 one or more notches of forbidden frequencies, the method comprising: receiving a plurality of information bits
 mapping the plurality of information bits to a plurality of constellation symbols corresponding to a plurality of OFDM subcarriers in the OFDM symbol, the plurality of constellation symbols including:(i) a first set of constellation symbols corresponding to a first set of OFDM subcarriers in the OFDM symbol, wherein the first set of OFDM subcarriers includes i) one or more OFDM subcarriers in the one or more notches, and ii) one or more OFDM subcarriers adjacent to the one or more notches where the one or more OFDM subcarriers adjacent to the one or more notches include at least some of the plurality of information bits, and(ii) a second set of constellation symbols corresponding to a second set of OFDM subcarriers in the OFDM symbol, wherein the second set of OFDM subcarriers are further in frequency from the one or more notches than the first set of OFDM subcarriers
 generating a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first set of constellation symbols
 generating a second timedomain signal by performing an IDFT based on the second set of constellation symbols
 modifying the first timedomain signal by applying a first windowing function corresponding to a first window transition width β1
 modifying the second timedomain signal by applying a second windowing function having a second window transition width β2, wherein the first window transition width β1 is different than the second window transition width β2 such that β1≠β2
 setting the first window transition width β1 to be longer than a guard interval width N_{GI }such that β1>N_{GI}
 setting the second window transition width β2 to be shorter than the guard interval width N_{GI }such that β2<
 N_{GI}
 and generating an output signal to include a sum of the first timedomain signal and the second timedomain signal, the output signal corresponding to the OFDM symbol.
Multiple transmission windows for OFDM symbol
Updated Time 12 June 2019
Patent Registration DataPublication Number
US10002995
Application Number
US14/481649
Application Date
09 September 2014
Publication Date
19 June 2018
Current Assignee
MARVELL WORLD TRADE LTD.
Original Assignee (Applicant)
MARVELL WORLD TRADE LTD.
International Classification
H01L33/60,H04L27/26,H01L33/10,H04L25/03,H01L25/075
Cooperative Classification
H01L33/60,H01L25/0753,H01L33/10,H04L25/03828,H04L27/2602
Inventor
IRANZO MOLINERO, SALVADOR
Patent Images
This patent contains figures and images illustrating the invention and its embodiment.
Abstract
A plurality of information bits are mapped to: (i) a first plurality of constellation symbols corresponding to a first plurality of OFDM subcarriers in an OFDM symbol, and (ii) a second plurality of constellation symbols corresponding to a second plurality of OFDM subcarriers in the OFDM symbol. A first timedomain signal is generated by performing an IDFT based on the first plurality of constellation symbols. A second timedomain signal is generated by performing an IDFT based on the second plurality of constellation symbols. The first timedomain signal is modified by applying a first windowing function corresponding to a first transition width β1. The second timedomain signal by applying a second windowing function having a second transition width β2. An output signal is generated to include a sum of the first timedomain signal and the second timedomain signal, the output signal corresponding to the OFDM symbol.
Claims
1. A method of generating an orthogonal frequency division multiplexing (OFDM) symbol for transmission in a communication channel containing one or more notches of forbidden frequencies, the method comprising: receiving a plurality of information bits;mapping the plurality of information bits to a plurality of constellation symbols corresponding to a plurality of OFDM subcarriers in the OFDM symbol, the plurality of constellation symbols including:(i) a first set of constellation symbols corresponding to a first set of OFDM subcarriers in the OFDM symbol, wherein the first set of OFDM subcarriers includes i) one or more OFDM subcarriers in the one or more notches, and ii) one or more OFDM subcarriers adjacent to the one or more notches where the one or more OFDM subcarriers adjacent to the one or more notches include at least some of the plurality of information bits, and(ii) a second set of constellation symbols corresponding to a second set of OFDM subcarriers in the OFDM symbol, wherein the second set of OFDM subcarriers are further in frequency from the one or more notches than the first set of OFDM subcarriers; generating a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first set of constellation symbols; generating a second timedomain signal by performing an IDFT based on the second set of constellation symbols; modifying the first timedomain signal by applying a first windowing function corresponding to a first window transition width β1; modifying the second timedomain signal by applying a second windowing function having a second window transition width β2, wherein the first window transition width β1 is different than the second window transition width β2 such that β1≠β2; setting the first window transition width β1 to be longer than a guard interval width N_{GI }such that β1>N_{GI}; setting the second window transition width β2 to be shorter than the guard interval width N_{GI }such that β2<N_{GI}; and generating an output signal to include a sum of the first timedomain signal and the second timedomain signal, the output signal corresponding to the OFDM symbol.
2. The method of claim 1, further comprising: modifying the first timedomain signal to include a first cyclic prefix having a first width N1_{CP}; modifying the second timedomain signal to include a second cyclic prefix having a width N2_{CP}; and setting the first window width β1, the second window width β2, first width N1_{CP}, and the second width N2_{CP }where the first and second timedomain signals each have a common OFDM symbol duration.
3. The method of claim 2, further comprising: generating a first plurality of OFDM symbols according to initial values of β1, N1CP, β2, N2_{CP}; after generating the first plurality of OFDM symbols, changing one or more of the values of β1, N1_{CP}, β2, N2_{CP}; and after changing one or more of the values of β1, N1_{CP}, β2, N2_{CP}, generating a second plurality of OFDM symbols according to one or more changed values of β1, N1_{CP}, β2, N2_{CP}.
