For a 40 MHz channel, the field is repeated once. For an 80 MHz channel, the field is repeated three times and a pad bit of 0 is appended. This process of repeating the signal field ensures that it occupies exactly one symbol.
Immediately following the physical layer header, the Data field begins to transmit the payload of the physical layer frame.
The format of the Data field is shown in Figure Because the Data field is transmitted following the header, it is transmitted at the data rate described by the physical layer header. The Data field carries a frame from higher protocol layers.
Before beginning transmission of the data from higher-layer protocols, there are a few housekeeping fields embedded in the physical layer frame:. The Service field is prepended to the higher-level protocol data before transmission.
The PSDU field contains a frame from from the It is variable length. To ensure that the number of bits passed to the transmitter will exactly match the number of bits required for a symbol, pad bits are added.
Tail bits are present when the physical layer frame is protected with a convolutional code and are used to ramp down the convolutional coder. If LDPC is used, the tail bits are not required. A block diagram for an This block diagram can be used to transmit both single-user and multi-user frames, but this chapter focuses on the single-user transmission use case.
When the MAC presents a frame for transmission, it is passed to the physical layer, and the following procedure is run:.
Preparation of Service field. To begin, the Service field that is prepended to the data for transmission is constructed. PHY padding. The first step in transmission is to pad the frame so that its length matches the number of bits required to end on a physical-level symbol boundary.
Scrambling and forward error correction FEC encoding. The scrambler reduces the probability of long strings of identical bits in its output, and is present because convolutional codes work best on data that does not have long runs of identical bits.
Stream parsing. The stream parser takes the output of the FEC encoder and divides up the encoded bits between each spatial stream. For example, if there are two spatial streams, the stream parser will divide up the encoded bits and assign each of them to one of the spatial streams. At this point, the bits flowing from the stream parser to the interleaver are a spatial stream. Output from the stream parser is sent to the interleaver, which is the first component in the radio chain.
Segment parsing. Convolutional code interleaving. Convolutional codes work best when errors are isolated, and errors on radio channels tend to affect several bits in a row.
The interleaver takes sequential bits from the carriers and separates them in the convolutional code bitstream to separate errors and make them easier to correct. LDPC has a similar function executed after constellation mapping. Constellation mapping. Bits are mapped onto QAM constellation points using the selected modulation. LDPC tone mapping. Tone mapping takes constellation points and ensures they are mapped to OFDM subcarriers separated by a sufficient distance.
It serves the same purpose as the interleaver for convolutional codes. For example, in a 40 MHz channel, two consecutive constellation points must be separated by at least six OFDM subcarriers to ensure that interference must be about 1. Segment deparsing. For MHz channels, the segment deparser brings the two frequency segments back together for transformation from constellation symbols into a set of spatial streams suitable for transmission.
Space-time block coding STBC. This optional step is used to transmit one spatial stream across multiple antennas for extra redundancy. The space-time block coder takes a single constellation symbol output and maps it onto multiple radio chains, transforming the spatial streams into space-time streams. Pilot insertion and cyclic shift diversity CSD. Constellation points for transmission are combined with the data for pilot subcarriers to create the complete data set for transmission.
When multiple data streams are present, they are each given a small phase shift to aid in distinguishing between them at the receiver. The phase shift is referred to as cyclic shift diversity because a slightly different phase shift is applied to each of the space-time streams. Spatial mapping. Space-time streams are mapped onto the transmit chains by the spatial mapper.
The simplest approach is a direct mapping that turns a spatial stream into a space-time stream for a single transmit chain. For higher performance, the spatial mapper may spread all of the space-time streams on to all of the transmission chains in a spatial expansion.
This process is a key component of beamforming, which can be used to shape a space-time stream to direct energy in the direction of a receiver. Inverse Fourier transform IFT. An inverse Fourier transform takes frequency-domain data from OFDM and converts it to time-domain data for transmission. Guard insertion and windowing. The guard interval is inserted at the start of each symbol, and each symbol is windowed to improve signal quality at the receiver.
Preamble construction. The preamble is created for each 20 MHz channel within the transmission channel. To guard against interference, each of the 20 MHz segments of the preamble are given a slight cyclic delay. RF and analog section. This prepares the data for transmission out an antenna, following the VHT preamble. The complex waveform that comes from the previous step is converted to a signal that can be placed on a carrier at the center frequency of the channel selected by the current AP.
A high power amplifier HPA increases the power so the signal can travel as far as needed, within regulatory limits. Receiving frames is the inverse of the transmission process. Incoming signals from the antenna are amplified by a low-noise amplifier LNA on each radio chain, and the preamble is used to set up the receiver to adjust for any frequency-specific fades that occur in the channel.
After compensating for the channel based on the reception of the preamble and pilot carriers, the incoming data is a series of constellation symbols. If STBC was used for transmission, multiple streams of constellation symbols will be combined into a single output bitstream; otherwise, each space-time stream becomes its own stream of constellation symbols.
Constellation symbols are turned into bits and processed by the FEC decoder, which will hopefully correct any resulting errors. Single-stream transmission is substantially simpler than multi-stream operation. When a device transmits multiple spatial streams, significant computational resources are applied to combine multiple spatial streams into one transmission. With only one spatial stream, however, the digital signal processing DSP work is not needed.
Eliminating the DSP requirement also substantially reduces power consumption, which is why many small battery-operated devices are single-stream only. Data rates are determined by the combination of channel width, modulation and coding, number of spatial streams, and the guard interval. I expect the first generation of products will be able to achieve this data rate, though the range at which they will do so is still to be determined.
With the same three spatial streams as you get in mainstream Or, with four-stream Another way to look at the speeds of At its most basic level, Each spatial stream adds proportionally to throughput. Wider channels also increase throughput proportionally. To get the speed of any MCS rate, take the basic 20 MHz stream, multiply by the number of spatial streams, and then multiply that result by a channel correction factor. Table shows how the calculation works. Take the MCS value from the lefthand column, and translate that to the building block data rate in the second column.
Multiply by the indicated factors in the next two columns to work out the resulting data rate. The three rightmost columns show the maximum data rates standardized in Roughly speaking, these combinations of MCS and channel width do not cleanly fit within the boundaries of the encoding and interleaving process used to assemble a frame.
The primary channel is used for communications with clients incapable of 40 MHz mode. When in 40 MHz mode, the center frequency is actually the mean of the primary and secondary channels.
Local regulations may restrict certain channels from operation. For example, Channels 12 and 13 are normally unavailable for use as either a primary or secondary channel in North America. For More Information on CableFree WiFi products, please Contact Us and our team will be delighted to advise on a precise solution to match your exact requirements.
You must be logged in to post a comment. Skip to content. WiFi Data Rates and Modulation for Backward compatibility When Deployment strategies To achieve maximum output, a pure BPSK is used for lower bit rates with A low bit-rate would be negotiated for many reasons, those discussed later in the document.
BPSK paired with half rate encoding, results in a bit-rate of 6Mbps. If the the signal strength exceeds the receiver sensitivity, a more complicated modulation scheme can be used.
It defines three operating modes:. Non-HT or Legacy Mode. In this mode the signal is transmitted in legacy In this mode, the burst has an initial legacy-compatible preamble, followed by an HT preamble and then HT data. This mode does not use a legacy-compatible preamble, it has only an HT preamble followed by HT data. The resulting burst preamble is slightly shorter than a HT-mixed preamble, so overall data transmission rates in this mode is slightly more efficient.
This information includes the total data length, the data modulation format, the number of data streams, the signal bandwidth, and whether a shortened guard interval is used. All operating modes use OFDM to transmit the data.
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