`Vijayan et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,221,680 B2
`May 22, 2007
`
`USOO7221680B2
`
`(54)
`
`(75)
`
`(73)
`
`(*)
`
`(21)
`(22)
`(65)
`
`(60)
`
`(51)
`
`(52)
`(58)
`
`MULTIPLEXING AND TRANSMISSION OF
`MULTIPLE DATA STREAMS IN A WRELESS
`MULTI-CARRIER COMMUNICATION
`SYSTEM
`Inventors: Rajiv Vijayan, San Diego, CA (US);
`Aamod Khandekar, San Diego, CA
`(US); Fuyun Ling, San Diego, CA
`(US); Gordon Kent Walker, Poway,
`CA (US); Ramaswamy Murali, San
`Diego, CA (US)
`QUALCOMM Incorporated, San
`Diego, CA (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 61 days.
`Appl. No.: 10/932,586
`Filed:
`Sep. 1, 2004
`
`Assignee:
`
`Notice:
`
`Prior Publication Data
`US 2005/OO58O89 A1
`Mar 17, 2005
`
`Related U.S. Application Data
`Provisional application No. 60/499,741, filed on Sep.
`2, 2003, provisional application No. 60/559,740, filed
`on Apr. 5, 2004.
`
`Int. C.
`(2006.01)
`H04B 7/26
`U.S. Cl. ....................... 370/441; 370/335; 370/342
`Field of Classification Search ................ 370/431,
`370/310.2, 335,342, 441,330, 208
`See application file for complete search history.
`
`
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`6,618,353 B2 * 9/2003 Merrill et al. .............. 370,225
`2002/0085486 A1* 7/2002 Ehrmann-Patin et al. ... 370/210
`2003/008.6366 A1* 5/2003 Branlund et al. ........... 370,208
`2004/0266351 A1* 12/2004 Chuah et al. ................. 455,62
`
`* cited by examiner
`Primary Examiner Chi Pham
`Assistant Examiner—Ronald Abelson
`(74) Attorney, Agent, or Firm Thomas R. Rouse; Sandip S.
`Minhas; Albert J. Harnois, Jr.
`
`(57)
`
`ABSTRACT
`
`Techniques for multiplexing and transmitting multiple data
`streams are described. Transmission of the multiple data
`streams occurs in “super-frames'. Each Super-frame has a
`predetermined time duration and is further divided into
`multiple (e.g., four) frames. Each data block for each data
`stream is outer encoded to generate a corresponding code
`block. Each code block is partitioned into multiple sub
`blocks, and each data packet in each code block is inner
`encoded and modulated to generate modulation symbols for
`the packet. The multiple subblocks for each code block are
`transmitted in the multiple frames of the same Super-frame,
`one Subblock per frame. Each data stream is allocated a
`number of transmission units in each Super-frame and is
`assigned specific transmission units to achieve efficient
`packing. A wireless device can select and receive individual
`data streams.
`
`35 Claims, 15 Drawing Sheets
`
`IPR2018-1581
`HTC EX1004, Page 1
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`U.S. Patent
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`May 22, 2007
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`Sheet 1 of 15
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`US 7,221,680 B2
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`
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`FIG.2
`
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`May 22, 2007
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`Sheet 2 of 15
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`US 7,221,680 B2
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`IPR2018-1581
`HTC EX1004, Page 3
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`U.S. Patent
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`May 22, 2007
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`Sheet 3 of 15
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`US 7,221,680 B2
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`U.S. Patent
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`May 22, 2007
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`US 7,221,680 B2
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`HTC EX1004, Page 5
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`U.S. Patent
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`May 22, 2007
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`HTC EX1004, Page 6
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`May 22, 2007
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`HTC EX1004, Page 7
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`U.S. Patent
`
`May 22, 2007
`
`Sheet 7 Of 15
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`US 7,221,680 B2
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`IPR2018-1581
`HTC EX1004, Page 8
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`
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`U.S. Patent
`
`May 22, 2007
`
`Sheet 8 of 15
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`US 7,221,680 B2
`
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`IPR2018-1581
`HTC EX1004, Page 9
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`
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`U.S. Patent
`
`May 22, 2007
`
`Sheet 9 Of 15
`
`US 7,221,680 B2
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`IPR2018-1581