4. The method of claim 3, wherein changing one or more of the values of β1, N1_{CP}, β2, N2_{CP }is performed as part of an optimization procedure to, at least one of: maximize data throughput to be greater than a predetermined throughput value; minimize power spectral density in the notch to be less than a predetermined spectral density value; minimize intersymbol interference (ISI) to be lower than a predetermined amount; and/or minimize errors due to channel delayspread.
5. An apparatus, comprising:one or more integrated circuit devices configured to: receive a plurality of information bits,map the plurality of information bits to a plurality of constellation symbols corresponding to a plurality of orthogonal frequency division multiplexing (OFDM) subcarriers in an OFDM symbol for transmission in a communication channel containing one or more notches of forbidden frequencies, the plurality of constellation symbols including:(i) a first set of constellation symbols corresponding to a first set of OFDM subcarriers in the OFDM symbol, wherein the first set of OFDM subcarriers includes i) one or more OFDM subcarriers in the one or more notches, and ii) one or more OFDM subcarriers adjacent to the one or more notches where the one or more OFDM subcarriers adjacent to the one or more notches include at least some of the plurality of information bits, and(ii) a second set of constellation symbols corresponding to a second set of OFDM subcarriers in the OFDM symbol, wherein the second set of OFDM subcarriers are further in frequency from the one or more notches than the first set of OFDM subcarriers, generate a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first set of constellation symbols, generate a second timedomain signal by performing an IDFT based on the second set of constellation symbols, modify the first timedomain signal by applying a first windowing function corresponding to a first window transition width β1, modify the second timedomain signal by applying a second windowing function having a second window transition width β2, wherein the first window transition width β1 is different than the second window transition width β2 such that β1≠β2, set the first window transition width β1 to be longer than a guard interval width N_{GI }such that β1>N_{GI}, set the second window transition width β2 to be shorter than the guard interval width N_{GI }such that β2<N_{GI}, and generate an output signal to include a sum of the first timedomain signal and the second timedomain signal, the output signal corresponding to the OFDM symbol.
6. The apparatus of claim 5, wherein the one or more integrated circuit devices are configured to: modify the first timedomain signal to include a first cyclic prefix having a first width N1_{CP}; modify the second timedomain signal to include a second cyclic prefix having a width N2_{CP}; and set the first window width β1, the second window width β2, first width N1_{CP}, and the second width N2_{CP }where the first and second timedomain signals each have a common symbol duration.
7. The apparatus of claim 6, wherein the one or more integrated circuit devices are configured to: generate a first plurality of OFDM symbols according to initial values of β1, N1_{CP}, β2, N2_{CP}; after generating the first plurality of OFDM symbols, change one or more of the values of β1, N1_{CP}, β2, N2_{CP}; and after changing one or more of the values of β1, N1_{CP}, β2, N2_{CP}, generate a second plurality of OFDM symbols according to one or more changed values of β1, N1_{CP}, β2, N2_{CP}.
8. The apparatus of claim 7, wherein the one or more integrated circuit devices are configured to change one or more of the values of β1, N1_{CP}, β2, N2_{CP }as part of an optimization procedure to, at least one of: maximize data throughput to be greater than a predetermined throughput value; minimize power spectral density in the notch to be less than a predetermined spectral density value; minimize intersymbol interference (ISI) to be lower than a predetermined amount; and/or minimize errors due to channel delayspread.
9. The apparatus of claim 6, wherein the one or more integrated circuit devices implement: a symbol mapping module configured to map information bits in the plurality of information bits to constellation symbols, a first serialtoparallel module coupled to the symbol mapping module, the first serialtoparallel module outputting the first plurality of constellation symbols, and a second serialtoparallel module coupled to the symbol mapping module, the second serialtoparallel module outputting the second plurality of constellation symbols.
10. The apparatus of claim 6, wherein the one or more integrated circuit devices implement: a first inverse discrete Fourier transform (IDFT) module configured to operate on the first plurality of constellation symbols, a first paralleltoserial module coupled to the first IDFT module, the first paralleltoserial module outputting the first timedomain signal, a second IDFT module configured to operate on the second plurality of constellation symbols, and a second paralleltoserial module coupled to the second IDFT module, the second paralleltoserial module outputting the second timedomain signal.
11. The apparatus of claim 6, further comprising a digitaltoanalog convert to convert the output signal to an analog signal.
12. A nontransitory computer readable storage medium having computer program instructions stored thereon that, when executed by one or more processors, cause the one or more processors to perform a method of generating an orthogonal frequency division multiplexing (OFDM) symbol for transmission in a communication channel containing one or more notches of forbidden frequencies, the method comprising: receiving a plurality of information bits;mapping the plurality of information bits to a plurality of constellation symbols corresponding to a plurality of OFDM subcarriers in the OFDM symbol, the plurality of constellation symbols including:(i) a first set of constellation symbols corresponding to a first set of OFDM subcarriers in the OFDM symbol, wherein the first set of OFDM subcarriers includes i) one or more OFDM subcarriers in the one or more notches, and ii) one or more OFDM subcarriers adjacent to the one or more notches where the one or more OFDM subcarriers adjacent to the one or more notches include at least some of the plurality of information bits, and(ii) a second set of constellation symbols corresponding to a second set of OFDM subcarriers in the OFDM symbol, wherein the second set of OFDM subcarriers are further in frequency from the one or more notches than the first set of OFDM subcarriers; generating a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first set of constellation symbols; generating a second timedomain signal by performing the IDFT based on the second set of constellation symbols; modifying the first timedomain signal by applying a first windowing function corresponding to a first transition having a first window transition width β1; modifying the second timedomain signal by applying a second windowing function corresponding to a second transition having a second window transition width β2, wherein the first window transition width β1 is different than the second window transition width β2 such that β1≠β2; setting the first window transition width β1 to be longer than a guard interval width N_{GI }such that β1>N_{GI}; setting the second window transition width β2 to be shorter than the guard interval width N_{GI }such that β2<N_{GI}; and adding the first timedomain signal and the second timedomain signal to generate an output signal corresponding to the OFDM symbol.
