TransferJet Physical Layer

- TransferJet physical layer including the DSSS, direct sequence spread spectrum RF signal format, modulation, and data encoding.

As with any wireless or radio system, the physical layer is required to transport the data over the air or radio interface.

The TransferJet physical layer defines the RF signal format modulation, and how the data is encoded onto the RF signal.

Although the physical layer uses some advanced techniques, these do not require large amounts of processing that will increase the processing required to a point where power consumption is very high. A balance has been achieved between performance, cost and power consumption.

TransferJet physical layer basics

The physical layer interfaces to the upper layers of the data stack. It organises the data that is sent and receives so that it can be taken from or passed to the required upper layer areas.

During transmission the TransferJet physical layer receives the digital data from the upper Connection Layer, CNL. It then converts the data to an analogue RF signal cantered at 4.48 GHz and with a 560 MHz bandwidth. The resulting signal is transferred to the induction coupler element for radiation from the TransferJet device.

The signal format is direct sequence spread spectrum, DSSS and uses Π/2 binary phase shift keying, BPSK. The transmitted power level is controlled by the system, but cannot exceed -70dBm / MHz.

Data is structured in the form of Service Data Units which are separated by interframe spaces. The Service Data Unit stream is about 2 ms long and starts with a preamble, sync and header followed by a variable-length payload. Forward error correction, FEC, spreading, scrambling are applied to the signal before transmission.

Key TransferJet Physical Layer Parameters
Feature Details
Transmission power Below -70dBm / MHz
This level corresponds to the low intensity radio wave regulation power limits in Japan and other countries.
Transmission rate 560 Mbps before error correction
375 Mbps effective throughput
The system can adjust the transmission rate dependent upon the wireless environment.
RF modulation Direct sequence spread spectrum with Π/2 BPSK
Carrier frequency 4.48 GHz centre frequency
The centre frequency and bandwidth of the TransferJet transmission fall within the requirements for UWB - Ultra-Wideband.
Signal bandwidth 560 MHz

Direct sequence spread spectrum

The direct sequence spread spectrum technique chosen for the TransferJet physical layer is a technique that is widely used in many areas of radio communications. It formed the basis of the CDMA multiple access system used in many 3G cellular telecommunications system as well as being used for many covert radio transmissions schemes..

The essence of the system is that data for transmission is multiplied with a predetermined code. This code sequence increases the bandwidth of the data and when modulated onto the RF carrier, it has the effect of increasing the signal bandwidth, but also lowering the spectral power density.

For the receiver, the recovered data is multiplied with the original predetermined code to recover the data. Only by knowing the code can the data be recovered. However it is possible to receive signals at very low levels making it an ideal choice for the TransferJet physical layer.

Note on CDMA:

CDMA, Code Division Multiple Access, is a multiple access scheme used by many 3G cellular technologies, and other forms of wireless technology. It uses a process called Direct Sequence Spread Spectrum where spreading codes are used to spread a signal out over a given bandwidth and then reconstituting the data in the receiver by using the same spreading code. By supplying different spreading codes to different users, several users are able to utilises the same frequency without mutual interference.

Click on the link for a CDMA tutorial

TransferJet modulation scheme

One of the key elements of the TransferJet physical layer is the modulation scheme that is used to place the data onto the RF carrier.

TransferJet Modulation Parameters
Feature Details
Chip rate 560 Mcps
Chip duration 1.786 nsec
Symbol rate 280 Msps
FEC 1/2 Convolutional code + Reed Solomon code

Frame format

The data transmitted over the TransferJet physical layer is split into frames. In this way, the system is able to manage the data and process it correctly.

The data packets are structured in the form of Service data Units, which are given the abbreviation PDSU and these are separated by interframe-spaces, IFS.

Each PSDU consists of a preamble followed by Sync data, the PHY header and then the CPDU which forms the payload.

FEC, spreading, scrambling and modulation with multi-rate support are applied to the PHY header and CPDU.

TransferJet PHY data structure
TransferJet PHY data structure

A CPDU frame is obtained from the higher CNL, connection layer. This si divided into 224 byte message blocks. As the first element of the error correction, Reed Solomon parity is added after each 224 Byte block. As the overall messages will not have a defined length, it means that the last block will be shorter. The parity for this is calculated by expanding the message to 224 Bytes by adding zeros. These zeros are not transmitted but will be assumed by the receiver.

Convolutional encoding is then applied to the data from the Reed Solomon stages. The output length from the convolutional coding is doubled as a result of the action of the encoder which as 1/2 convolutional encoding.

After the convolutional encoding, tail bits are assed which include all the zero bits. These tail bits are used to initialise the state of the Viterbi decoder used in the receiver.

The final element of the TransfetJet physical layer encoding is to spread and scramble the data. This is then added to the carrier using Π/2 binary phase shift keying. This payload data is then combined with the Preamble and Sync to provide the completed transmitted data packet.

By Ian Poole

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