| Ofdm multiple sub-channel communication system -> Monitor Keywords |
|
|
  Ofdm multiple sub-channel communication systemUSPTO Application #: 20070183308Title: Ofdm multiple sub-channel communication system Abstract: A transmitter for transmitting an OFDM signal in a communications channel, including: (a) a plurality of base-band OFDM modulators, each for modulating a respective baseband data signal onto a plurality of orthogonal sub-carriers and outputting a respective baseband sub-channel OFDM signal, and (b) a fast convolution filter and up-converter for applying fast convolution filtering and digital up-conversion to the sub-channel OFDM signals to output a combined OFDM signal that includes each of the sub-channel OFDM signals, the fast convolution and up-converter filtering each of the sub-channel OFDM signals and frequency shifting all of the sub-channel signals to respective designated frequencies within the combined OFDM signal. (end of abstract) Agent: Hodgson Russ LLP The Guaranty Building - Buffalo, NY, US Inventors: Dmitri Korobkov, Randy Howell, Alex Dolgonos USPTO Applicaton #: 20070183308 - Class: 370208000 (USPTO) Related Patent Categories: Multiplex Communications, Generalized Orthogonal Or Special Mathematical Techniques, Particular Set Of Orthogonal Functions The Patent Description & Claims data below is from USPTO Patent Application 20070183308. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation of and claims priority to U.S. patent application Ser. No. 10/166,872, filed Jun. 11, 2002, and issued Apr. 17, 2007 as U.S. Pat. No. 7,206,350. BACKGROUND OF THE INVENTION [0002] The present invention relates to the transmission and reception of orthogonal frequency division multiplexed (OFDM) signals, and more particularly, to communications systems using OFDM to communicate information in a multiple user two-way communication system. [0003] OFDM is a bandwidth efficient multi-carrier modulation technique where a data stream is divided into a set of lower rate digital data streams, each of which is modulated onto a separate data signal. Each data signal or sub-carrier have distinct carrier frequencies. OFDM is currently used in one-way wireless broadcasts of digital television and digital radio signals where it has demonstrated its robustness to certain types of channel impairments such as multi-path fading. [0004] In OFDM, to avoid mutual interference between the set of sub-carriers the frequency spacing .DELTA.f between sub-carriers is chosen such that over time interval T the spectrum of each sub-carrier has a null at the other sub-carrier frequencies (orthogonal sub-carriers). For a system generating baseband samples of the OFDM signal at a sampling rate of N/T and where the number of sub-carriers is less than or equal to N, this orthogonality constraint can be efficiently realized by setting .DELTA.f=1/T so that exactly N baseband samples of the OFDM signal is generated over time T. If the number of samples N over the orthogonality interval T is a power-of-two value (i.e., N=2.sup.k where k is an integer greater than zero) the process of modulating the data streams onto the set of sub-carriers can be efficiently implemented via an Inverse Fast Fourier Transform (IFFT). To recover the set of data streams from the set of sub-carriers at the OFDM receiver a Fast Fourier Transform (FFT) can be employed. [0005] Often, the communication channel into which the transmitter 10 transmits is divided into a number of frequency sub-channels to permit multiple users to access the system. A limitation of prior OFDM systems that propose the use of multiple sub-channels that the transmitters and receivers of these systems are configured such that once the sub-channels have been allocated the sub-channels are static in that they can not be dynamically varied in quick response to changes in demands placed on system resources, leading to inefficient use of bandwidth. [0006] Thus, there is a need for a transmitter and receiver architecture that is adapted for a multiple user communications system that enables the use of band-width efficient transmissions. More particularly, there is a need for an adequate filtering and upconversion structure to permit an OFDM transmitter to create multiple sub-channels in a dynamic fashion where the frequency position and frequency width of the sub-channels can quickly change from one time instance to the next. Similarly, a filtering and downconversion process is required at the OFDM receiver to recover the data from the various sub-channels it receives. SUMMARY OF THE INVENTION [0007] According to one aspect of the invention, there is provided a transmitter for transmitting an OFDM signal in a communications channel, including: (a) a plurality of base-band OFDM modulators, each for modulating a respective baseband data signal onto a plurality of orthogonal sub-carriers and outputting a respective baseband sub-channel OFDM signal, and (b) a fast convolution filter and up-converter for applying fast convolution filtering and digital up-conversion to the sub-channel OFDM signals to output a combined OFDM signal that includes each of the sub-channel OFDM signals, the fast convolution and up-converter filtering each of the sub-channel OFDM signals and frequency shifting all of the sub-channel signals to respective designated frequencies within the combined OFDM signal. [0008] According to another aspect of the invention is a method for transmitting an OFDM signal including a plurality of sub-channel OFDM signals in a communications channel, comprising, at a transmitter: (a) receiving data and segmenting the data into a plurality of parallel baseband data signals; (b) generating in parallel a plurality of baseband sub-channel OFDM signals, each baseband sub-channel OFDM signal including a respective