`HTC EX1004, Page 10
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`U.S. Patent
`
`May 22, 2007
`
`Sheet 10 of 15
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`US 7,221,680 B2
`
`Packet Interleaving across Code Blocks
`
`952
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`IPR2018-1581
`HTC EX1004, Page 11
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`
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`U.S. Patent
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`May 22, 2007
`
`Sheet 11 of 15
`
`US 7,221,680 B2
`
`1000
`
`ldentify active PLCs for
`the Current Super-frame
`
`PrOCeSS at least One data block
`for each active PLC to obtain at
`least One Code block for the PLC
`
`012
`
`1014
`
`1016
`
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`specific number of transmission
`units (or slots) based on its payload
`1018
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`in the Current Super-frame (e.g.,
`in a rectangular pattern or a zigzag
`segment) to each active PLC
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`1020
`
`Partition each Code block into
`multiple subblocks, one subblock for
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`1022
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`the packets in each subblock to obtain
`modulation symbols for the subblock
`1024
`For each frame of the current super
`frame, multiplex the data symbols
`in the Subblock(s) to be sent in that
`frame for each active PLC onto the
`transmission units assigned to the PLC
`f026
`Form a composite symbol stream with
`the multiplexed data symbols for all active
`PLCs and overhead symbols for the PLCs
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`IPR2018-1581
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`HTC EX1004, Page 13
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`U.S. Patent
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`US 7,221,680 B2
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`US 7,221,680 B2
`
`1.
`MULTIPLEXING AND TRANSMISSION OF
`MULTIPLE DATA STREAMS IN A WRELESS
`MULTI-CARRIER COMMUNICATION
`SYSTEM
`
`2
`There is therefore a need in the art for techniques to
`transmit multiple data streams in a multi-carrier system Such
`that they can be received by wireless devices, with minimal
`power consumption.
`
`This application claims the benefit of provisional U.S.
`Application Ser. No. 60/499,741, entitled “A Method for
`Multiplexing and Transmitting Multiple Multimedia
`Streams to Mobile Terminals over Terrestrial Radio Links.”
`filed Sep. 2, 2003, and provisional U.S. Application Ser. No.
`60/559,740, entitled “Multiplexing and Transmission of
`Multiple Data Streams in a Wireless Multi-Carrier Commu
`nication System,” filed Apr. 5, 2004.
`
`10
`
`BACKGROUND
`
`15
`
`SUMMARY
`
`Techniques for multiplexing and transmitting multiple
`data streams in a manner to facilitate power-efficient and
`robust reception of individual data streams by wireless
`devices are described herein. Each data stream is processed
`separately based on a coding and modulation scheme (e.g.,
`an outer code, an inner code, and a modulation scheme)
`selected for that stream to generate a corresponding data
`symbol stream. This allows the data streams to be individu
`ally recovered by the wireless devices. Each data stream is
`also allocated certain amount of resources for transmission
`of that stream. The allocated resources are given in “trans
`mission units’ on a time-frequency plane, where each trans
`mission unit corresponds to one Subband in one symbol
`period and may be used to transmit one data symbol. The
`data symbols for each data stream are mapped directly onto
`the transmission units allocated to the stream. This allows
`the wireless devices to recover each data stream indepen
`dently, without having to process the other data streams
`being transmitted simultaneously.
`In an embodiment, transmission of the multiple data
`streams occurs in “super-frames', with each Super-frame
`having predetermined time duration (e.g., on the order of a
`second or few seconds). Each super-frame is further divided
`into multiple (e.g., two, four, or some other number of)
`frames. For each data stream, each data block is processed
`(e.g., outer encoded) to generate a corresponding code
`block. Each code block is partitioned into multiple sub
`blocks, and each Subblock is further processed (e.g., inner
`encoded and modulated) to generate a corresponding Sub
`block of modulation symbols. Each code block is transmit
`ted in one super-frame, and the multiple subblocks for the
`code block are transmitted in the multiple frames of the
`Super-frame, one Subblock per frame. The partitioning of
`each code block into multiple subblocks, the transmission of
`these subblocks over multiple frames, and the use of block
`coding across the subblocks of the code block provide robust
`reception performance in slowly time-varying fading chan
`nels.
