that are evenly distributed over the OFDM bandwidth so that 30 limited bandwidth interference is not likely to corrupt all of the carriers. The interleaving can also ensure that the redundant information is modulated onto more than one symbol so that broadband but short duration interference is not likely to corrupt all of the symbols. 8 The encoder module 220 includes a buffer that can be used to temporarily store data and parity bits from the Turbo encoder 302, to be read out by the interleaver 304 in a different order than the order in which they were stored. For example, a buffer can include includes k "data sub-banks" of m/k bits each and k "parity sub-banks" of n/k 5 bits each (e.g., the sub-banks can correspond to logical regions of memory). In the case of k= 4, the data bits are divided into four equal sub-blocks of m/4 bits, and the parity bits are divided into 4 equal sub-blocks of n/4 bits (where both m and n are selected to be divisible by 4). The Turbo encoder 302 writes the first m/4 data bits (in natural order) to the first data sub-bank, the next m/4 data bits to the second data sub-bank, and 10 so on. The Turbo encoder 302 writes the first n/4 parity bits (in natural order) to the first parity sub-bank, the next n/4 parity bits to the second parity sub-bank, and so on. The interleaver 304 generates a stream of bits to be modulated onto carriers of data symbols by reading from the sub-banks in a predetermined order. For example, the four data sub-banks of length m/4 may be thought of as a matrix consisting of m/4 15 rows and four columns, with column 0 representing the first sub-bank, column 1 representing the second sub-bank, and so on. Groups of four bits on the same row (one bit from each sub-block) are read out from the matrix at a time, starting with row 0. After a row has been read out, a row pointer is incremented by StepSize before performing the next row read. After m/4/StepSize row reads, the end of the matrix has 20 been reached. The process is then repeated for different rows until all bits from the matrix have been read out. The parity bits can be interleaved in a similar manner. In some implementations, the data bits and the parity bits can also interleaved with each other in a predetermined manner. In some modes of communication, called ROBO modes, the interleaver 304 25 performs additional processing to generate increased redundancy in the output data stream. For example, ROBO mode can introduce further redundancy by reading each sub-bank location multiple times at different cyclic shifts to represent each encoded bit by Multiple bits at the output of the interleaver 304. Other types of encoders and/or interleavers can be used that also provide 30 redundancy to enable each portion of an MPDU to be recovered from fewer than all of the modulated carriers or fewer than all of the modulated symbols. Referring again to FIG. 2, the encoded data is fed into a mapping module 222 that takes groups of data bits (e.g., 1, 2, 3, 4, 6, 8, or 10 bits), depending on the 9 constellation used for the current symbol (e.g., a BPSK, QPSK, 8-QAM, 16-QAM constellation), and maps the data value represented by those bits onto the corresponding amplitudes of in-phase (I) and quadrature-phase (Q) components of a carrier waveform of the current symbol. This results in each data value being associated with a 5 corresponding complex number Ci= Ai exp(jli) whose real part corresponds to the I component and whose imaginary part corresponds to the Q component of a carrier with peak frequencyfi. Alternatively, any appropriate modulation scheme that associates data values to modulated carrier waveforms can be used. The mapping module 222 also determines which of the carrier frequenciesfi,..., 10 fN (or "tones") within the OFDM bandwidth are used by the system 200 to transmit information according to a "tone mask." For example, some carriers that are likely to interfere with licensed entities in a particular region (e.g., North America) can be avoided, and no power is radiated on those carriers. Devices sold in a given region can be programmed to use a tone mask configured for that region. The mapping module 15 222 also determines the type of modulation to be used on each of the carriers in the tone mask according to a "tone map." The tone map can be a default tone map (e.g., for redundant broadcast communication among multiple stations), or a customized tone map determined by a receiving station that has been adapted to characteristics of the communication medium 204 (e.g., for more efficient unicast communication between 20 two stations). If a station determines (e.g., during channel adaptation) that a carrier in the tone mask is not suitable for use (e.g., due to fading or noise) the tone map can specify that the carrier is not to be used to modulate data, but instead can use pseudorandom noise for that carrier (e.g., coherent BPSK modulated with a binary value from a Pseudo Noise (PN) sequence). For two stations to communicate, they 25 should use the same tone mask and tone map, or at least know what tone mask and tone map the other device is using so that the signals can be demodulated properly. A modulation module 224 performs the modulation of the resulting set of N complex numbers (some of which may be zero for unused carriers) determined by the mapping module 222 onto N orthogonal carrier waveforms having peak frequencies 30 fi,...,fN. The modulation module 224 performs an inverse discrete Fourier transform (IDFT) to form a discrete time symbol waveform S(n) (for a sampling ratefa), which can be written as N 10 S(n) = E AiexpUj(21in/N+1|bi)] Eq.