Patent Publication Number: US-2012026924-A1

Title: MAC Packet Data Unit Construction for Wireless Systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is the first application for the present disclosure. 
     MICROFICHE APPENDIX 
     Not applicable. 
     TECHNICAL FIELD 
     This application relates to wireless communication techniques in general, and to technique of the disclosure, in particular. 
     ART RELATED TO THE APPLICATION 
     Draft IEEE 802.16m System Description Document, IEEE 802.16m-08/003r1, dated Apr. 15, 2008, it is stated that:
         This [802.16m] standard amends the IEEE 802.16 WirelessMAN-OFDMA specification to provide an advanced air interface for operation in licensed bands. It meets the cellular layer requirements of IMT-Advanced next generation mobile networks. This amendment provides continuing support for legacy WirelessMAN-OFDMA equipment.   And the standard will address the following purpose:
           i. The purpose of this standard is to provide performance improvements necessary to support future advanced services and applications, such as those described by the ITU in Report ITU-R M.2072.   
               

       FIGS. 7-13  of the present application correspond to  FIGS. 1-7  of IEEE 802.16m-08/003r1. 
     SUMMARY 
     Aspects and features of the present application will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of a disclosure in conjunction with the accompanying drawing figures and appendices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present application will now be described, by way of example only, with reference to the accompanying drawing figures, wherein: 
         FIG. 1  is a block diagram of a cellular communication system; 
         FIG. 2  is a block diagram of an example base station that might be used to implement some embodiments of the present 5 application; 
         FIG. 3  is a block diagram of an example wireless terminal that might be used to implement some embodiments of the present application; 
         FIG. 4  is a block diagram of an example relay station that might be used to implement some embodiments of the present application; 
         FIG. 5  is a block diagram of a logical breakdown of an example OFDM transmitter architecture that might be used to implement some embodiments of the present application; 
         FIG. 6  is a block diagram of a logical breakdown of an example OFDM receiver architecture that might be used to implement some embodiments of the present application; 
         FIG. 7  is  FIG. 1  of IEEE 802.16m-08/003r1, an Example of overall network architecture; 
         FIG. 8  is  FIG. 2  of IEEE 802.16m-08/003r1, a Relay Station in overall network architecture; 
         FIG. 9  is  FIG. 3  of IEEE 802.16m-08/003r1, a System Reference Model; 
         FIG. 10  is  FIG. 4  of IEEE 802.16m-08/003r1, The IEEE 802.16m Protocol Structure; 
         FIG. 11  is  FIG. 5  of IEEE 802.16m-08/003r1, The IEEE 802.16m MS/BS Data Plane Processing Flow; 
         FIG. 12  is  FIG. 6  of IEEE 802.16m-08/003r1, The IEEE 802.16m MS/BS Control Plane Processing Flow; and 
         FIG. 13  is  FIG. 7  of IEEE 802.16m-08/003r1, Generic protocol architecture to support multicarrier system. 
     
    
    