`Each data stream may be “allocated a variable number of
`transmission units in each Super-frame depending on the
`stream's payload in the super-frame, the availability of
`transmission units in the Super-frame, and possibly other
`factors. Each data stream is also “assigned specific trans
`mission units within each Super-frame using an assignment
`scheme that attempts to (1) pack the transmission units for
`all data streams as efficiently as possible, (2) reduce the
`transmission time for each data stream, (3) provide adequate
`time-diversity, and (4) minimize the amount of signaling to
`indicate the specific transmission units assigned to each data
`stream. Overhead signaling for various parameters of the
`data streams (e.g., the coding and modulation scheme used
`for each data stream, the specific transmission units assigned
`to each data stream, and so on) may be transmitted prior to
`each super-frame and may also be embedded within the data
`payload of each data stream. This allows a wireless device
`to determine the time-frequency location of each desired
`data stream in the upcoming Super-frame. The wireless
`device may power on only when the desired data stream is
`transmitted, using the embedded overhead signaling, and
`thereby minimize power consumption.
`
`I. Field
`The present invention relates generally to communication,
`and more specifically to techniques for multiplexing and
`transmitting multiple data streams in a wireless multi-carrier
`communication system.
`II. Background
`A multi-carrier communication system utilizes multiple
`carriers for data transmission. These multiple carriers may
`be provided by orthogonal frequency division multiplexing
`(OFDM), some other multi-carrier modulation techniques,
`or some other construct. OFDM effectively partitions the
`overall system bandwidth into multiple orthogonal sub
`bands. These Subbands are also referred to as tones, carriers,
`subcarriers, bins, and frequency channels. With OFDM,
`each subband is associated with a respective subcarrier that
`may be modulated with data.
`A base station in a multi-carrier system may simulta
`neously transmit multiple data streams for broadcast, mul
`ticast, and/or unicast services. A data stream is a stream of
`data that may be of independent reception interest to a
`wireless device. A broadcast transmission is sent to all
`wireless devices within a designated coverage area, a mul
`ticast transmission is sent to a group of wireless devices, and
`a unicast transmission is sent to a specific wireless device.
`For example, a base station may broadcast a number of data
`streams for multimedia (e.g., television) programs via a
`terrestrial radio link for reception by wireless devices. This
`system may employ a conventional multiplexing and trans
`mission scheme Such as, for example, Digital Video Broad
`casting-Terrestrial (DVB-T) or Integrated Services Digital
`Broadcasting-Terrestrial (ISDB-T). Such a scheme would
`first multiplex all of the data streams to be transmitted onto
`a single high-rate composite stream and then process (e.g.,
`encode, modulate, and up-convert) the composite stream to
`generate a modulated signal for broadcast via the radio link.
`A wireless device within the coverage area of the base
`station may be interested in receiving only one or few
`specific data streams among the multiple data streams car
`ried by the composite stream. The wireless device would
`need to process (e.g., down-convert, demodulate, and
`decode) a received signal to obtain a high-rate decoded data
`stream and then demultiplex this stream to obtain the one or
`few specific data streams of interest. This type of processing
`may not be a problem for receiver units intended to be
`powered on all the time. Such as those used in homes.
`However, many wireless devices are portable and powered
`by internal batteries. Continuous demodulation and decod
`ing of the high-rate composite stream to recover just one or
`few data streams of interest consumes significant amounts of
`power. This can greatly shorten the “ON” time for the
`wireless devices, which is undesirable.
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`Various aspects and embodiments of the invention are
`described in further detail below.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The features and nature of the present invention will
`become more apparent from the detailed description set
`forth below when taken in conjunction with the drawings in
`which like reference characters identify correspondingly
`throughout and wherein:
`FIG. 1 shows a wireless multi-carrier system;
`FIG. 2 shows an exemplary Super-frame structure;
`FIGS. 3A and 3B illustrate transmission of one data block
`and multiple data blocks, respectively, on a physical layer
`channel (PLC) in a super-frame;
`FIG. 4 shows a frame structure in a time-frequency plane;
`FIG. 5A shows a burst-TDM (time division multiplex)
`scheme;
`FIG. 5B shows a cycled-TDM scheme:
`FIG. 5C shows a burst-TDM/FDM (frequency division
`multiplex) scheme:
`FIG. 6 shows an interlaced subband structure;
`FIG. 7A shows assignment of slots to PLCs in rectangular
`patterns;
`FIG. 7B shows assignment of slots to PLCs in “Zigzag
`Segments;
`FIG. 7C shows assignment of slots to two joint PLCs in
`rectangular patterns;
`FIG. 8 illustrates coding of a data block with an outer
`code;
`FIGS. 9A and 9B show assignment of slots for one data
`block using one Subband group and a maximum allowable
`number of Subband groups, respectively;
`FIG. 9C shows assignment of slots for six data blocks;
`FIGS. 9D and 9E show assignment of slots to two joint
`PLCs with rectangular patterns stacked horizontally and
`vertically, respectively;
`FIG. 10 shows a process for broadcasting multiple data
`Streams;
`FIG. 11 shows a block diagram of a base station;
`FIG. 12 shows a block diagram of a wireless device:
`FIG. 13 shows a block diagram of a transmit (TX) data
`processor, a channelizer, and an OFDM modulator at the
`base station; and
`FIG. 14 shows a block diagram of a data stream processor
`for one data stream.
