Abstract:
The disclosed embodiments relate to a system and method for processing communication signals. An exemplary method of processing a received orthogonal frequency division multiplexing (OFDM) broadcast signal having a frequency spectrum comprises demodulating the OFDM broadcast signal over a subset of the frequency spectrum to create a first demodulated signal component corresponding to a first signal component the first signal component being representative of a lower resolution version of a second signal component and providing data corresponding to the first demodulated signal component.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to improving the reception of transmitted communication signals, including orthogonal frequency division multiplexed (OFDM) signals in a multi-carrier system. 
       BACKGROUND OF THE INVENTION 
       [0002]    This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
         [0003]    Manufacturers of wireless communication equipment have a range of transmission technologies to choose from when designing a system or product. Some exemplary technologies are multicarrier systems, spread spectrum systems, narrowband systems, and infrared systems. An exemplary multicarrier transmission technology is orthogonal frequency division multiplexing (OFDM). 
         [0004]    OFDM is a robust technique for efficiently transmitting data over a channel having a frequency spectrum. The technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit data. These sub-carriers are arranged for optimal bandwidth efficiency compared to conventional frequency division multiplexing (FDM), which can waste portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier. 
         [0005]    The transmission of data through a channel via OFDM signals also provides several other advantages over more conventional transmission techniques. Some of these advantages are a tolerance to multipath delay spread and frequency selective fading, efficient spectrum usage, simplified sub-channel equalization, and good interference properties. 
         [0006]    Some wireless communication systems, such as satellite systems, employ large reception bandwidths. This makes them unsuitable for difficult reception conditions such as mobile TV or reception in a car. 
         [0007]    Additionally, different devices that are intended to receive the same communication signal may have different practical uses, resulting in a need for different levels of “robustness” in terms of signal reception. For example, a high definition television intended for home use will likely need to accurately receive a high resolution signal to function as intended. A mobile television with a small screen may, however, be able to perform effectively with a lower resolution signal. The performance of the mobile television may suffer under difficult reception conditions because it is adapted to receive a high resolution signal that is not really needed for it to function effectively. A system and method that improves the performance of OFDM communications under these circumstances is desirable. 
       SUMMARY OF THE INVENTION 
       [0008]    The disclosed embodiments relate to a system and method for transmitting and/or receiving communication signals. An exemplary method of processing a received orthogonal frequency division multiplexing (OFDM) broadcast signal having a frequency spectrum comprises demodulating the OFDM broadcast signal over a subset of the frequency spectrum to create a first demodulated signal component corresponding to a first signal component, the first signal component being representative of a lower resolution version of a second signal component, and providing data corresponding to the first demodulated signal component. 
         [0009]    An exemplary alternative method comprises encoding a first signal component to create an encoded first signal component, modulating the encoded first signal component across a subset of a frequency spectrum to create a modulated first signal component, encoding a second signal component to create an encoded second signal component, the encoded second signal component comprising data corresponding to the first signal component, and modulating the encoded second signal component across the frequency spectrum to create a modulated second signal component. The alternative exemplary embodiment further comprises transmitting the modulated first signal component and the modulated second signal component as a broadcast signal. 
         [0010]    An exemplary system may be adapted to process a received orthogonal frequency division multiplexing (OFDM) broadcast signal having a frequency spectrum. Such a system may comprise a circuit that is adapted to demodulate the OFDM broadcast signal over a subset of the frequency spectrum to create a first demodulated signal component corresponding to a first signal component, the first signal component being representative of a lower resolution version of a second signal component, and a circuit that is adapted to provide data corresponding to the first demodulated signal component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    In the drawings: 
           [0012]      FIG. 1  is a diagram showing an exemplary format in accordance with which data may be transmitted in an OFDM system; 
           [0013]      FIG. 2  is a graph showing an exemplary OFDM transmission waveform; 
           [0014]      FIG. 3  is a graph showing an OFDM waveform having a first signal component and a second signal component in accordance with an exemplary embodiment of the present invention; 
           [0015]      FIG. 4  is a block diagram of a system for transmitting and receiving OFDM signals in accordance with an exemplary embodiment of the present invention; 
           [0016]      FIG. 5  is a process flow diagram illustrating the operation of an exemplary embodiment of the present invention; and 
           [0017]      FIG. 6  is a process flow diagram illustrating the operation of an alternative exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]      FIG. 1  is a diagram showing an exemplary format in accordance with which data may be transmitted in an OFDM system. An exemplary symbol frame  1  illustrates the use of a training sequence  2 , multiple cyclic prefixes  4  and multiple blocks of user data  4 . The training sequence or symbol  2  may contain known transmission values for each subcarrier in the OFDM symbol, and a predetermined number of a cyclic prefixes  4  and user data pairs  6 . For example, the proposed ETSI-BRAN HIPERLAN/2 (Europe) and IEEE 802.11a (USA) wireless LAN standards, herein incorporated by reference, assigns 64 known values or subsymbols (i.e., 52 non-zero values and 12 zero values) to selected training symbols of a training sequence (e.g., “training symbol C” of the proposed ETSI standard and “long OFDM training symbol” of the proposed IEEE standard). 
