Changes between Version 6 and Version 7 of OFDM/MIMO/Docs/PHYDetails/FrameFormat
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- Aug 29, 2009, 9:08:36 PM (15 years ago)
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OFDM/MIMO/Docs/PHYDetails/FrameFormat
v6 v7 2 2 3 3 == Overview == 4 The frame format for the WARP MIMO OFDM core is based loosely on the frame format specified in the [http://ieeexplore.ieee.org/xpl/standardstoc.jsp?isnumber=17645&isYear=1999 IEEE 802.11a] wireless networking standard. The WARP MIMO OFDM core supports both single (SISO) and two antenna (MIMO) modes. The frame formats for these two modes are detailed below.4 The frame format for the WARP MIMO OFDM core is based loosely on the frame format specified in the [http://ieeexplore.ieee.org/xpl/standardstoc.jsp?isnumber=17645&isYear=1999 IEEE 802.11a] wireless networking standard. 5 5 6 6 Each frame is divided into four sections: … … 9 9 10 10 === Preamble === 11 The preamble is a hard-coded 320-sample sequence used by the receiver for AGC, carrier frequency offset estimation and symbol timing estimation. The preamble consists of two 160-sample sections. The first is a repitition of 10 16-sample sequences, called short training symbols (STS). The second half is 2.5 repititions of a 64-sample sequence, called the long training symbol (LTS). The preamble is defined in the OFDM models's initialization script ([source:/ResearchApps/PHY/MIMO_OFDM/ofdm_tx_mimo_init.m@L#L48 ofdm_tx_mimo_init]).11 The preamble is a hard-coded 320-sample sequence used by the receiver for AGC, carrier frequency offset estimation and symbol timing estimation. The preamble consists of two 160-sample sections. The first is a repitition of 10 16-sample sequences, called short training symbols (STS). The second half is 2.5 repititions of a 64-sample sequence, called the long training symbol (LTS). The preamble is defined in the OFDM models's initialization script. 12 12 13 In SISO mode, the preamble is transmitted only on antenna A. In MIMO mode, the preamble is always transmitted from antenna A and optionally from antenna B. This option is controlled by the [wiki:OFDM/MIMO/Docs/ModelRegisters#DISABLE_ANTB_PREAMBLE DISABLE_ANTB_PREAMBLE] field in the [wiki:OFDM/MIMO/Docs/ModelRegisters#Tx_ControlBits Tx_ControlBits] register.13 In SISO mode, the preamble is transmitted only from the active antenna. In Alamouti and multiplexing modes, the preamble is always transmitted from antenna A and optionally from antenna B. This option is controlled by user software. 14 14 15 In both modes, the preamble is scaled by a programmed constant. This scaling allows the full-scale swing of the preamble (+/-1 default) to match the swing of the IFFT output. The scaling constant is programmed via the [wiki:OFDM/MIMO/Docs/ModelRegisters#Tx_PreambleScaling Tx_PreambleScaling] register.15 In both modes, the preamble is scaled by a programmed constant. This scaling allows the full-scale swing of the preamble (+/-1 default) to match the swing of the IFFT output. The scaling constant is programmed by user software. 16 16 17 17 [[Image(OFDM/MIMO/Docs/Images:preamble.png, 500)]] 18 18 19 19 === Training === 20 The OFDM core uses hard-coded training sequences for channel estimation. Each repitition of the sequence is treated as a full OFDM symbol at the receiver. The number of repititions is programmable but must be known to both the transmitter and receiver ahead of time. The training sequence is defined in the OFDM models's initialization script ([source:/ResearchApps/PHY/MIMO_OFDM/ofdm_tx_mimo_init.m@L#L37 ofdm_tx_mimo_init]).20 The OFDM core uses hard-coded sequence of BPSK symbols for channel training. Each repetition of the sequence is treated as a full OFDM symbol at the receiver. The number of repetitions is programmable but must be known to both the transmitter and receiver ahead of time. The training sequence is defined in the OFDM model's initialization script. 21 21 22 For SISO links, the programmed number of training symbols are inserted between the preamble and base-rate symbols. For MIMO links, the programmed number of training symbol periods are dedicated to channel estimation and are split evenly between the transmit antennas. See the SISO vs. MIMO frame format sectionsbelow for more details.22 For SISO mode, the programmed number of training symbols are inserted between the preamble and base-rate symbols. For Alamouti and multiplexing modes, the programmed number of training symbol periods are dedicated to channel estimation and are split evenly between the transmit antennas. See the frame format section below for more details. 