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`DETAILED DESCRIPTION
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`4
`the coverage area of the system. A wireless device may be
`fixed or mobile and may also be referred to as a user
`terminal, a mobile station, user equipment, or some other
`terminology. A wireless device may also be a portable unit
`Such as a cellular phone, a handheld device, a wireless
`module, a personal digital assistant (PDA), and so on.
`Each base station 110 may broadcast multiple data
`streams simultaneously to wireless devices within its cov
`erage area. These data streams may be for multimedia
`content Such as video, audio, tele-text, data, video/audio
`clips, and so on. For example, a single multimedia (e.g.,
`television) program may be sent in three separate data
`streams for video, audio, and data. A single multimedia
`program may also have multiple audio data streams, e.g., for
`different languages. For simplicity, each data stream is sent
`on a separate physical layer channel (PLC). There is thus a
`one-to-one relationship between data streams and PLCs. A
`PLC may also be called a data channel, a traffic channel, or
`Some other terminology.
`FIG. 2 shows an exemplary super-frame structure that
`may be used for broadcast system 100. Data transmission
`occurs in units of Super-frames 210. Each Super-frame has a
`predetermined time duration, which may be selected based
`on various factors such as, for example, the desired statis
`tical multiplexing for the data streams, the desired amount of
`time diversity, acquisition time for the data streams, buffer
`requirements for the wireless devices, and so on. A larger
`super-frame size provides more time diversity and better
`statistical multiplexing of the data streams being transmit
`ted, so that less buffering may be required for individual data
`streams at the base station. However, a larger Super-frame
`size also results in a longer acquisition time for a new data
`stream (e.g., at power-on or when Switching between data
`streams), requires larger buffers at the wireless devices, and
`also has longer decoding latency or delay. A Super-frame
`size of approximately one second may provide good tradeoff
`between the various factors described above. However,
`other Super-frame sizes (e.g., a quarter, a half, two, or four
`seconds) may also be used. Each Super-frame is further
`divided into multiple equal-sized frames 220. For the
`embodiment shown in FIG. 2, each super-frame is divided
`into four frames.
`The data stream for each PLC is encoded and modulated
`based on a coding and modulation scheme selected for that
`PLC. In general, a coding and modulation scheme comprises
`all of the different types of encoding and modulation to be
`performed on a data stream. For example, a coding and
`modulation scheme may comprise a particular coding
`scheme and a particular modulation scheme. The coding
`scheme may comprise error detection coding (e.g., a cyclic
`redundancy check (CRC)), forward error correction coding,
`and so on, or a combination thereof. The coding scheme may
`also indicate a particular code rate of a base code. In an
`embodiment that is described below, the data stream for each
`PLC is encoded with a concatenated code comprised of an
`outer coder and an inner code and is further modulated based
`on a modulation scheme. As used herein, a “mode” refers to
`a combination of an inner code rate and a modulation
`scheme.
`FIG. 3A illustrates the transmission of a data block on a
`PLC in a super-frame. The data stream to be sent on the PLC
`is processed in data blocks. Each data block contains a
`particular number of information bits and is first encoded
`using an outer code to obtain a corresponding code block.
`Each code block is partitioned into four subblocks, and the
`bits in each Subblock are further encoded using an inner code
`and then mapped to modulation symbols, based on the mode
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`The word “exemplary' is used herein to mean “serving as
`an example, instance, or illustration.” Any embodiment or
`design described herein as “exemplary' is not necessarily to
`be construed as preferred or advantageous over other
`embodiments or designs.
