Weightless M2M Air Interface & Physical Layer, PHY
- an overview of the 'Weightless' air interface / radio interface / physical layer and how it operates to provide M2M communications.
The Weightless M2M radio interface / physical layer has been designed to meet the requirements of the overall Weightless system.
The Weightless air interface provides the radio bearer and physical elements to enable the data to be transmitted to and from the base station by the terminal machine. The radio interface uses a form of single carrier modulation and is aimed at use within an unused 6 or 8 MHz television channel, i.e. it does not use a multicarrier modulation format such as OFDM.
The Weightless physical layer takes the binary data from the MAC layer, reformats it and then transmits it over the air.
The radio interface has to accommodate the propagation issues as well as interference and varying signal levels.
To achieve this the Weightless radio interface utilises techniques including error correction, selection of modulation type, and frequency hopping.
Weightless modulation scheme and radio format
The modulation scheme used within the Weightless M2M system is a key to its successful operation.
There is a variety of formats of modulation that can be used depending upon the requirements and the signal levels, etc. Modulation schemes used are DBPSK, BPSK, QPSK and 16-QAM.
When generating the RF signal the data undergoes a number of processes as shown in the diagram below:
Weightless M2M Physical Layer Processes
The different processes included in the Weightless M2M physical layer and radio interface are described in more detail below.
Forward Error Correction, FEC
The forward error correction adds additional redundant bits into the data sent, beyond any redundancy introduced in the MAC layer. This enables errors that may be introduced in the radio transmission path to be detected and corrected.
As with any FEC system, the one used within the Weightless physical layer needs to balance the level of error correction with the additional overhead required to provide it.
The process of whitening the Weightless radio signal involves XORing the data to be transmitted with a pseudorandom data sequence and this makes the signal approximate to white noise.
This process helps overcome interference issues and any problems that may occur if data strings include data that does not change from 1 to 0 which can confuse synchronisation streams.
Weightless modulation formats
The Weightless radio interface uses a variety of modulation formats dependent upon the link conditions (signal to noise ratio) and data requirements.
Both phase shift keying and quadrature amplitude modulation are used.
The modulation mapping encodes the data onto the symbols that are carried by the modulation format used. These symbols represent the complex points on the modulation constellation.
|Overview of Modulation & Coding usedon Weightless M2M Physical Layer|
|Modulation Scheme||Coding Rate||Spreading Factor||Downlink PHY Data Rate
The spreading process within the Weightless physical layer multiplies the data by a code-word to create a longer data sequence. It is used where there is insufficient signal level to support communications via an un-spread signal. In this way it reduced data rate to gain additional range.
Cyclic prefix insertion
The cyclic prefix insertion in Weightless adds a repetition of the end of frame to the beginning of the frame. This helps the system to overcome many of the effects of multipath.
The synchronisation process adds known patterns of data to the transmission to enable the receiver to synchronisation of the clocks to the received signal.
A process of root raised cosine pulse shaping is used. This turns the square wave produced by the data stream into a more sinusoidal type signal. This reduces the out of band emissions because the sharp transitions in square waves create high levels of sidebands that extend out either side of the carrier.
The Weightless physical layer provides for frequency hopping. The hopping is applied at the frame rate to ensure ease of operation and to maximise the data rate with the hopping.
The use of frequency hopping within Weightless provides some mitigation against interference as well as for propagation characteristics where some frequencies may experience deep fades as a result of multiple transmission paths.
By Ian Poole
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