     Like reference numerals are used in different figures to denote similar elements. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Wireless System Overview 
     Referring to the drawings,  FIG. 1  shows a base station controller (BSC)  10  which controls wireless communications within multiple cells  12 , which cells are served by corresponding base stations (BS)  14 . In some configurations, each cell is further divided into multiple sectors  13  or zones (not shown). In general, each base station  14  facilitates communications using OFDM with mobile and/or wireless terminals  16 , which are within the cell  12  associated with the corresponding base station  14 . The movement of the mobile terminals  16  in relation to the base stations  14  results in significant fluctuation in channel conditions. As illustrated, the base stations  14  and mobile terminals  16  may include multiple antennas to provide spatial diversity for communications. In some configurations, relay stations  15  may assist in communications between base stations  14  and wireless terminals  16 . Wireless terminals  16  can be handed off  18  from any cell  12 , sector  13 , zone (not shown), base station  14  or relay  15  to an other cell  12 , sector  13 , zone (not shown), base station  14  or relay  15 . In some configurations, base stations  14  communicate with each and with another network (such as a core network or the internet, both not shown) over a backhaul network  11 . In some configurations, a base station controller  10  is not needed. 
     With reference to  FIG. 2 , an example of a base station  14  is illustrated. The base station  14  generally includes a control system  20 , a baseband processor  22 , transmit circuitry  24 , receive circuitry  26 , multiple antennas  28 , and a network interface  30 . The receive circuitry  26  receives radio frequency signals bearing information from one or more remote transmitters provided by mobile terminals  16  (illustrated in  FIG. 3 ) and relay stations  15  (illustrated in  FIG. 4 ). A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  22  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. As such, the baseband processor  22  is generally implemented in one or more digital signal processors (DSPs) or application-specific integrated circuits (ASICs). The received information is then sent across a wireless network via the network interface  30  or transmitted to another mobile terminal  16  serviced by the base station  14 , either directly or with the assistance of a relay  15 . 
     On the transmit side, the baseband processor  22  receives digitized data, which may represent voice, data, or control information, from the network interface  30  under the control of control system  20 , and encodes the data for transmission. The encoded data is output to the transmit circuitry  24 , where it is modulated by one or more carrier signals having a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signals to the antennas  28  through a matching network (not shown). Modulation and processing details are described in greater detail below. 
     With reference to  FIG. 3 , an example of a mobile terminal  16  is illustrated. Similarly to the base station  14 , the mobile terminal  16  will include a control system  32 , a baseband processor  34 , transmit circuitry  36 , receive circuitry  38 , multiple antennas  40 , and user interface circuitry  42 . The receive circuitry  38  receives radio frequency signals bearing information from one or more base stations  14  and relays  15 . A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  34  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. The baseband processor  34  is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs). 
     For transmission, the baseband processor  34  receives digitized data, which may represent voice, video, data, or control information, from the control system  32 , which it encodes for transmission. The encoded data is output to the transmit circuitry  36 , where it is used by a modulator to modulate one or more carrier signals that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  40  through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the mobile terminal and the base station, either directly or via the relay station. 
     In OFDM modulation, the transmission band is divided into multiple, orthogonal carrier waves. Each carrier wave is modulated according to the digital data to be transmitted. Because OFDM divides the transmission band into multiple carriers, the bandwidth per carrier decreases and the modulation time per carrier increases. Since the multiple carriers are transmitted in parallel, the transmission rate for the digital data, or symbols, on any given carrier is lower than when a single carrier is used. 
     OFDM modulation utilizes the performance of an Inverse Fast Fourier Transform (IFFT) on the information to be transmitted. For demodulation, the performance of a Fast Fourier Transform (FFT) on the received signal recovers the transmitted information. In practice, the IFFT and FFT are provided by digital signal processing carrying out an Inverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform (DFT), respectively. Accordingly, the characterizing feature of OFDM modulation is that orthogonal carrier waves are generated for multiple bands within a transmission channel. The modulated signals are digital signals having a relatively low transmission rate and capable of staying within their respective bands. The individual carrier waves are not modulated directly by the digital signals. Instead, all carrier waves are modulated at once by IFFT processing. 
     In operation, OFDM is preferably used for at least downlink transmission from the base stations  14  to the mobile terminals  16 . Each base station  14  is equipped with “n” transmit antennas  28  (n&gt;=1), and each mobile terminal  16  is equipped with “m” receive antennas  40  (m&gt;=1). Notably, the respective antennas can be used for reception and transmission using appropriate duplexers or switches and are so labelled only for clarity. 
     When relay stations  15  are used, OFDM is preferably used for downlink transmission from the base stations  14  to the relays  15  and from relay stations  15  to the mobile terminals  16 . 