`The multiplexing and transmission techniques described
`55
`herein may be used for various wireless multi-carrier com
`munication systems. These techniques may also be used for
`broadcast, multicast, and unicast services. For clarity, these
`techniques are described for an exemplary multi-carrier
`broadcast system.
`FIG. 1 shows a wireless multi-carrier broadcast system
`100. System 100 includes a number of base stations 110 that
`are distributed throughout the system. A base station is
`generally a fixed station and may also be referred to as an
`access point, a transmitter, or Some other terminology.
`Neighboring base stations may broadcast the same or dif
`ferent content. Wireless devices 120 are located throughout
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`selected for the PLC. The four subblocks of modulation
`symbols are then transmitted in the four frames of one
`Super-frame, one Subblock per frame. The transmission of
`each code block over four frames provides time diversity
`and robust reception performance in a slowly time-varying
`fading channel.
`FIG. 3B illustrates the transmission of multiple (N) data
`blocks on a PLC in a super-frame. Each of the N data
`blocks is encoded separately using the outer code to obtain
`a corresponding code block. Each code block is further
`partitioned into four subblocks, which are inner encoded and
`modulated based on the mode selected for the PLC and then
`transmitted in the four frames of one super-frame. For each
`frame, N subblocks for the N code blocks are transmitted
`in a portion of the frame that has been allocated to the PLC.
`Each data block may be encoded and modulated in
`various manners. An exemplary concatenated coding
`scheme is described below. To simplify the allocation and
`assignment of resources to the PLCs, each code block may
`be divided into four equal-sized subblocks that are then
`transmitted in the same portion or location of the four frames
`in one Super-frame. In this case, the allocation of a Super
`frame to the PLCs is equivalent to the allocation of a frame
`to the PLCs. Hence, resources can be allocated to the PLCs
`once every Super-frame.
`Each PLC may be transmitted in a continuous or non
`continuous manner, depending on the nature of the data
`stream being carried by that PLC. Thus, a PLC may or may
`not be transmitted in any given Super-frame. For each
`Super-frame, an “active PLC is a PLC that is being trans
`mitted in that super-frame. Each active PLC may carry one
`or multiple data blocks in the super-frame.
`Referring back to FIG. 2, each super-frame 210 is pre
`ceded by a pilot and overhead section 230. In an embodi
`ment, section 230 includes (1) one or more pilot OFDM
`symbols used by the wireless devices for frame synchroni
`Zation, frequency acquisition, timing acquisition, channel
`estimation, and so on, and (2) one or more overhead OFDM
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`symbols used to carry overhead signaling information for
`the associated (e.g., immediately following) Super-frame.
`The overhead information indicates, for example, the spe
`cific PLCs being transmitted in the associated Super-frame,
`the specific portion of the Super-frame used to send the data
`block(s) for each PLC, the outer code rate and mode used for
`each PLC, and so on. The overhead OFDM symbol(s)
`carries overhead signaling for all PLCs sent in the Super
`frame. The transmission of the pilot and overhead informa
`tion in a time division multiplexed (TDM) manner allows
`50
`the wireless devices to process this section with minimal ON
`time. In addition, overhead information pertaining to each
`PLC’s transmission in the next super-frame may be embed
`ded in one of the PLC’s transmitted data blocks in the
`current super-frame. The embedded overhead information
`allows the wireless device to recover the PLC’s transmission
`in the next Super-frame without having to check the over
`head OFDM symbol(s) sent in that super-frame. Thus, the
`wireless devices may initially use the overhead OFDM
`symbols to determine the time-frequency location of each
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`desired data stream, and may subsequently power on only
`during the time that the desired data stream is transmitted
`using the embedded overhead signaling. These signaling
`techniques may provide significant savings in power con
`Sumption and allow the wireless devices to receive content
`using standard batteries. Since the outer code rate and mode
`used for each PLC typically do not vary on a super-frame
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`basis, the outer code rate and mode may be sent on a separate
`control channel and do need not be sent in every Super
`frame.
`FIG. 2 shows a specific Super-frame structure. In general,
`a Super-frame may be defined to be of any time duration and
`may be divided into any number of frames. Pilot and
`overhead information may also be sent in other manners
`different from the manner shown in FIG. 2. For example,
`overhead information may be sent on dedicated subbands
`using frequency division multiplexing (FDM).
