WHDI RF Physical Layer
- the Wireless Home Digital Interface, WHDI physical layer enables the information to be transmitted from one unit to another. It is the area that interfaces with the radio signal.
The WHDI physical layer is a key area of the whole WHDI system.
The physical layer is that area of the overall WHDI system that enables communication with the external world. For WHDI that transmits data wirelessly, it contains the RF signal generation and it deals with the packet formats on this signal, and may other aspects pertaining to the external interface.
WHDI physical layer basics
The WHDI standard was aimed at allowing multimedia devices to connect easily over a wireless interface. Normally wired interfaces have been sued in the past and these often require thick and bulky cables for their interconnection. Behind most home television sets, this is a nest of wires that are not always easy to hide.
However multimedia, and in particular, video requires high data streaming rates. To enable the required data rates to be achieved the WHDI physical layer employs a number of techniques used by other high data rate systems.
- OFDM: The WHDI physical layer employs OFDM, orthogonal frequency division multiplex, as the overall signal format. This has been shown to provide robust communications for data systems where high data rates are employed.
Note on OFDM:
Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects..
Click on the link for an OFDM tutorial
- MIMO : MIMO or Multiple Input Multiple Output is also used by the WHDI physical layer. It allows much higher data rates to be achieved than would be possible without it.
Note on MIMO:
Two major limitations in communications channels can be multipath interference, and the data throughput limitations as a result of Shannon's Law. MIMO provides a way of utilising the multiple signal paths that exist between a transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth. By using multiple antennas at the transmitter and receiver along with some complex digital signal processing, MIMO technology enables the system to set up multiple data streams on the same channel, thereby increasing the data capacity of a channel.
Click on the link for a MIMO tutorial
Devices operate in the 5 GHz ISM, Industrial, Scientific and Medical frequency band. As such they operate alongside other standards including IEEE 802.11a, 802.11n and 802.11ac as well as other license free devices.
Ranges can be up to about 30 metres and the system can deliver full HD video. It is also scalable for higher resolution video and also for 3D.
WHDI physical layer data compression
AV content delivered between wired units like DVD recorders and the like to a display such as a television is uncompressed. Not only is the quality compromised somewhat, but there are also issues with copyright infringement, and interoperability between the plethora of codec formats that would be used..
As a result the data transferred across the WHDI physical layer is undertaken in an uncompressed format which means that high data transfer rates are required.
The WHDI physical layer uses the same channel for uplink and downlink as this provides a far better use of the available frequency spectrum. Typically around 98% of the data is transferred in the downlink direction, i.e. from the data source to the display. This is because the payload data is transferred in this direction only and the uplink only carries control information, e.g. acknowledgements and other management messages.
To ensure that the WHDI TDD techniques operate with the maximum efficiency, the uplink transmissions occur during the vertical blanking period when there is no data in the downlink direction.
WHDI physical layer frames
In order to ensure the data is transmitted in a standardised format that can be decoded at the remote end, the WHDI physical layer transmission is split into frames.
- Downlink: During the downlink transmission period, Downlink PHY Data Units, DLPDUs are transmitted. These DLPDUs contain A/V and control information.
- Uplink: During the uplink transmission Uplink Initialization PHY Data Units, UPIPDUs are transmitted and Uplink Control PHY Data Units, ULCPDUs.
Modem frames comprise an uplink WHDI frame and a downlink WHDI frame and they are aligned with the video frames. In this way, both video and modem frames have the same rate. There may also be a short silence and RF turnaround time at the end of the uplink frame and before the next downlink frame starts.
Within the data transmission, the active video period is defined as the time between the transmission of the start of the first pixel in a video frame to the end of the last. In this way, the data transmission occurs during the active video period.
Using the active video period in this way gives a small amount of time for buffering and it also helps reduce latency.
By Ian Poole
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