         [0019]    The user data  6  may comprise a predetermined number of pilots  8 , also containing known transmission values, embedded on predetermined subcarriers. For example, the proposed ETSI and IEEE standards have four pilots located at bins or subcarriers ±7 and ±21. 
         [0020]      FIG. 2  is a graph showing an exemplary OFDM transmission waveform. The graph is generally referred to by the reference number  10 . The graph comprises an x-axis  12 , which corresponds to frequency, and a y-axis  14 , which corresponds to the amplitude of a signal. The overall OFDM spectrum of the illustrated OFDM channel is indicated by a bracket  18 . An OFDM signal  16  comprises multiple subcarriers, identified as a, b, c and so on in  FIG. 1 . Those of ordinary skill in the art will appreciate that the use of OFDM allows overlapping subcarrier bands (as shown in  FIG. 2 ) to be received and decoded accurately. 
         [0021]      FIG. 3  is a graph showing an OFDM waveform having a first signal component and a second signal component in accordance with an exemplary embodiment of the present invention. The graph is generally referred to by the reference number  100 . In order to improve reception of a portion of the OFDM spectrum, embodiments of the present invention may include a first signal component and a second signal component, as illustrated in  FIG. 3 . 
         [0022]    The graph  100  includes an x-axis  122 , which corresponds to frequency, and a y-axis  124 , which corresponds to the amplitude of a signal. An OFDM signal  125  comprises a first signal component, as illustrated by a bracket  126 , and a second signal component, as illustrated by an arrow  128 . The frequency spectrum of the entire OFDM channel comprises the combination of the first signal component  126  and the second signal component  128 , as illustrated by a bracket  130 . 
         [0023]    An exemplary embodiment of the present invention may be adapted to tune a portion of the frequency spectrum corresponding to the first signal component  126  to improve reception. Reception of the first signal component  126  may be performed in a first mode, while reception of the entire frequency spectrum (corresponding to the first and second signal components  130 ) may be performed in a second mode of operation. Moreover, the first signal component  126  may be a subset of the second signal component  128 . 
         [0024]    In the exemplary embodiment illustrated in  FIG. 3 , the first signal component  126  is separated from the second signal component  128  at either end by a guard band or guard interval, as illustrated by a bracket  132  and a bracket  134 . The first signal component  126  may be constructed such that it contains correct OFDM properties, such as a training sequence, a cyclic prefix, pilot signals, the use of 2 n  carriers, a guard band that complies with existing standards, and the like. 
         [0025]    As shown in  FIG. 3 , bandwidth associated with the first signal component  126  is smaller than the bandwidth associated with the second frequency component  128 . Accordingly, the first signal component  126  may be distributed among a smaller number of carriers than the entire frequency spectrum required for the channel. The smaller signal bandwidth results in improved reception properties. 
         [0026]    The second signal component  128  may also employ the carriers in the first signal component  126 , with no data. Additional carrier frequencies outside the bandwidth of the first signal component  126  may be employed by the second signal component  128 . Accordingly, the total number of carriers for the second signal component  128  (including the first signal component  126 ) may be a larger power of two than the number of carriers required for the first signal component  126  alone. By way of example, the first signal component  126  may be carried by 64 carriers of which 52 are active, and the second signal component  128  may add an additional 448 carriers (400 active), for a total of 512 carriers (452 active). 
         [0027]    The first signal component  126  may comprise a lower resolution version of the information carried by the entire frequency spectrum (the second signal component  128 ). In this case, embodiments of the present invention may include multiple resolution structures to support scaling, such as the specification(s) of the Joint Video Team (JVT) regarding advanced video coding. In low resolution mode, minimal information may be sent (as for a low resolution display). Additional resolution may be sent in portions of the second signal component  128 . This means that hardware adapted to employ lower resolution (e.g. a relatively small video screen) may tune only the first signal component, which would result in improved reception compared to tuning the entire frequency spectrum corresponding to the second signal component. The need for more complex tuning circuitry having a bandwidth reception capability suited to the reception of the entire broadcast frequency spectrum (including the second signal component) is not required in such devices. On the other hand, devices that can usefully employ sufficient bandwidth to accommodate the entire second signal component may nonetheless include alternative circuitry to tune and use only the first signal component under conditions where reception capability is reduced. 
         [0028]      FIG. 4  is a block diagram of a system for transmitting and receiving OFDM signals in accordance with an exemplary embodiment of the present invention. The block diagram is generally referred to by the reference number  200 . The functional blocks illustrated in  FIG. 4  may be implemented in hardware, software or some combination of both. The functions performed by each block may be split up and performed separately, or incorporated into other functional blocks with other functions. 
         [0029]    A transmitter portion of the system is indicated by an arrow  31 , and a receiver portion of the system is indicated by an arrow  33 . The transmitter portion  31  and receiver portion  33  may be implemented in a single transceiver unit, which would be capable of both sending and receiving OFDM signals. 
