Patent Application
20080165892
The present invention relates, in general, to a wireless communications system, and more particularly, to using the preamble in an orthogonal frequency division multiple access (OFDMA)-based wireless communication system to indicate the number of guard tones.
In wideband wireless communication systems, the signal often tends to weaken from frequency selective fading due to multi-path transmissions. Frequency selective fading is a radio propagation anomaly generally caused by the partial cancellation of a radio signal by itself. As the signal arrives at the receiver by multiple different paths, and at least one of the paths is changing (lengthening or shortening), the combination of the multiple signals sometimes causes partial signal cancellations.
Orthogonal frequency division multiplexing (OFDM) systems have been proposed to overcome the problem of frequency selective fading by dividing the total bandwidth into multiple subcarriers, such that the bandwidth on each subcarrier is sufficiently narrow to enable the data modulation symbols carried by that subcarrier to experience relatively flat fading. An OFDMA system uses the OFDM modulation technique to multiplex the data traffic of several mobile stations in both frequency and time.
Superframe 103 is made up from preamble frame 101 and traffic frames 102 through 103. In an OFDM system, preamble frame 101 and traffic frame 102 consists of multiple OFDM symbols. For example, traffic frame 103 contains OFDM symbol 1-104, OFDM symbol 2-105, through OFDM symbol N-106. Each OFDM symbol, such as OFDM symbol 105, includes inverse fast Fourier transform (IFFT) symbol 109, which is the result of an IFFT operation on the modulation data sequence, cyclic prefix (CP) 108, which is a copy of the last portion of IFFT symbol 109 and is inserted before the IFFT symbol 109, and two windowing periods 107 and 110, which shape the modulation pulse so that the radio spectrum of the transmitted signal meets the emission mask requirement set forth by the radio regulatory body, such as the Federal Communication Commission (FCC) in the United States.
Preamble 101 of superframe 103 provides control information for a mobile station to acquire the base station signals in the power-up procedure or to continue to receive the signaling of the updated system parameters after the mobile station becomes active in the system.
When the carrier bandwidth of the particular system is 5 MHz or less, the FFT size of the preamble is typically the same size as the traffic frames, which is usually 512. However, in systems where the carrier bandwidth is greater than 5 MHz, the FFT size for the traffic frames may be 2 or more times higher than the preamble FFT size. Therefore, if the FFT size used in the preamble frames is 512, the mobile station will still need to decode the PBCCH in order to extract the exact FFT size used on the traffic frames. In step 301, the mobile station decodes the acquisition pilot, TDM2, to acquire the PilotPhase or PilotPN information, depending on the synchronization of the network.
The mobile station descrambles the acquisition pilot, TDM3, using the detected information contents in TDM2, as the scrambling seed, then, in step 302, decodes the information on TDM3. The information bits included in TDM3 are typically: (1) a 1-bit Sync/Asynch bit to indicate if the system is synchronous or asynchronous; (2) a 1-bit half-duplex bit to indicate if the half-duplex operation is supported; (3) a 1-bit frequency-reuse on preamble bit to indicate if the frequency-reuse is used on the PBCCH and SBCCH; and (4) the four least significant bits (LSBs) of the system time to indicate when the first sub-packet of the PBCCH encoded packet starts in an asynchronous sector.
In step 303, the mobile station decodes the PBCCH, which carries various information, including the exact FFT size used on the traffic frames, the number of guard tones used in the traffic frames, and the nine LSBs of the system time to enable the mobile stations to convert the PilotPhase to PilotPN for a synchronous system. In step 304, the mobile station decodes the SBCCH in the even-numbered superframes, which include enough sector configuration information to enable the mobile station to demodulate the forward link traffic channels. In step 305, the mobile station decodes the additional system configuration parameters that are broadcast in the overhead signaling messages via the traffic channels and enable the mobile station to start the random access procedure on the reverse link.
In a direct spread spectrum communication system, such as code division multiple access (CDMA)-based systems, because the energy is spread over the entire bandwidth, the CDMA channel occupies a certain bandwidth given a certain spreading factor or chip rate. In order to adapt the CDMA system to a variety of channel bandwidths efficiently, the chip rate is typically changed. Considering this operation, one advantage of OFDM or OFDMA-based systems is that two zones of guard tones (or guard sub-carriers) on the two edges of the bandwidth may be set up, such that there is generally no energy transmitted on these guard tones. Therefore, even though the FFT size of an OFDM system is limited to a few choices of 2's power, the effective occupied channel bandwidth may be very flexible by defining the number of guard tones, thereby allowing OFDM-based systems to fit into a wide range of spectrum easily.