23 23 24 24 === Base Rate === 25 In order to maintain compatibility with nodes communicating over various distances (i.e. capable of communicating at various rates), the OFDM core allows a number of OFDM symbols to be modulated using a base-rate. The number of base-rate symbols and the assignment of base-rate modulation schemes to subcarriers must be known to both the transmitter and receiver ahead of time. In MIMO mode, both antennas transmit the same base-rate symbols using the same modulation schemes.25 In order to maintain compatibility with nodes communicating over various distances (i.e. capable of communicating at various rates), the OFDM core allows a number of OFDM symbols to be modulated using a base-rate. The number of base-rate symbols and the assignment of base-rate modulation schemes to subcarriers must be known to both the transmitter and receiver ahead of time. In multiplexing mode, the base-rate symbols are transmitted only from antenna A. 26 26 27 Generally, these base-rate symbols contain the packet's header, which enables every receiving node to know the packet's destination and length.27 Generally, these base-rate symbols contain the packet's header, which enables every receiving node to know the packet's source, destination, full-rate modulation and length. 28 28 29 In simulation, the number of base-rate symbols is defined in the OFDM models' initialization script ([source:/ResearchApps/PHY/MIMO_OFDM/ofdm_tx_mimo_init.m@L#L150 ofdm_tx_mimo_init.m]). In hardware, the number must be written to the [wiki:OFDM/MIMO/Docs/ModelRegisters#NUM_BASERATE_SYMS1 NUM_BASERATE_SYMS] field in the OFDM transmitter's [wiki:OFDM/MIMO/Docs/ModelRegisters#Tx_OFDM_SymCounts Tx_OFDM_SymCounts] register. 30 31 See [wiki:OFDM/MIMO/Docs/ModelSharedMem#ProgrammableModulationSchemes Programmable Modulation Schemes] for example code which illustrates how to program the base-rate modulation scheme. 29 The number of base-rate symbols and base-rate modulation scheme is programmed by user code and must correspond to the length of the header defined by the user's packet format. 32 30 33 31 === Full Rate === 34 The remaining OFDM symbols in each packet contain data modulated at the full-rate. The full-rate modulation schemes are programmable across subcarriers and antennas. In the current model, the assignment of full-rate modulation schemes to subcarriers and antennas must be known to both the transmitter and receiver ahead of time. A future version of the model will support dynamic assignments per-packet, including the full-rate modulation assignments in packet's header in the base-rate symbols.32 The remaining OFDM symbols in each packet contain data modulated at the full-rate. The full-rate modulation schemes are programmable across subcarriers and antennas. The assignment of full-rate modulation schemes to subcarriers and antennas must be known to both the transmitter and receiver ahead of time. 35 33 36 In simulation, the number of full-rate symbols is defined in the OFDM models' initialization script ([source:/ResearchApps/PHY/MIMO_OFDM/ofdm_tx_mimo_init.m@L#L150 ofdm_tx_mimo_init.m]). In hardware, the number must be written to the [wiki:OFDM/MIMO/Docs/ModelRegisters#NUM_PYLD_SYMS NUM_PYLD_SYMS] field in the OFDM transmitter's [wiki:OFDM/MIMO/Docs/ModelRegisters#Tx_OFDM_SymCounts Tx_OFDM_SymCounts] register. 37 38 See [wiki:OFDM/MIMO/Docs/ModelSharedMem#ProgrammableModulationSchemes Programmable Modulation Schemes] for example code which illustrates how to program the full-rate modulation schemes for the two antennas. 34 The number of full-rate symbols is calculated automatically by the transmitter based on the modulation rate and packet length specified by user code. The maximum number of full-rate symbols is 2047 (way more than enough for very long packets). 39 35 40 36 == SISO Frame Format == … … 42 38 [[Image(OFDM/MIMO/Docs/Images:SISO_Frame_Format.png, 684)]] 43 39 44 == M IMOFrame Format ==40 == Multiplexing Frame Format == 45 41 46 42 [[Image(OFDM/MIMO/Docs/Images:MIMO_Frame_Format.png)]]