     With reference to  FIG. 4 , an example of a relay station  15  is illustrated. Similarly to the base station  14 , and the mobile terminal  16 , the relay station  15  will include a control system  132 , a baseband processor  134 , transmit circuitry  136 , receive circuitry  138 , multiple antennas  130 , and relay circuitry  142 . The relay circuitry  142  enables the relay  14  to assist in communications between a base station  16  and mobile terminals  16 . The receive circuitry  138  receives radio frequency signals bearing information from one or more base stations  14  and mobile terminals  16 . A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  134  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. The baseband processor  134  is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs). 
     For transmission, the baseband processor  134  receives digitized data, which may represent voice, video, data, or control information, from the control system  132 , which it encodes for transmission. The encoded data is output to the transmit circuitry  136 , where it is used by a modulator to modulate one or more carrier signals that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  130  through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the mobile terminal and the base station, either directly or indirectly via a relay station, as described above. 
     With reference to  FIG. 5 , a logical OFDM transmission architecture will be described. Initially, the base station controller  10  will send data to be transmitted to various mobile terminals  16  to the base station  14 , either directly or with the assistance of a relay station  15 . The base station  14  may use the channel quality indicators (CQIs) associated with the mobile terminals to schedule the data for transmission as well as select appropriate coding and modulation for transmitting the scheduled data. The CQIs may be directly from the mobile terminals  16  or determined at the base station  14  based on information provided by the mobile terminals  16 . In either case, the CQI for each mobile terminal  16  is a function of the degree to which the channel amplitude (or response) varies across the OFDM frequency band. 
     Scheduled data  44 , which is a stream of bits, is scrambled in a manner reducing the peak-to-average power ratio associated with the data using data scrambling logic  46 . A cyclic redundancy check (CRC) for the scrambled data is determined and appended to the scrambled data using CRC adding logic  48 . Next, channel coding is performed using channel encoder logic  50  to effectively add redundancy to the data to facilitate recovery and error correction at the mobile terminal  16 . Again, the channel coding for a particular mobile terminal  16  is based on the CQI. In some implementations, the channel encoder logic  50  uses known Turbo encoding techniques. The encoded data is then processed by rate matching logic  52  to compensate for the, data expansion associated with encoding. 
     Bit interleaver logic  54  systematically reorders the bits in the encoded data to minimize the loss of consecutive data bits. The resultant data bits are systematically mapped into corresponding symbols depending on the chosen baseband modulation by mapping logic  56 . Preferably, Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key (QPSK) modulation is used. The degree of modulation is preferably chosen based on the CQI for the particular mobile terminal. The symbols may be systematically reordered to further bolster the immunity of the transmitted signal to periodic data loss caused by frequency selective fading using symbol interleaver logic  58 . 
     At this point, groups of bits have been mapped into symbols representing locations in an amplitude and phase constellation. When spatial diversity is desired, blocks of symbols are then processed by space-time block code (STC) encoder logic  60 , which modifies the symbols in a fashion making the transmitted signals more resistant to interference and more readily decoded at a mobile terminal  16 . The STC encoder logic  60  will process the incoming symbols and provide “n” outputs corresponding to the number of transmit antennas  28  for the base station  14 . The control system  20  and/or baseband processor  22  as described above with respect to  FIG. 5  will provide a mapping control signal to control STC encoding. At this point, assume the symbols for the “n” outputs are representative of the data to be transmitted and capable of being recovered by the mobile terminal  16 . 
     For the present example, assume the base station  14  has two antennas  28  (n=2) and the STC encoder logic  60  provides two output streams of symbols. Accordingly, each of the symbol streams output by the STC encoder logic  60  is sent to a corresponding IFFT processor  62 , illustrated separately for ease of understanding. Those skilled in the art will recognize that one or more processors may be used to provide such digital signal processing, alone or in combination with other processing described herein. The IFFT processors  62  will preferably operate on the respective symbols to provide an inverse Fourier Transform. The output of the IFFT processors  62  provides symbols in the time domain. The time domain symbols are grouped into frames, which are associated with a prefix by prefix insertion logic  64 . Each of the resultant signals is up-converted in the digital domain to an intermediate frequency and converted to an analog signal via the corresponding digital up-conversion (DUC) and digital-to-analog (D/A) conversion circuitry  66 . The resultant (analog) signals are then simultaneously modulated at the desired RF frequency, amplified, and transmitted via the RF circuitry  68  and antennas  28 . Notably, pilot signals known by the intended mobile terminal  16  are scattered among the sub-carriers. The mobile terminal  16 , which is discussed in detail below, will use the pilot signals for channel estimation. 
     Reference is now made to  FIG. 6  to illustrate reception of the transmitted signals by a mobile terminal  16 , either directly from base station  14  or with the assistance of relay  15 . Upon arrival of the transmitted signals at each of the antennas  40  of the mobile terminal  16 , the respective signals are demodulated and amplified by corresponding RF circuitry  70 . For the sake of conciseness and clarity, only one of the two receive paths is described and illustrated in detail. Analog-to-digital (A/D) converter and down-conversion circuitry  72  digitizes and downconverts the analog signal for digital processing. The resultant digitized signal may be used by automatic gain control circuitry (AGC)  74  to control the gain of the amplifiers in the RF circuitry  70  based on the received signal level. 