Claim Tree

11. A method of generating an orthogonal frequency division multiplexing (OFDM) symbol for transmission in a communication channel containing

2. The method of claim 1, further comprising:
 modifying the first timedomain signal to include a first cyclic prefix having a first width N1_{CP}
 modifying the second timedomain signal to include a second cyclic prefix having a width N2_{CP}
 and setting the first window width β1, the second window width β2, first width N1_{CP}, and the second width N2_{CP }where the first and second timedomain signals each have a common OFDM symbol duration.


55. An apparatus, comprising:
 one or more integrated circuit devices configured to: receive a plurality of information bits,map the plurality of information bits to a plurality of constellation symbols corresponding to a plurality of orthogonal frequency division multiplexing (OFDM) subcarriers in an OFDM symbol for transmission in a communication channel containing one or more notches of forbidden frequencies, the plurality of constellation symbols including:(i) a first set of constellation symbols corresponding to a first set of OFDM subcarriers in the OFDM symbol, wherein the first set of OFDM subcarriers includes i) one or more OFDM subcarriers in the one or more notches, and ii) one or more OFDM subcarriers adjacent to the one or more notches where the one or more OFDM subcarriers adjacent to the one or more notches include at least some of the plurality of information bits, and(ii) a second set of constellation symbols corresponding to a second set of OFDM subcarriers in the OFDM symbol, wherein the second set of OFDM subcarriers are further in frequency from the one or more notches than the first set of OFDM subcarriers, generate a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first set of constellation symbols, generate a second timedomain signal by performing an IDFT based on the second set of constellation symbols, modify the first timedomain signal by applying a first windowing function corresponding to a first window transition width β1, modify the second timedomain signal by applying a second windowing function having a second window transition width β2, wherein the first window transition width β1 is different than the second window transition width β2 such that β1≠β2, set the first window transition width β1 to be longer than a guard interval width N_{GI }such that β1>N_{GI}, set the second window transition width β2 to be shorter than the guard interval width N_{GI }such that β2<
 N_{GI}, and generate an output signal to include a sum of the first timedomain signal and the second timedomain signal, the output signal corresponding to the OFDM symbol.

6. The apparatus of claim 5, wherein
 the one or more integrated circuit devices are configured to: modify the first timedomain signal to include a first cyclic prefix having

1212. A nontransitory computer readable storage medium having
 computer program instructions stored thereon that, when executed by one or more processors, cause the one or more processors to perform a method of generating an orthogonal frequency division multiplexing (OFDM) symbol for transmission in a communication channel containing one or more notches of forbidden frequencies, the method comprising: receiving a plurality of information bits
 mapping the plurality of information bits to a plurality of constellation symbols corresponding to a plurality of OFDM subcarriers in the OFDM symbol, the plurality of constellation symbols including:(i) a first set of constellation symbols corresponding to a first set of OFDM subcarriers in the OFDM symbol, wherein the first set of OFDM subcarriers includes i) one or more OFDM subcarriers in the one or more notches, and ii) one or more OFDM subcarriers adjacent to the one or more notches where the one or more OFDM subcarriers adjacent to the one or more notches include at least some of the plurality of information bits, and(ii) a second set of constellation symbols corresponding to a second set of OFDM subcarriers in the OFDM symbol, wherein the second set of OFDM subcarriers are further in frequency from the one or more notches than the first set of OFDM subcarriers
 generating a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first set of constellation symbols
 generating a second timedomain signal by performing the IDFT based on the second set of constellation symbols
 modifying the first timedomain signal by applying a first windowing function corresponding to a first transition having a first window transition width β1
 modifying the second timedomain signal by applying a second windowing function corresponding to a second transition having a second window transition width β2, wherein the first window transition width β1 is different than the second window transition width β2 such that β1≠β2
 setting the first window transition width β1 to be longer than a guard interval width N_{GI }such that β1>N_{GI}
 setting the second window transition width β2 to be shorter than the guard interval width N_{GI }such that β2<
 N_{GI}
 and adding the first timedomain signal and the second timedomain signal to generate an output signal corresponding to the OFDM symbol.
Description
FIELD OF THE DISCLOSURE
The present disclosure relates generally to communication networks and, more particularly, to multiband data transmission systems and methods.
BACKGROUND
In most communication systems, increasing bandwidth translates into a greater transmission capacity. However, the physical medium or communication channel usually has some characteristics that vary in frequency, and only certain fragments or frequency bands can be used. For example, in an electrical grid, the power that can be injected starting from 30 MHz may vary in comparison to the power that can be injected at frequencies lower than 30 MHz. Also, an electrical grid has other characteristics that make an electrical grid less desirable as a transmission medium. For example, in an electrical grid, the noise floor decreases with the frequency, the attenuation increases with the frequency, and there is significant noise in the frequencymodulation (FM) radio band (from 88.5 to 108 MHz). Thus, there is a need to avoid certain parts of the frequency spectrum.