         [0030]    In the transmitter portion  31 , a data stream  32  that is intended to be transmitted is delivered to an encoder  34 . The encoder  34  separates the data stream  32  into information corresponding to an encoded first signal component  36  and an encoded second signal component  38 . The encoded first signal component  36  and the encoded second signal component  38  may correspond respectively to the first signal component  126  and the second signal component  128  illustrated in  FIG. 2 . As set forth above, the information that comprises the encoded first signal component  36  may be a subset of the encoded second signal component  38 . 
         [0031]    The encoded first signal component  36  a modulator and inverse fast forward Fourier transform block  40 . As shown in  FIG. 3 , the portion of the OFDM frequency spectrum represented by the encoded first signal component  36  may be thought of as a “core” of the entire frequency spectrum represented by the encoded second signal component  38 . The encoded second signal component  38  is delivered by the encoder  34  to a modulator and inverse fast Fourier transform block  42 . The modulator and inverse fast Fourier transform blocks  40  and  42  respectively deliver a modulated first signal component  41  and a modulated second signal component  43  to an RF up-converter block  44 . The RF up-converter block  44  is adapted to transmit the information in an OFDM format via an antenna  46 . 
         [0032]    An OFDM broadcast signal  48  is transmitted from the antenna  46  to a receiving antenna  50  of the receiving portion  33  of the system  200 . Upon receipt by the antenna  50 , the OFDM broadcast signal  48  is delivered to an RF receiver  52 . The RF receiver  52  delivers the signal to a fast Fourier transform block  54  and a fast Fourier transform block  56 . The fast Fourier transform block  54  may be adapted to process only the portion of the received frequency spectrum corresponding to the first signal component  126  ( FIG. 3 ). The fast Fourier transform block  56  may be adapted to process information corresponding to the second signal component  128  ( FIG. 3 ). 
         [0033]    The fast Fourier transform block  54  delivers output to a demodulator block  58 . Because the information processed by the fast Fourier transform block  54  represents a smaller bandwidth of the OFDM frequency spectrum of the channel being transmitted, that signal represents information that has a lower data rate than the entire frequency spectrum. As a result, a sample rate conversion may be needed to properly recover the signal. That sample rate conversion may be performed, for example, by a time base correction block  60 , which receives input from the demodulator block  58 . The time base correction block  60  then delivers input to a first signal decoder  62 . The first signal decoder  62  produces an output signal that corresponds to information contained in the first signal component  126  ( FIG. 3 ). 
         [0034]    After processing, the fast Fourier transform block  56  delivers output to a demodulator block  60 , which in turn provides an output to a second signal decoder  64 . The information processed by the demodulator block  60  and the second signal decoder  64  correspond to the second signal component  128  ( FIG. 3 ), which embodies the entire OFDM frequency spectrum for the channel that was received. 
         [0035]      FIG. 5  is a process flow diagram illustrating the operation of an exemplary embodiment of the present invention. The process is generally represented by the reference number  300 . 
         [0036]    At block  72 , the process begins. At block  74 , a first signal component is encoded to produce an encoded first signal component, such as the encoded first signal component  36  illustrated in  FIG. 4 . The encoded first signal component  36  is modulated, as shown at block  76 , to produce a modulated first signal component  41 . In an exemplary embodiment of the present invention, the modulated first signal component  41  is modulated across a subset of a frequency spectrum prior to transmission as an OFDM broadcast signal  48 . As set forth above, the resulting reduction in bandwidth relative to the full frequency spectrum may improve reception characteristics of data corresponding to the first signal component. 
         [0037]    At block  78 , a second signal component is encoded to create an encoded second signal component  38  ( FIG. 4 ). The encoded second signal component  38  comprises a superset of data corresponding to the first signal component  36  ( FIG. 4 ). At block  80 , the second signal component is modulated across the entire frequency spectrum corresponding to the broadcast signal to create a modulated second signal component  43  ( FIG. 4 ). The modulated first signal component  41  and the modulated second signal component  43  are then transmitted as an OFDM broadcast signal  48  ( FIG. 4 ), as shown at block  82 . At block  84  the process ends. 
         [0038]      FIG. 6  is a process flow diagram illustrating the operation of an alternative exemplary embodiment of the present invention. The process is generally referred to by the reference number  400 . 
         [0039]    At block  92 , the process begins. An OFDM broadcast signal is demodulated over a subset of its broadcast frequency spectrum, as shown at block  94 . The OFDM broadcast signal is also demodulated over the entire broadcast frequency spectrum, as illustrated at block  96 . As a result of the demodulation over a subset of the frequency spectrum, data corresponding to a first demodulated signal is provided at block  98 . This data corresponds to the first signal component  126  ( FIG. 3 ), which may be a lower resolution version of the data represented by the second signal component  128  ( FIG. 3 ). Additionally, data-corresponding to a second demodulated signal is provided at block  100  as a result of the demodulation of the OFDM broadcast signal over the entire broadcast frequency spectrum (block  96 ). At block  102 , the process ends. As set forth above, a user device that is suitable for displaying a lower resolution may be adapted to have improved reception by tuning only the portion of the broadcast spectrum corresponding to the first signal component. 
         [0040]    While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.