(1) i= 1 where the time index n goes from 1 to N, Ai is the amplitude and 4i is the phase of the 5 carrier with peak frequencyfi =(i/N)fR, andj = *1. In some implementations, the discrete Fourier transform corresponds to a fast Fourier transform (FFT) in which N is a power of 2. A post-processing module 226 combines a sequence of consecutive (potentially overlapping) symbols into a "symbol set" that can be transmitted as a continuous block 10 over the communication medium 204. The post-processing module 226 prepends a preamble to the symbol set that can be used for automatic gain control (AGC) and symbol timing synchronization. To mitigate intersymbol and intercarrier interference (e.g., due to imperfections in the system 200 and/or the communication medium 204) the post-processing module 226 can extend each symbol with a cyclic prefix that is a 15 copy of the last part of the symbol. The post-processing module 226 can also perform other functions such as applying a pulse shaping window to subsets of symbols within the symbol set (e.g., using a raised cosine window or other type of pulse shaping window) and overlapping the symbol subsets. The modulation module 224 or the post-processing module 226 can include a 20 spectral shaping module that further modifies the spectrum of a signal that includes modulated symbols according to an "amplitude mask." While the tone mask can be changed by exchanging messages among stations in a network, the amplitude mask enables a station to attenuate power transmitted on certain carriers without needing to exchange messages among the stations. Thus, the spectral shaping module enables 25 dynamic spectral shaping in response to dynamic spectral constraints by changing the amplitude of carriers that may cause interference. In some cases, the spectral shaping module sets the amplitude of the frequency component below a predetermined limit in response to an event such as detecting a transmission from a licensed entity. Referring to FIG. 4, an exemplary implementation of the modulation module 30 224 includes a spectral shaping module 400 coupled to an IDFT module 402. The spectral shaping module 400 modifies the amplitude Ai for the carriers that are to be attenuated, providing an attenuated amplitude A' to the IDFT module 402. The value of the phase and Pi for the attenuated carriers can be passed through the spectral shaping 11 module 400 without modification. Thus, in this example, the IDFT module 402 performs a discrete Fourier transform that includes the attenuated carrier frequencies. The amplitude mask specifies an attenuation factor a for the amplitude A'i= cxAi according to the amount by which the power is to be attenuated (e.g., 2 dB in 5 amplitude for each 1 dB in power). The amplitude A' is set below a predetermined amplitude that is normally used for modulating the information (e.g., according to a predetermined constellation) such that the resulting radiated power does not interfere with other devices. The amplitude mask entry may also indicate that a carrier is to be nulled completely with the corresponding amplitude set to zero. The attenuated carriers 10 are still processed by the receiving station even if they are transmitted with zero amplitude so that the modulation and encoding scheme is preserved. Generally, for two stations to communicate, they don't necessarily need to know what amplitude mask the other station is using (or whether the station is using an amplitude mask at all). Even though no modification of the modulation scheme 15 between a transmitter and a receiver is necessary to partially attenuate or fully attenuate (i.e., turn off) a carrier using the amplitude mask, in some cases, when a receiving station updates a tone map (which determines how carriers within the tone mask are to be. modulated) the receiving station will detect a very poor signal-to-noise ratio on the attenuated carriers and may exclude them from the updated tone map (indicating that 20 those carriers are not to be used for modulating data). In alternative implementations, the spectral shaping module can be included in the post-processing module 226, for example, as a programmable notch filter that reduces the amplitude of one or more narrow frequency bands in the signal. An Analog Front End (AFE) module 228 couples an analog signal containing a 25 continuous-time (e.g., low-pass filtered) version of the symbol set to the communication medium 204. The effect of the transmission of the continuous-time version of the waveform SQ) over the communication medium 204 can be represented by convolution with a function g(rt) representing an impulse response of transmission over the communication medium. The communication medium 204 may add noise 30 n(t), which may be random noise and/or narrowband noise emitted by ajammer. At the receiver 206, modules implementing the PHY layer receive a signal from the communication medium 204 and generate an MPDU for the MAC layer. An AFE module 230 operates in conjunction with an Automatic Gain Control (AGC) module 12 C :\NRonblDCDEu?8293_I.DOC-21AWI I - 13 232 and a time synchronization module 234 to provide sampled signal data and timing information to a discrete Fourier transform (DFT) module 236. After removing the cyclic prefix, the receiver 206 feeds the sampled discrete- time symbols into DFT module 236 to extract the sequence of N complex numbers representing 5 the encoded data values (by performing an N-point DFT). Demodulator/Decoder module 238 maps the complex numbers onto the corresponding bit sequences and performs the appropriate decoding of the bits (including deinterleaving, error correction, and descrambling). The data that was modulated onto carriers that were subsequently attenuated by the spectral shaping module 400 can be 10 recovered due to the redundancy in the encoding scheme. Any of the modules of the communication system 200 including modules in the transmitter 202 or receiver 206 can be implemented in hardware, software, or a combination of hardware and software. Many other implementations of the invention other than those described above are 15 within the invention, which is defined by the following claims. The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general 20 knowledge in the field of endeavour to which this specification relates. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or 25 steps.