In the field of wireless communications, there is also a need to avoid certain parts of the frequency spectrum. In the context of Wireless Local Area Networks (WLAN), work has begun on a two new standards, IEEE 802.11ah and IEEE 802.11af, each of which will specify wireless network operation in sub1 GHz frequencies. Low frequency communication channels are generally characterized by better propagation qualities and extended propagation ranges compared to transmission at higher frequencies. In the past, sub1 GHz ranges have not been utilized for wireless communication networks because such frequencies were reserved for other applications (e.g., licensed TV frequency bands, radio frequency band, etc.). There are few frequency bands in the sub1 GHz range that remain unlicensed, with different specific unlicensed frequencies in different geographical regions. The IEEE 802.11 ah Standard will specify wireless operation in available unlicensed sub1 GHz frequency bands. The IEEE 802.11af Standard will specify wireless operation in TV White Space (TVWS), i.e., unused TV channels in sub1 GHz frequency bands.
Thus, in both wired and wireless communication systems, it is necessary to configure the system to avoid certain bands of frequencies (e.g., a frequency band having a notch) so as to avoid interfering with other existing communication systems or to avoid frequencies at which the medium displays undesirable characteristics.
SUMMARY
Orthogonal frequency division multiplexing (OFDM) is an advantageous technique for communicating information over both wired and wireless communication systems. At least some embodiments disclosed herein mitigate problems related to generating OFDM symbols for information transmission in frequency bands in which some frequencies are to be avoided, e.g., contain notches. At least some disclosed embodiments allow for an optimization of system performance with respect to one or more metrics including, for example, one or more of: increased data throughput, minimization of intersymbol interference (ISI), minimization of errors due to channel delayspread, and minimization of power spectral density of bands at frequencies that are to be avoided.
In one embodiment, a method is generally directed to generating an orthogonal frequency division multiplexing (OFDM) symbol based on the use of different time domain windows for different spectral bands. The method includes receiving a plurality of information bits and mapping the plurality of information bits to (1) a first plurality of constellation symbols corresponding to a first plurality of OFDM subcarriers in the OFDM symbol, and (2) a second plurality of constellation symbols corresponding to a second plurality of OFDM subcarriers in the OFDM symbol. The method also includes generating a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first plurality of constellation symbols and generating a second timedomain signal by performing an IDFT based on the second plurality of constellation symbols. The method additionally includes modifying the first timedomain signal by applying a first window having a first width β1, and modifying the second timedomain signal by applying a second window having a second width β2, wherein the first width is different than the second width β2 such that β1≠β2. The method further includes adding the first timedomain signal and the second timedomain signal to generate the OFDM symbol.
In a further embodiment, the first plurality of constellation symbols is generated to correspond to subcarriers having frequencies in a first band of frequencies containing a notch or adjacent to a notch, such that subcarriers in the notch carry reduced power spectral density in comparison to subcarriers not in the notch. Further, the second plurality of constellation symbols is generated to correspond to subcarriers having frequencies in a second band of frequencies that does not contain a notch.
In another embodiment, an apparatus is provided for generating an orthogonal frequency division multiplexing (OFDM) symbol based on the use of different time domain windows for different spectral bands. The apparatus includes a digital signal processor configured to include an input module, a constellation mapping module, an IDFT module, a windowing module, a signal addition module, and an output module. The input module is configured to receive a plurality of information bits and to provide these information bits to the constellation mapping module.
The constellation mapping module is configured to map the plurality of information bits to (1) a first plurality of constellation symbols corresponding to a first plurality of OFDM subcarriers in the OFDM symbol, and (2) a second plurality of constellation symbols corresponding to a second plurality of OFDM subcarriers in the OFDM symbol.
The IDFT module is configured to generate a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first plurality of constellation symbols, and to generate a second timedomain signal by performing an IDFT based on the second plurality of constellation symbols.
The windowing module is configured to modify the first timedomain signal by applying a first window having a first width β1, and to modify the second timedomain signal by applying a second window having a second width β2, wherein the first width is different than the second width β2 such that β1≠β2.
The signal addition module is configured to add the first timedomain signal and the second timedomain signal to generate the OFDM symbol, and the output module is configured to provide the OFDM symbol as output to a network interface, to a communications device, or to a nontransitory computer readable storage medium for storage of the OFDM symbol.
In further embodiments, a nontransitory computer readable storage medium having computer program instructions stored thereon is provided. The computer program instructions are such that, when executed by a processor, they cause the processor to perform a method of generating an orthogonal frequency division multiplexing (OFDM) symbol based on the use of different time domain windows for different spectral bands.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates one way in which OFDM symbols may be windowed and overlapped, as is done, for example, in the ITUT G.hn standard.
FIG. 2 illustrates an example transmission block diagram of a conventional OFDM system using a windowing and overlap scheme as shown in FIG. 1.
FIG. 3 illustrates an apparatus for applying windows of different time duration to subcarriers corresponding to different frequency bands, according to an embodiment.
FIG. 4 illustrates a method for generating an OFDM symbol based on the use of different time domain windows for different spectral bands, according to an embodiment.
Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
Embodiments disclosed herein are described in the context of communication systems that operate using frequency bands containing notches of forbidden frequencies. As such, embodiments described herein utilize a multiband approach in which frequency components of a signal at frequency bands containing notches or at frequency bands adjacent to notches may be treated differently from other frequency components in the signal.
