11 | | A hard-coded 320-sample sequence used by the receiver for AGC, carrier frequency offset estimation and symbol timing estimation |
12 | | [source:/ResearchApps/PHY/MIMO_OFDM/ofdm_tx_mimo_init.m#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 ([source:/ResearchApps/PHY/MIMO_OFDM/ofdm_tx_mimo_init.m@L#L48 ofdm_tx_mimo_init]). |
| 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. |
| 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. |
| 16 | |
| 17 | [[Image(OFDM/MIMO/Docs/Images:preamble.png, 500)]] |
19 | | A programmable number of OFDM symbols containing a packet's header, modulated using the user-specified base-rate modulation scheme. |
20 | | [source:/ResearchApps/PHY/MIMO_OFDM/ofdm_tx_mimo_init.m#L150] |
| 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. |
| 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. |
| 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. |
23 | | A programmable number of OFDM symbols containing a packet's payload, modulated using the user-specified full-rate modulation scheme. |
24 | | [source:/ResearchApps/PHY/MIMO_OFDM/ofdm_tx_mimo_init.m#L150] |
| 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. |
| 35 | |
| 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. |