     Initially, the digitized signal is provided to synchronization logic  76 , which includes coarse synchronization logic  78 , which buffers several OFDM symbols and calculates an auto-correlation between the two successive OFDM symbols. A resultant time index corresponding to the maximum of the correlation result determines a fine synchronization search window, which is used by fine synchronization logic  80  to determine a precise framing starting position based on the headers. The output of the fine synchronization logic  80  facilitates frame acquisition by frame alignment logic  84 . Proper framing alignment is important so that subsequent FFT processing provides an accurate conversion from the time domain to the frequency domain. The fine synchronization algorithm is based on the correlation between the received pilot signals carried by the headers and a local copy of the known pilot data. Once frame alignment acquisition occurs, the prefix of the OFDM symbol is removed with prefix removal logic  86  and resultant samples are sent to frequency offset correction logic  88 , which compensates for the system frequency offset caused by the unmatched local oscillators in the transmitter and the receiver. Preferably, the synchronization logic  76  includes frequency offset and clock estimation logic  82 , which is based on the headers to help estimate such effects on the transmitted signal and provide those estimations to the correction logic  88  to properly process OFDM symbols. 
     At this point, the OFDM symbols in the time domain are ready for conversion to the frequency domain using FFT processing logic  90 . The results are frequency domain symbols, which are sent to processing logic  92 . The processing logic  92  extracts the scattered pilot signal using scattered pilot extraction logic  94 , determines a channel estimate based on the extracted pilot signal using channel estimation logic  96 , and provides channel responses for all sub-carriers using channel reconstruction logic  98 . In order to determine a channel response for each of the sub-carriers, the pilot signal is essentially multiple pilot symbols that are scattered among the data symbols throughout the OFDM sub-carriers in a known pattern in both time and frequency. Continuing with  FIG. 6 , the processing logic compares the received pilot symbols with the pilot symbols that are expected in certain sub-carriers at certain times to determine a channel response for the sub-carriers in which pilot symbols were transmitted. The results are interpolated to estimate a channel response for most, if not all, of the remaining sub-carriers for which pilot symbols were not provided. The actual and interpolated channel responses are used to estimate an overall channel response, which includes the channel responses for most, if not all, of the sub-carriers in the OFDM channel. 
     The frequency domain symbols and channel reconstruction information, which are derived from the channel responses for each receive path are provided to an STC decoder  100 , which provides STC decoding on both received paths to recover the transmitted symbols. The channel reconstruction information provides equalization information to the STC decoder  100  sufficient to remove the effects of the transmission channel when processing the respective frequency domain symbols. 
     The recovered symbols are placed back in order using symbol de-interleaver logic  102 , which corresponds to the symbol interleaver logic  58  of the transmitter. The de-interleaved symbols are then demodulated or de-mapped to a corresponding bitstream using de-mapping logic  104 . The bits are then de-interleaved using bit de-interleaver logic  106 , which corresponds to the bit interleaver logic  54  of the transmitter architecture. The de-interleaved bits are then processed by rate de-matching logic  108  and presented to channel decoder logic  110  to recover the initially scrambled data and the CRC checksum. Accordingly, CRC logic  112  removes the CRC checksum, checks the scrambled data in traditional fashion, and provides it to the de-scrambling logic  114  for descrambling using the known base station de-scrambling code to recover the originally transmitted data  116 . 
     In parallel to recovering the data  116 , a CQI, or at least information sufficient to create a CQI at the base station  14 , is determined and transmitted to the base station  14 . As noted above, the CQI may be a function of the carrier-to-interference ratio (CR), as well as the degree to which the channel response varies across the various sub-carriers in the OFDM frequency band. For this embodiment, the channel gain for each sub-carrier in the OFDM frequency band being used to transmit information is compared relative to one another to determine the degree to which the channel gain varies across the OFDM frequency band. Although numerous techniques are available to measure the degree of variation, one technique is to calculate the standard deviation of the channel gain for each sub-carrier throughout the OFDM frequency band being used to transmit data. 
     In some embodiments, a relay station may operate in a time division manner using only one radio, or alternatively include multiple radios. 
       FIGS. 1 to 6  provide one specific example of a communication system that could be used to implement embodiments of the application. It is to be understood that embodiments of the application can be implemented with communications systems having architectures that are different than the specific example, but that operate in a manner consistent with the implementation of the embodiments as described herein. 
     Overview of Current Draft 802.16M 
       FIGS. 7-13  of the present application correspond to  FIGS. 1-7  of IEEE 802.16m-08/003r1. 
       FIGS. 14 to 21  depict further details of the present invention. 
     The description of these figures in of IEEE 802.16m-08/003r1 is incorporated herein by reference. 
     Further Details of Present Disclosure 
     Details of embodiments of the present disclosure are in the attached description. 
     The above-described embodiments of the present application are intended to be examples only. Those of skill in the art may effect alterations, modifications and variations to the particular embodiments without departing from the scope of the application. 
     Design Principle 
     Two types of MAC PDUs formats are considered
         MAC PDU with payload encapsulated   MAC PDU without payload encapsulated
 