Example implementations are discussed below in the context of two frequency bands being considered, one with or adjacent to a notch and one without notches or not adjacent to notches. These examples are presented merely for the purpose of illustration and are not limiting. One of ordinary skill in the art, upon reading the present disclosure, will appreciate that techniques discussed herein may be utilized in the context of more than two frequency bands.
Many communication systems that use OFDM, for example systems that conform to the International Telecommunication Union's (ITU's) G.hn family of standards, veryhighbitrate digital subscriber line (VDSL) communication, etc., define the use of a transmission window. FIG. 1 illustrates an OFDM symbol 200 that is overlapped with other OFDM symbols, as is done, for example, in the ITUT G.hn standard. The OFDM signal 200 is formed from an original data signal having a time duration N (202). In forming the OFDM symbol 200, the original data signal is modified to include a cyclic prefix having a time duration N_{CP }(204), thus creating an extended signal having a duration 210. A windowing function is applied to the extended signal to create transition periods 208, each having a duration β. A guard interval (GI) 206 is a time interval between an end of a preceding OFDM symbol 214 and a beginning of the original OFDM symbol. After windowing and overlapping with adjacent OFDM symbols, the OFDM symbol 200 has a duration 212 given by Ns=N+N_{CP}−β.
A smoother transition between symbols provides a sharper spectrum in frequency, which may be especially important in cases in which notches in the spectrum are required in order to coexist with, for example, HAM radio services and/or to avoid undesirable characteristics of the transmission medium. U.S. Pat. Nos. 7,483,365 and 8,265,176 discuss issues related to power line communication systems coexisting with HAM radio services.
As the window width, β, increases, the sharpness of notches in a frequency spectrum generally increase. The presence of the window, however, increases the symbol duration and thus reduces the throughput. Additionally, as compared to an OFDM symbol with the same symbol duration 212 but no windowing, the windowing reduces the ability of the Guard Interval 206 to mitigate the channel delayspread and to reduce inter symbol interference (ISI).
FIG. 2 is a block diagram of an example transmitter 300 of a conventional OFDM system using a windowing and overlap scheme such as discussed above with respect to FIG. 1. Information bits 302 are received by the transmitter 300 and are provided to a channel coding module 304 configured to implement channel coding. A symbol mapping module 306 maps encoded bits to constellation points, e.g., the symbol mapping module 306 maps encoded bits to transmission symbols according to a suitable modulation scheme such as binary phase shift keying (BPSK), quadrature amplitude modulation (QAM), etc. The resulting constellation points are provided to a serial to parallel module 308 that parses constellation points to different inputs of an IDFT module 310 that performs an Inverse Discrete Fourier Transform (IDFT), e.g., an inverse fast Fourier transform (IFFT). The different inputs of the IDFT module 310 correspond to different OFDM subchannels. An output of the IDFT module 310 is provided to a parallel to serial module 312 that generates a timedomain signal. The output of the parallel to serial module 312 is provided to a cyclic prefix module 314 which adds a cyclic prefix to the timedomain signal. A windowing function is then applied by a windowing module 316. Finally an analog representation of the OFDM symbol is generated by a digital to analog converter (DAC) 318.
Since the windowing function is applied in the time domain (e.g., after the IFFT), in such a conventional system, the windowing function is the same for all the subcarriers in the time domain signal. In other words, the transition period corresponding to the windowing function is the same for all the subcarriers in the time domain signal.
The subcarriers that contribute the most to the power spectral density in a notch are the subcarriers that are relatively closer (in frequency) to the notch, whereas subcarriers relatively farther (in frequency) from the notch contribute less to the power spectral density in a notch. Thus, unlike the transmitter 300 of FIG. 2, embodiments described below apply a first windowing function for subcarriers that are relatively close (in frequency) to a notch and apply a second windowing function corresponding to a shorter transition period (or no windowing is applied) to subcarriers that are relatively farther (in frequency) from the notch.
FIG. 3 is a block diagram of an example transmitter apparatus 400, according to an embodiment. As described in more detail below, the transmitter 400 applies different windowing functions corresponding to different transition time durations to subcarriers corresponding to different frequency bands.
Information bits 402 are provided to a channel coding module 404, which encodes the information bits 402 to generate encoded bits. Symbol mapping is performed by a symbol mapping module 406. The symbol mapping module 406 maps encoded bits to constellation points, e.g., the symbol mapping module 406 maps encoded bits to transmission symbols according to a suitable modulation scheme such as BPSK, QAM, etc.
The constellation points are parsed to one of two transmit chains 407, where each transmit chain 407 corresponds to a respective subset of subchannels. For example, in an embodiment, the transmit chain 407a corresponds to a first subset of subchannels relatively closer, in frequency, (e.g., as compared to other subchannels not in the first subset) to a notch and for which a first windowing function is to be applied, where the first windowing function corresponds to a relatively longer transition period (e.g., as compared to a second windowing function applied to other subchannels or to no windowing function); whereas the transmit chain 407b corresponds to a second subset of subchannels relatively farther, in frequency, (e.g., as compared to the first subset of channels) from the notch and for which a second windowing function (or no windowing function) is to be applied, where the second windowing function corresponds to a relatively shorter transition period (e.g., as compared to the first windowing function).