Usually, a particular FFT size corresponds to a certain channel bandwidth if half of that FFT size cannot fit into the same channel bandwidth. This means that the number of guard tones can be almost as large as half of the FFT size.
As described previously, the mobile station acquires the exact number of guard tones in the traffic frames and SBCCH from the information contained in the PBCCH. However, the OFDM preamble may also contain a number of guard tones, but not necessarily the same number of guard tones as contained in the traffic frames and SBCCH.
A problem arises when the mobile station tries to demodulate and decode the PBCCH without knowing the number of guard tones that the base station may have applied. Without knowledge of the appropriate number of guard tones, the decoding may have erroneous results. One compromised solution in use provides for the mobile station to use the worst case scenario for determining the number of guard tones when decoding the PBCCH. Using this worst case assumption, when the allowable channel bandwidth is slightly larger than what half of the FFT size can fit into, the mobile station may ignore almost half of the modulation symbols on the PBCCH during decoding. The result of this compromise is the unnecessary loss of decoding performance on the PBCCH, thereby prolonging system acquisition delay.
Representative embodiments of the present invention provide methods for acquiring signal in an OFDMA-based network that includes receiving an OFDMA signal stream from a base station, decoding one or more acquisition pilots in a superframe preamble prior to decoding a primary broadcast control channel (PBCCH) symbol, detecting a guard tone code within the decoded one or more acquisition pilots, and decoding the PBCCH symbol using the guard tone code.
Additional representative embodiments of the present invention provide methods that include detecting a number of guard tones used in coding a superframe preamble in an OFDM network, encoding the number of guard tones into one or more acquisition pilots of the superframe preamble, and transmitting the superframe preamble in an OFDM data stream.
Additional representative embodiments of the present invention provide mobile stations that are made up from a processor, memory, a first decoding component stored in the memory, where the processor operates the first decoding component to decode one or more acquisition pilots in a superframe preamble of an OFDMA-based network communication stream, and where the first decoding component directs the mobile station to decode the one or more acquisition pilots prior to decoding a PBCCH symbol of the superframe preamble in an OFDMA-based network. The mobile stations also include a guard tone code table stored in the memory, where the processor accesses the guard tone code table during execution of the first decoding component upon detection of a guard tone code in the decoded one or more acquisition pilots, and a second decoding component in the memory, where the processor operates the second decoding component to decode the PBCCH symbol of the superframe preamble using information in the guard tone code table corresponding to the guard tone code.
Additional representative embodiments of the present invention provide base stations in an OFDMA-based network that include a processor, memory, a coding component operable by the processor to encode a number of guard tones used to code a superframe preamble, where the encoded number of guard tones is placed into one or more acquisition pilots of the superframe preamble, and a transmitter for transmitting a communication stream including at least the superframe preamble to a plurality of mobile stations.
Additional representative embodiments of the present invention provide computer program products having a computer readable medium with computer program logic recorded thereon, including code for receiving an OFDMA signal stream from a base station in an OFDMA-based network, code for decoding one or more acquisition pilots in a superframe preamble prior to decoding a PBCCH symbol, code for detecting a guard tone code within the decoded one or more acquisition pilots, and code for decoding the PBCCH symbol using the guard tone code.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
The present invention provides a unique method and system for indicating the number of guard tones on the preamble in an OFDM or OFDMA based communication system. It is understood, however, that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components, signals, messages, protocols, and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to limit the invention from that described in the claims. Well known elements are presented without detailed description in order not to obscure the present invention in unnecessary detail. For the most part, details unnecessary to obtain a complete understanding of the present invention have been omitted inasmuch as such details are within the skills of persons of ordinary skill in the relevant art. Details regarding control circuitry described herein are omitted, as such control circuits are within the skills of persons of ordinary skill in the relevant art.
In accordance with one aspect of the present invention, a method for indicating the number of guard tones on the preamble in an OFDM or OFDMA based communication system is disclosed, the method comprising: indicating the number of guard tones that is used on the preamble frames by the base station using at least one acquisition pilot; indicating the number of guard tones that is used on the traffic frames by the base station using the PBCCH; decoding the acquisition pilot(s) by the mobile station before decoding the PBCCH; using the information of the number of guard tones detected from the acquisition pilot(s) to demodulate and to decode the PBCCH; using the information of the number of guard tones detected from the acquisition pilot(s) to demodulate and to decode the SBCCH if the PBCCH is decoded correctly; and using the information of the number of guard tones detected from the PBCCH to demodulate and to decode the traffic frames.