`FIG. 4 shows the structure of one frame on a time
`frequency plane. The horizontal axis represents time, and the
`vertical axis represents frequency. Each frame has a prede
`termined time duration, which is given in units of OFDM
`symbol periods (or simply, symbol periods). Each OFDM
`symbol period is the time duration to transmit one OFDM
`symbol (described below). The specific number of symbol
`periods per frame (N) is determined by the frame duration
`and the symbol period duration, which in turn is determined
`by various parameters such as the overall system bandwidth,
`the total number of subbands (N), and the cyclic prefix
`length (described below). In an embodiment, each frame has
`a duration of 297 symbol periods (or N-297). Each frame
`also covers the N, total Subbands, which are given indices
`of 1 through N.
`With OFDM, one modulation symbol may be sent on each
`Subband in each symbol period, i.e., each transmission unit.
`Of the N, total Subbands, N, Subbands may be used for
`data transmission and are referred to as “data Subbands,
`N subbands may be used for pilot and are referred to as
`“pilot Subbands, and the remaining N, Subbands may be
`used as "guard” Subbands (i.e., no data or pilot transmis
`sion), where N. Na+N+N. The number of “usable”
`Subbands is equal to the number of data and pilot Subbands,
`or N. Na+N. In an embodiment, broadcast system
`100 utilizes an OFDM structure having 4096 total subbands
`(N=4096), 3500 data subbands (N=3500), 500 pilot
`subbands (N=500), and 96 guard subbands (N=96).
`Other OFDM structures with different number of data, pilot,
`usable, and total subbands may also be used. In each OFDM
`symbol period, N, data symbols may be sent on the N.
`data Subbands, N, pilot symbols may be sent on the N.,
`pilot Subbands, and N, guard symbols are sent on the N.,
`guard Subbands. As used herein, a "data symbol is a
`modulation symbol for data, a “pilot symbol is a modula
`tion symbol for pilot, and a 'guard symbol is a signal value
`of Zero. The pilot symbols are known a priori by the wireless
`devices. The N, data symbols in each OFDM symbol may
`be for one or multiple PLCs.
`In general, any number of PLCs may be transmitted in
`each Super-frame. For a given Super-frame, each active PLC
`may carry one or multiple data blocks. In one embodiment,
`a specific mode and a specific outer code rate is used for each
`active PLC, and all data blocks for the PLC are encoded and
`modulated in accordance with this outer code rate and mode
`to generate corresponding code blocks and Subblocks of
`modulation symbols, respectively. In another embodiment,
`each data block may be encoded and modulated in accor
`dance with a specific outer code rate and mode to generate
`a corresponding code block and Subblocks of modulation
`symbols, respectively. In any case, each code block contains
`a specific number of data symbols, which is determined by
`the mode used for that code block.
`Each active PLC in a given super-frame is allocated a
`specific amount of resources to transmit that PLC in the
`Super-frame. The amount of resources allocated to each
`active PLC is dependent on (1) the number of code blocks
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`to be sent on the PLC in the super-frame, (2) the number of
`data symbols in each code block, and (3) the number of code
`blocks, along with the number of data symbols per code
`block, to be sent on other PLCs. Resources may be allocated
`in various manners. Two exemplary allocation schemes are
`described below.
`FIG. 5A shows a burst-TDM allocation scheme. For this
`scheme, each active PLC is allocated all N, data Subbands
`in one or more OFDM symbol periods. For the example
`shown in FIG. 5A, PLC 1 is allocated all data Subbands in
`symbol periods 1 through 3, PLC 2 is allocated all data
`subbands in symbol periods 4 and 5, and PLC 3 is allocated
`all data subbands in symbol periods 6 through 9. For this
`scheme, each OFDM symbol contains data symbols for only
`one PLC. The bursts of OFDM symbols for different PLCs
`are time division multiplexed within a frame.
`If consecutive OFDM symbols are assigned to each active
`PLC, then the burst-TDM can minimize the transmission
`time for the PLCs. However, the short transmission time for
`each PLC also results in less time diversity. Since an entire
`OFDM symbol is allocated to one PLC, the granularity of
`the resource allocation (i.e., the Smallest unit that may be
`allocated to a PLC) for each frame is one OFDM symbol.
`The number of information bits t