The transmit chain 407a includes a serial to parallel module 408a, an IDFT module 410a, a parallel to serial module 412a, a cyclic prefix generation module 414a, and a windowing module 416a. Similarly, the transmit chain 407b includes a serial to parallel module 408b, an IDFT module 410b, a parallel to serial module 412b, a cyclic prefix generation module 414b, and a windowing module 416b. Each transmit chain 407 operates in a manner similar to corresponding modules in the transmitter 300 of FIG. 2, except that each transmit chain 407 operates to generate a respective time domain signal corresponding to only a respective subset of subchannels. Additionally, the windowing module 416a applies a first windowing function, whereas the windowing module 416b applies a second windowing function. In an embodiment, no windowing function is applied by the transmit chain 407b and the windowing module 416b is omitted or bypassed. In an embodiment, the first windowing function corresponds to a longer transition period as compared to the second windowing function. In an embodiment, both transmit chains 407 use a same OFDM symbol duration (e.g., Ns 212 as described with reference to FIG. 1) so that the time domain signals generated by the chains 407 are correctly aligned in time.
Timedomain signals generated by the transmit chains 407 are summed by a summing module 418. A DAC 420 converts the summed signal to an analog signal.
The transmitter apparatus 400 of FIG. 3 may be implemented using dedicated transmit chains 407. In other embodiments, however, in which a communication device utilizes more chains in reception than in transmission, processing resources that would otherwise remain idle during transmission (e.g., processing resources utilized for additional receive chains) may be utilized to implement transmit chain 407b, for example, during transmission. U.S. Patent Application Publication No. 2012/0134375, entitled “MultiBand Data Transmission,” which is incorporated herein by reference, describes how processing resources in a communication device can be timeshared between a transmit mode and a receive mode. Similarly, U.S. patent application Ser. No. 14/180,178, entitled “MultiBand Transmission System,” which is incorporated herein by reference, describes how processing resources in a communication device can be timeshared between a transmit mode and a receive mode.
In some embodiments it may be advantageous to reduce the GI between OFDM symbols so that a window transition in subcarriers close to a notch may be longer than the GI, while a window transition for other subcarriers, e.g., subcarriers relatively further from the notch, is shorter than the GI. Due to the window transition being longer than the GI for some subcarriers there may be distortion in the vicinity of the subcarriers with the long window transition (e.g., because the flat part of the window may be shorter than N), but the other subcarriers will have no such distortion and hence will have a higher signal to noise ratio (SNR).
In some embodiments it may be advantageous to reduce the window transition duration for subcarriers far from a notch to increase the effective GI and eliminate more ISI, while keeping a longer window duration just at or below the duration of the GI for subcarriers closer to the notch.
In some embodiments, the transmitter apparatus 400 is modified to include a suitable number of transmit chains 407 more than two, e.g., three transmit chains 407, four transmit chains 407, five transmit chains 407, etc. In such embodiments, the summing module 418 is modified to sum outputs from the multiple transmit chains 407. In some embodiments, one or more of the transmit chains 407 may omit applying a windowing function.
FIG. 4 is a flow diagram of an example method 500 for generating an OFDM symbol, according to an embodiment. The method 500 may be implemented by the example transmitter apparatus 400 of FIG. 3, in an embodiment, and the method 500 is discussed with reference to FIG. 3 merely for explanatory purposes. In other embodiments, however, the method 500 is implemented by another suitable apparatus.
At block 504, a plurality of information bits are received. In an embodiment, the plurality of information bits are for inclusion in an OFDM symbol.
At block 508, information bits received at block 504 are mapped to i) a first plurality of constellation symbols (sometimes referred to herein as constellation points) corresponding to a first plurality of OFDM subcarriers, and ii) a second plurality of constellation symbols corresponding to a second plurality of OFDM subcarriers. For example, in the transmitter apparatus 400, the symbol mapping module 406 and the serialtoparallel modules 408 map information bits (e.g., after having been encoded) to i) a first plurality of constellation points corresponding to a first plurality of OFDM subcarriers, and ii) a second plurality of constellation symbols corresponding to a second plurality of OFDM subcarriers.
At block 512, a first timedomain signal is generated by performing an IDFT on the first plurality of constellation symbols. For example, in the transmitter apparatus 400, the IDFT module 410a performs an IDFT on the first plurality of constellation points. Similarly, at block 516, a second timedomain signal is generated by performing an IDFT on the second plurality of constellation symbols. For example, in the transmitter apparatus 400, the IDFT module 410b performs an IDFT on the second plurality of constellation points.
At block 520, the first timedomain signal is modified by applying a first windowing function corresponding to a transition width β1. For example, in the transmitter apparatus 400, the windowing module 416a applies a first windowing function. Similarly, at block 524, the second timedomain signal is modified by applying a second windowing function corresponding to a transition width β2. For example, in the transmitter apparatus 400, the windowing module 416b applies a second windowing function. In some embodiments, β1≠β2. In some embodiments in which the first plurality of subcarriers are closer in frequency to a notch as compared to the second plurality of subcarriers, β1>β2.
In some embodiments, the first windowing function has a different shape than the second windowing function. For example, in an embodiment, the first windowing function has a first shape selected from a set of suitable shapes including two or more of i) a raised cosine shape, ii) a triangular shape, iii) a hamming window shape, etc.; whereas the second windowing function has a second shape selected from the set of suitable shapes, where the first shape is different than the second shape. In other embodiments, the first shape and the second shape are the same.
At block 528, the OFDM symbol is generated to include a sum of the first timedomain signal and the second timedomain signal. For example, in the transmitter apparatus 400, timedomain signals generated by the chains 407 are summed by the summing module 418, and the DAC 420 converts the summed signal to an analog signal.