MAC PDU with payload encapsulated
   Two versions of header are considered, to optimize the overhead for different types of traffic
           Short version—for small packet (e.g. VolP) type of traffic   Normal version   
           The version used is negotiated during connection setup
 
Control MAC PDU (without payload)
   Multiple types of control MAC header   Fixed length
 
MAC sub-header is used to carry additional control information for MAC PDU with and without payload
       

     Short Version of MAC Header with Payload 
     Suitable for service flow with
         No encryption required   No ARQ required   No fragmentation required. Can be used for packing of fixed length SDU.   Limited types of lengths Example service: VoIP       

     Design Principle 
     
         
         
           
             Minimum header size 
             Option 1: SDU packing/concatenation is done outside of MAC PDU, i.e. each MAC PDU contains one SDU and multiple MAC PDU are concatenated to form a PHY SDU. 
             Option 2: SDU packing/concatenation is done within a MAC PDU, i.e. each MAC PDU contains multiple fixed length SDU. (see  FIG. 14 ) 
           
         
       
    
     Header Format 
     
         
         
           
             HT: Header type; ‘1’ indicates MAC PDU with payload or with subheaders only, ‘0’ indicates MAC PDU without payload 
             FID: flow ID 
             For the last 3 bits in the header, there are two options:
           Option 1 where SDU packing/concatenation is done outside of MAC PDU, the last 3 bits of the header is defined as ‘Length type’ to indicates 8 different lengths (negotiated definition of type and corresponding length at connection setup)   Option 2 where SDU packing/concatenation is done within a MAC PDU, the last 3 bits of the header is defined as ‘number of SDUs’ to indicate the number of SDUs concatenated within the MAC PDU.   
         
           
         
       
    
     Normal Version of MAC Header with Payload 
     For service flow where
         Encryption is required   Fragmentation and packing are possible   Any value of length is possible
 
Design principle
   Consolidate per MAC PDU information into the MAC header
           Aggregate per-SDU information together in the MAC header to reduce overhead, i.e. no need for packing subheader   The SDU fragment sequence number is defined per service flow instead of per SDU, to reduce the overhead   
           Concatenate/packing of multiple SDUs within a MAC PDU to save security encryption overhead
 
Issue of 802.16e MAC PDU with packing of multiple SDUs within a MAC PDU
   Packing sub-header (PSH) is per-SDU based where the FC ( 2 ) and FSN/BSN ( 11 ) and length ( 11 ) present separately for every SDU/fragment
           FC field and FSN/BSN field can be replaced by packing format field and FSN/BSN field in the proposed MAC header described in the next 3 slides.   Lengths of each SDU can be collectively indicated in the proposed Length SH showin shown in  FIGS. 15 and 16 .
 