one of the baseband data signals modulated onto a plurality of orthogonal sub-carriers; and (c) applying fast convolution filtering and digital up-conversion to the baseband sub-channel OFDM signals to output a combined OFDM signal that includes each of the sub-channel OFDM signals, including filtering each of the baseband sub-channel OFDM signals and frequency shifting each of the sub-channel baseband OFDM signals to respective designated frequencies within the combined OFDM signal. BRIEF DESCRIPTION OF THE DRAWINGS [0009] FIG. 1 is a block diagram of an OFDM baseband transmission engine. [0010] FIG. 2 is a block diagram of an OFDM communications system in accordance with one embodiment of the present invention. [0011] FIG. 3 is a block diagram of a multi-channel OFDM transmitter of the communications system of FIG. 2. [0012] FIG. 4 shows a block diagram of a baseband Overlap-and-Save Fast Convolution filtering method. [0013] FIG. 5 shows a block diagram of a baseband Overlap-and-Add Fast Convolution filtering method. [0014] FIG. 6 shows a block diagram of the filtering and IF upsampling section of the multi-channel OFDM transmitter of FIG. 3. [0015] FIG. 7 shows a block diagram of a multi-channel OFDM receiver in accordance with an embodiment of the present invention that can be used in the communications system of FIG. 2. [0016] FIG. 8 shows a block diagram of the filtering and IF downsampling section of the multi-channel OFDM receiver of FIG. 7. DETAILED DESCRIPTION OF THE INVENTION [0017] A description of the preferred embodiment of the present invention is discussed in detail. Many modifications to this preferred embodiment are possible without departing from the general spirit and scope of the invention, and such modifications will be obvious to those skilled in the art. [0018] The general principles of FFT-based OFDM signal transmission can be described with reference to FIG. 1, representing a block diagram of a typical OFDM transmission engine. The OFDM transmission engine 10 receives a data stream 12 from a source. A Forward Error Correction (FEC) block 14 receives that input data stream and applies a coding scheme to introduce error-correcting and/or error-detecting redundancy into the data stream. As FEC coding typically spans the set of OFDM sub-carriers, an OFDM system can take advantage of frequency diversity to mitigate losses due to frequency selective fading. Immediately after FEC coding, a spectrum builder block 16 divides the sequence of coded bits into segments of m*.sub..M bits which is further sub-divided into M sub-segments of m bits (M=number of modulated carriers). Each sub-segment of m bits is mapped to a complex number according to a given signal constellation rule. For example, quadrature phase shift keying (QPSK) with m=2 or any 2.sup.m-ary quadrature amplitude modulation (QAM) scheme such as 16-QAM (m=4) and 64-QAM (m=6). The length M set of m-bit data symbols will be employed to modulate M OFDM sub-carriers. The set of M data symbols are written to a length N sequence (N=total number of sub-carriers), initially filled with zeros (M.ltoreq.N). Each position of the length N sequence uniquely represents a single OFDM sub-carrier. Some of the positions can be occupied by a set of M.sub.pilot values (commonly referred to as pilot tones) known to the OFDM receiver in order to assist it to overcome channel impairments. The remaining N-M-M.sub.pilot positions remain as zero values to represent blank carriers. Such blank carriers are useful to set a frequency guard space between adjacent channels or sub-channels to avoid mutual interference. [0019] After spectrum building 16, each successive sequence of N complex values generated from successive segments of m*.sub..M coded bits are Inverse Fast Fourier Transformed (IFFT) in IFFT block 18. This modulates the set of N complex values onto the N OFDM sub-carriers. By dividing the input data stream into N sub-carriers the symbol duration T will be become long relative to that of the multipath delay spread. As a result the problem of multipath induced inter-symbol interference (ISI) is substantially reduced for an OFDM system. To almost completely eliminate inter-symbol interference a guard time is introduced for each OFDM symbol. This guard time usually takes the form of a cyclic extension of the OFDM symbol. For example, L samples at the beginning of the N sample OFDM symbol can be copied to the end of the symbol. This procedure is denoted in Cyclic Extension block 20. This serves to lengthen each baseband OFDM symbol to N+L samples. The overall OFDM symbol duration becomes T.sub.OFDM=T+T.sub.Guard where T.sub.Guard is the duration of the guard interval of L baseband samples. After adding the cyclic extension the baseband OFDM symbol undergoes filtering and upconversion 22 to convert the baseband waveform to an appropriate intermediate frequency (IF) form prior to conversion to a radio frequency (RF) form suitable for transmission. Continue reading... Full patent description for Ofdm multiple sub-channel communication system Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Ofdm multiple sub-channel communication system patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Ofdm multiple sub-channel communication system or other areas of interest. ### Previous Patent Application: Method and apparatus for initial acquisition and cell search for an ofdma system Next Patent Application: Orthogonal frequency division multiplex transmission method Industry Class: Multiplex communications ### FreshPatents.com Support Thank you for viewing the Ofdm multiple sub-channel communication system patent info. IP-related news and info Results in 1.43298 seconds Other interesting Feshpatents.com categories: Novartis , Pfizer , Philips , Polaroid , Procter & Gamble , |
||