In some embodiments, the method 500 is modified to include additional processing blocks such as discussed above, e.g., GI insertion, cyclic prefix insertion, generation of additional timedomain signals, etc. In some embodiments, block 524 is omitted such that no windowing function is applied to the second timedomain signal.
In some embodiments, a data stream is produced by generating a plurality of ODFM symbols according to initial values of β1, N1_{CP}, β2, N2_{CP }and overlapping the symbols as described above. The generated data stream can be characterized in terms of its throughput, intersymbol interference (ISI), rate of errors resulting from channel delayspread, as well as by the value of the power spectral density in bands lying within the notch. An optimization procedure can be performed by varying the values of β1, N1_{CP}, β2, N2_{CP }so as to optimize one or more metrics of the data stream. For example, the optimization procedure may, in various embodiments, involve any suitable combination of one or more of the following:
 maximize data throughput to be greater than a predetermined throughput value;
 minimize power spectral density of bands in the notch to be less than a predetermined spectral density value; minimize intersymbol interference (ISI) to be lower than a predetermined amount; and
 minimize errors due to channel delayspread.
In a further embodiment, it may be advantageous to set the first window width β1 to be longer than the guard window width N_{GI }such that β1>N_{GI}, and to set the second window width β2 to be shorter than the guard window width N_{GI }such that β2≤N_{GI}. For certain applications, this particular choice may have beneficial effects for an optimization procedure such as described above.
In further embodiments, a nontransitory computer readable storage medium having computer program instructions stored thereon is provided. The computer program instructions are such that, when executed by a processor, they cause the processor to perform a method of generating an orthogonal frequency division multiplexing (OFDM) symbol based on the use of different time domain windows for different spectral bands, as described above with regard to FIGS. 3 and 4.
In an embodiment, a method of generating an orthogonal frequency division multiplexing (OFDM) symbol includes receiving a plurality of information bits; mapping the plurality of information bits to: (i) a first plurality of constellation symbols corresponding to a first plurality of OFDM subcarriers in the OFDM symbol, and (ii) a second plurality of constellation symbols corresponding to a second plurality of OFDM subcarriers in the OFDM symbol. The method further includes: generating a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first plurality of constellation symbols; generating a second timedomain signal by performing an IDFT based on the second plurality of constellation symbols; modifying the first timedomain signal by applying a first windowing function corresponding to a first transition width β1; modifying the second timedomain signal by applying a second windowing function having a second transition width β2, wherein the first transition width β1 is different than the second transition width β2 such that β1≠β2; and generating an output signal to include a sum of the first timedomain signal and the second timedomain signal, the output signal corresponding to the OFDM symbol.
In other embodiments, the method includes none of, one of, or any suitable combination of two or more of the following features.
The first plurality of OFDM subcarriers correspond to a first frequency band containing or adjacent to a notch in which signal power corresponding to the OFDM symbol is to be minimized; and the second plurality of OFDM subcarriers correspond to a second frequency band that is further in frequency from the notch as compared to the first frequency band.
The first transition width β1 is longer than the second transition width β2 such that β1>β2.
The method further includes modifying the first timedomain signal to include a first cyclic prefix having a first width N1_{CP}; modifying the second timedomain signal to include a second cyclic prefix having a width N2_{CP}; and setting the first window width β1, the second window width β2, first width N1_{CP}, and the second width N2_{CP }so that the first and second timedomain signals each have a common OFDM symbol duration.
The method further includes generating a first plurality of OFDM symbols according to initial values of β1, N1_{CP}, β2, N2_{CP}; after generating the first plurality of OFDM symbols, changing one or more of the values of β1, N1_{CP}, β2, N2_{CP}; and after changing one or more of the values of β1, N1_{CP}, β2, N2_{CP}, generating a second plurality of OFDM symbols according to one or more changed values of β1, N1_{CP}, β2, N2_{CP}.
Changing one or more of the values of β1, N1_{CP}, β2, N2_{CP }is performed as part of an optimization procedure to, at least one of: maximize data throughput to be greater than a predetermined throughput value; minimize power spectral density in the notch to be less than a predetermined spectral density value; minimize intersymbol interference (ISI) to be lower than a predetermined amount; and/or minimize errors due to channel delayspread.
The method further includes setting the first window transition width β1 to be longer than a guard interval width N_{GI }such that β1>N_{GI}; and setting the second window transition width β2 to be shorter than the guard interval width N_{GI }such that β2≤N_{GI}.
In another embodiment, an apparatus comprises one or more integrated circuit devices configured to: receive a plurality of information bits, map the plurality of information bits to: (i) a first plurality of constellation symbols corresponding to a first plurality of orthogonal frequency division multiplexing (OFDM) subcarriers in an OFDM symbol, and (ii) a second plurality of constellation symbols corresponding to a second plurality of OFDM subcarriers in the OFDM symbol. The one or more integrated circuit devices are also configured to: generate a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first plurality of constellation symbols, generate a second timedomain signal by performing an IDFT based on the second plurality of constellation symbols, modify the first timedomain signal by applying a first windowing function corresponding to a first transition width β1, modify the second timedomain signal by applying a second windowing function having a second transition width β2, wherein the first transition width β1 is different than the second transition width β2 such that β1≠β2, and generate an output signal to include a sum of the first timedomain signal and the second timedomain signal, the output signal corresponding to the OFDM symbol.
In other embodiments, the apparatus includes none of, one of, or any suitable combination of two or more of the following features.