Claims (21)

1. A method for operating a communicating station in order to transmit information through a communication medium using a set of channels, wherein the channels have respective carrier frequencies, the method comprising: determining a list of one or more of the carrier frequencies over which transmissions are to be attenuated; encoding information bits according to an error correction code in order to obtain coded bits, wherein the coded bits represent the received information bits with redundancy; interleaving the coded bits to obtain interleaved bits; mapping blocks of the interleaved bits to corresponding sets of complex numbers, wherein each set of complex numbers includes one complex number for each of the carrier frequencies; modifying each set of complex numbers by attenuating one or more of the complex numbers in the set that correspond to the one or more carrier frequencies on the list, wherein said attenuating includes scaling the one or more complex numbers so that their amplitudes are less than a predetermined amplitude level; computing an inverse Fourier transform of each modified set of complex numbers to obtain a sequence of time-domain samples; and transmitting a time-domain signal over the communication medium based on the sequence of time-domain samples.

2. The method of claim 1, wherein the one or more carrier frequencies on the list are determined without negotiating with any other communicating station over the communication medium. C \NRPonbIDCC\MKA\413X72_ I DOC.X02/21)12 - 15

3. A method of as in any one of the preceding claims, wherein the one or more carrier frequencies on the list are determined without exchanging messages with any other communicating station over the communication medium.

4. A method of as in any one of the preceding claims, wherein the list is not provided to a receiving station that receives and demodulates the time-domain signal.

5. A method of as in any one of the preceding claims, wherein the predetermined amplitude level is zero.

6. A method of as in any one of the preceding claims, wherein the predetermined amplitude level is an amplitude level that based on a regulatory limit for radiated power.

7. A method of as in any one of the preceding claims, wherein the predetermined amplitude level is a level that ensures non-interference with one or more other communication stations.

8. A method of as in any one of the preceding claims, wherein the said interleaving spreads coded bits across different blocks of the interleaved bits.

9. A method of as in any one of the preceding claims, wherein said interleaving spreads coded bits over the carrier frequencies.

10. A method of as in any one of the preceding claims, wherein said interleaving spreads coded bits uniformly over the carrier frequencies. CANRPonbl\DCC\MKA\411X71_ I DOC -12/2012 - 16

11. A method of as in any one of the preceding claims, further comprising detecting transmission from another communication station over the communication medium, wherein said determining the list includes adding one or more of the carrier frequencies to the list in response to detecting said transmission.

12. A method of as in any one of the preceding claims, wherein said mapping of blocks of the interleaved bits to corresponding sets of complex numbers includes applying a tone mask to avoid one or more frequencies specified by the tone mask, wherein one or more complex numbers in each set that correspond to the one or more specified frequencies are set to zero and are not determined by the interleaved bits, wherein the tone mask is determined by negotiation with another communication station over the communication medium.

13. A method of as in any one of the preceding claims, further comprising: scrambling the information bits prior said encoding the information bits.

14. A communicating station for transmitting information through a communication medium using a set of channels, wherein the channels have respective carrier frequencies, the system comprising: memory for storing a list of one or more of the carrier frequencies over which transmissions are to be attenuated; an encoding unit configured to encode information bits according to an error correction code in order to obtain coded bits, wherein the coded bits represent the received information bits with redundancy; an interleaver configured to interleave the coded bits to obtain interleaved bits; a modulation module configured to: C:NRPonb\DCC\MKA\J3K070_ DOC.R,2/2012 - 17 map blocks of the interleaved bits to corresponding sets of complex numbers, wherein each set of complex numbers includes one complex number for each of the carrier frequencies; modify each set of complex numbers by attenuating one or more of the complex numbers in the set that correspond to the one or more carrier frequencies on the list, wherein said attenuating includes scaling the one or more complex numbers so that their amplitudes are less than a predetermined amplitude level; and compute an inverse Fourier transform of each modified set of complex numbers to obtain a sequence of time-domain samples.

15. The communicating station of claim 14, wherein the transmitter is configured to determine the one or more carrier frequencies on the list without negotiating a change in modulation scheme with a receiving station over the communication medium.

16. The communicating station of claim 14, wherein the transmitter is configured to determine the one or more carrier frequencies on the list without exchanging messages with any other communicating station over the communication medium.

17. The communicating station of claim 14, wherein the list is not provided to a receiving station that receives and demodulates the time-domain signal.

18. The communicating station of claim 14, wherein the predetermined amplitude level is zero.

19. The communicating station of claim 14 further comprising: a front end configured to transmit a time-domain signal over the communication medium based on the sequence of time-domain samples. C.NRPonbl\DCC\MKA\4 I I070_ DOC-/02/201 2 - 18

20. A communication station implementing the method of any one of claims I to 13.

21. A method for operating a communicating station or a communicating station, substantially as hereinbefore described with reference to the accompanying drawings.