In accordance with another aspect of the present invention, a second method for indicating the range of the number of guard tones on the preamble in an OFDM or OFDMA based communication system is disclosed, the method comprising: dividing all possible choices of the number of guard tones that can be used on the preamble frames into at least two exclusive groups wherein each group has at least one choice of number of guard tones that can be used on the preamble frames; indicating the group index to which the exact number of guard tones that is used on the preamble frames belongs by the base station using at least one acquisition pilot; indicating the number of guard tones that is used on the traffic frames by the base station using the PBCCH; decoding the acquisition pilot(s) by the mobile station before decoding the PBCCH; using the worse case scenario number of guard tones within the group of the group index that is detected from the acquisition pilot(s) to demodulate and to decode the PBCCH and SBCCH; and using the information of the number of guard tones used on the traffic that is detected from the PBCCH to demodulate and to decode the traffic frames.
Considering that the granularity of the traffic resource allocation is 16 sub-carriers, the granularity of guard tone allocation is 32 sub-carriers. Therefore, a 3-bit field (referred to as Bits 3, 2, and 1) of Number of Guard Tones on Preamble can indicate 8 choices of guard tones. For example, the 3-bit field of “000” corresponds to 0 guard tones, “001” corresponds to 32 guard tones, “010” corresponds to 64 guard tones, “011” corresponds to 96 guard tones, “100” corresponds to 128 guard tones, “101” corresponds to 160 guard tones, “110” corresponds to 192 guard tones, and “111” corresponds to 224 guard tones. If more guard tones are still needed, the system will try the 256 FFT size instead of 512.
Acquisition pilot TDM3402 carries the information regarding the number of guard tones used on the preamble frames. Acquisition pilot TDM3402 carries a total of 7 bits of information to fit into the smallest FFT size. For a larger FFT size, the time domain waveform is repeated to fit in. Among these 7 bits, a Sync/Async bit is located at a fixed bit position. For example, at the most significant bit (MSB) or Bit 7.
The Sync/Async bit indicates an asynchronous system for purposes of the embodiment illustrated in
In the synchronous system, the rest of the bits in TDM3405 are, for example, 1-bit Half-Duplex, Bits 3, 2, and 1 of the Number of Guard Tone on Preamble field, 1-bit to indicate the Frequency Reuse on Preamble, and 1-bit reserved bit. In the synchronous system, the complete field of the Number of Guard Tones on Preamble is indicated in TDM3405. Therefore, the mobile station, after decoding TDM3405, can use the exact number of guard tones to demodulate the PBCCH without losing any modulation symbols that the base station sends out.
Base station 700 also includes processor 708 and memory 709. When generating the communication stream, coding component 710, stored in memory 709, is executed by processor 708 to encode the number of guard tones used in coding the preamble superframe into the acquisition pilots. Coding component 710 may either provide a guard tone code that represents the actual number of guard tones used, or one that represents a number of subsets of guard tones that could be used on base station 700.
The various illustrative logical blocks, modules, and circuits described in connection with the embodiment disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein.
Bus 1002 is also coupled to input/output (I/O) controller card 1005, communications adapter card 1011, user interface card 1008, and display card 1009. The I/O adapter card 1005 connects storage devices 1006, such as one or more of a hard drive, a CD drive, a floppy disk drive, a tape drive, to computer system 1000. The I/O adapter 1005 is also connected to a printer (not shown), which would allow the system to print paper copies of information such as documents, photographs, articles, and the like. Note that the printer may be a printer (e.g., dot matrix, laser, and the like), a fax machine, scanner, or a copier machine.
The steps of a method or algorithm described in connection with the embodiments disclosed herein may be implemented or performed directly in hardware, in a software module executed by a processor, or in combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, or any other form of storage medium in the art.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, many of the features and functions discussed above can be implemented in software, hardware, or firmware, or a combination thereof. As another example, it will be readily understood by those skilled in the art that the techniques disclosed in the present invention can be used in a frequency division duplex (FDD) system as well as in a time division duplex (TDD) system or even further varied while remaining within the scope of the present invention.
Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
This application claims the benefit of U.S. Provisional Application No. 60/884,412, filed on Jan. 10, 2007, entitled “METHOD FOR INDICATING THE NUMBER OF GUARD TONES ON THE PREAMBLE IN AN OFDM BASED COMMUNICATION SYSTEM,” which application is hereby incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60884412 | Jan 2007 | US |