The first plurality of OFDM subcarriers correspond to a first frequency band containing or adjacent to a notch in which signal power corresponding to the OFDM symbol is to be minimized; and the second plurality of OFDM subcarriers correspond to a second frequency band that is further in frequency from the notch as compared to the first frequency band.
The first transition width β1 is longer than the second transition width β2 such that β1>β2.
The one or more integrated circuit devices are configured to: modify the first timedomain signal to include a first cyclic prefix having a first width N1_{CP}; modify the second timedomain signal to include a second cyclic prefix having a width N2_{CP}; and set the first window width β1, the second window width β2, first width N1_{CP}, and the second width N2_{CP }so that the first and second timedomain signals each have a common symbol duration.
The one or more integrated circuit devices are configured to: generate a first plurality of OFDM symbols according to initial values of β1, N1_{CP}, β2, N2_{CP}; after generating the first plurality of OFDM symbols, change one or more of the values of β1, N1_{CP}, β2, N2_{CP}; and after changing one or more of the values of β1, N1_{CP}, β2, N2_{CP}, generate a second plurality of OFDM symbols according to one or more changed values of β1, N1_{CP}, β2, N2_{CP}.
The one or more integrated circuit devices are configured to change one or more of the values of β1, N1_{CP}, β2, N2_{CP }as part of an optimization procedure to, at least one of: maximize data throughput to be greater than a predetermined throughput value; minimize power spectral density in the notch to be less than a predetermined spectral density value; minimize intersymbol interference (ISI) to be lower than a predetermined amount; and/or minimize errors due to channel delayspread.
The one or more integrated circuit devices are configured to: set the first window transition width β1 to be longer than a guard interval width N_{GI }such that β1>N_{GI}; and set the second window transition width β2 to be shorter than the guard interval width N_{GI }such that β2≤N_{GI}.
The one or more integrated circuit devices implement: a symbol mapping module configured to map information bits in the plurality of information bits to constellation symbols, a first serialtoparallel module coupled to the symbol mapping module, the first serialtoparallel module outputting the first plurality of constellation symbols, and a second serialtoparallel module coupled to the symbol mapping module, the second serialtoparallel module outputting the second plurality of constellation symbols.
The one or more integrated circuit devices implement: a first inverse discrete Fourier transform (IDFT) module configured to operate on the first plurality of constellation symbols, a first paralleltoserial module coupled to the first IDFT module, the first paralleltoserial module outputting the first timedomain signal, a second IDFT module configured to operate on the second plurality of constellation symbols, and a second paralleltoserial module coupled to the second IDFT module, the second paralleltoserial module outputting the second timedomain signal.
The apparatus further comprises a digitaltoanalog convert to convert the output signal to an analog signal.
In another embodiment, a nontransitory computer readable storage medium having computer program instructions stored thereon that, when executed by one or more processors, cause the one or more processors to perform a method of generating an orthogonal frequency division multiplexing (OFDM) symbol, the method comprising: receiving a plurality of information bits; mapping the plurality of information bits to: (i) a first plurality of constellation symbols corresponding to a first plurality of OFDM subcarriers in the OFDM symbol, and (ii) a second plurality of constellation symbols corresponding to a second plurality of OFDM subcarriers in the OFDM symbol. The method further comprises generating a first timedomain signal by performing an Inverse Discrete Fourier Transform (IDFT) based on the first plurality of constellation symbols; generating a second timedomain signal by performing the IDFT based on the second plurality of constellation symbols; modifying the first timedomain signal by applying a first windowing function corresponding to a first transition having a first width β1; modifying the second timedomain signal by applying a second windowing function corresponding to a second transition having a second width β2, wherein the first width β1 is different than the second width β2 such that β1≠β2; and adding the first timedomain signal and the second timedomain signal to generate an output signal corresponding to the OFDM symbol.
In other embodiments, the storage medium includes none of, one of, or any suitable combination of two or more of the following features.
The storage further comprises computer program instructions stored thereon that, when executed by a processor, cause the processor to perform the following method steps: the first plurality of OFDM subcarriers correspond to a first frequency band containing or adjacent to a notch in which signal power corresponding to the OFDM symbol is to be minimized; and the second plurality of OFDM subcarriers correspond to a second frequency band that is further in frequency from the notch as compared to the first frequency band.
The first transition width β1 is longer than the second transition width β2 such that β1>β2.
At least some of the various blocks, operations, and techniques described above may be implemented utilizing hardware, a processor executing firmware instructions, a processor executing software instructions, or any combination thereof. When implemented utilizing a processor executing software or firmware instructions, the software or firmware instructions may be stored in any tangible, nontransitory computer readable memory such as on a magnetic disk, an optical disk, or other storage medium, in a RAM or ROM or flash memory, processor, hard disk drive, optical disk drive, tape drive, etc.
Likewise, the software or firmware instructions may be delivered to a user or a system via any known or desired delivery method including, for example, on a computer readable disk or other transportable computer storage mechanism or via communication media.
Thus, the software or firmware instructions may be delivered to a user or a system via a communication channel such as a telephone line, a DSL line, a cable television line, a fiber optics line, a wireless communication channel, the Internet, etc. The software or firmware instructions may include machine readable instructions that, when executed by the processor, cause the processor to perform various acts.
When implemented in hardware, the hardware may comprise one or more of discrete components, an integrated circuit, an applicationspecific integrated circuit (ASIC), a programmable logic device, etc.
While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, changes, additions and/or deletions may be made to the disclosed embodiments without departing from the scope of the claims.
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