Issue of concatenating upper laxer data on MAC PDU level, i.e. one MAC PDU contains one SDU and multiple MAC PDUs are concatenated to form one PHY SDU
   
           Security related information (e.g., PN ( 4 ) and IV ( 8 )) incurs substantial overhead on each MAC PDU since 802.16e performs encryption on each MAC PDU
 
 FIG. 15  shows the overhead reduction by introducing the new Normal version of MAC header
   The overhead reduction is about 50% excluding encryption overhead.       

     Refer to FIGS. 17,  18   a  and  18   b     
     FID (4 bits)
         Flow ID of a MS
 
Number of SDU (3 bits)
   Option 1: Indicate the number of SDUs in the payload ( 0 - 7 )
             0  means only the MAC PDU only contains control subheaders, no payload   
           Option 2: Indicate the number of SDUs in the payload ( 1 - 8 )
           This option does not allow subheader-only transmission, without payload
 
FSN/BSN (11 bits)
   
           The fragment sequence number or ARQ block sequence number of the first fragment of an SDU or the first ARQ block.
 
Packing format (2 bits)—refer to  FIGS. 16   a  and  16   b.  
   For the case of two or more SDU
           Bit  1 =1 indicates the first SDU is fragmented;  0  indicates first SDU is not fragmented   Bit  2 =1 indicates the last SDU is fragmented;  0  indicates last SDU is not fragmented   
           For the case of single SDU
           Bit  1 ,  2 =10 indicates the payload is the first fragment of a SDU   Bit  1 ,  2 =01 indicates the payload is the last fragment of a SDU   Bit  1 ,  2 =00 indicates the payload is an entire SDU without fragmented   Bit  1 ,  2 =11 indicates the payload is a middle fragment of a SDU
 
PI—padding indicator (1 bit)
   
           Indicate whether there are padding bits.
           If PI=1, there is padding and a Length sub-header is present after the 3-byte MAC header.   If P=0, there is no padding. If ‘number of SDUs’ is 1, Length subheader is not present. If ‘number of SDUs’ is greater than 1, Length sub-header is present after the 3-byte MAC header to indicate the length of the first (‘number of SDUs’−1) SDUs.   
           Option 1:
           Length subheader includes (‘Number of SDUs’×11) bits   
           Option 2:
           Length subheader consists of length sub-fields where each sub-field correspond to one SDU   Each length sub-field consists of Length type ( 1 ) and length (7 or 11 bits)   Length subheader consists of ‘Number of SDUs’ of length sub-fields   
           Length SH is octet aligned
 
SHI—Sub-Header Indicator (1 bit)
   Indicate whether other control sub-header(s) are present
 
EKS (1 bits)—security key sequence number (two keys are assumed)
 
Note: for per MS MAC PDU the FID can be moved into Length sub-header (4 FID+1 Length)
       

     MAC PDU with Sub-Header Only 
     MAC PDU with sub-header only, without payload (see  FIG. 19 ). 
     MAC Sub-Header 
     Multiple types of sub-header should be considered to carry control information. ARQ feedback information can be one type of sub-header.
 
For each type
         Fixed length   Follow the MAC header if no Length sub-header or follows the Length sub-header.
 
Sub-header format
   Sub-header type (4 bits): indicate 16 different sub-header   Last (1 bit): indicate whether this is the last sub-header   Control info (3 to variable number of bits depending on the type) (see  FIG. 20 )       

     MAC Control Header 
     Multiple types of MAC control header should be considered
         Format (fixed length)   Sent on UL either with other MAC PDU or stand alone following a ranging code.       

     transmission. The fixed length design allows BS to assign fixed UL resource following ranging from the MS.
         Sent on DL either with other MAC PDU or stand alone (see  FIG. 21 )       

     Key Features 
     Multiple version of MAC header to best match different type of traffic and methods of encapsulation
         short/normal version
 
Short version header
   Type of length field enable shorten the length field for VolP type of service which has limited type of length of packets
 
Normal version header
   Put always required per MAC PDU information into header   Aggregate information per SDU together to reduce overhead   The SDU fragment sequence shall be defined per service flow, instead of per SDU, to reduce the overhead   Packet or SDU concatenation is done at the SDU level prior to adding the MAC header to form a MAC PDU
 
Sub-header can be transmitted in a MAC PDU without payload