4.3 Wireless solutions - WHDI (Wireless Home Digital Interface)

WHDI (Wireless Home Digital Interface) is presented as a wireless alternative to state-of-the-art interfaces for the transmission of uncompressed video between video sources, players and displays (e.g. HDMI).

The objective of WHDI technology is to enable wireless streaming of uncompressed HD video and audio between CE devices such as LCD and plasma HDTVs, multimedia projectors, A/V receivers, DVD and BD players, set-top boxes (STBs), game consoles and PCs.

The main characteristics of this incoming standard are:

• Use of 40 MHz channel within the 5 GHz band
• 30 meters of maximum coverage
• Very low latency (<1ms)
• Up to 3 Gbps video transmission (enough for 1080p HD)

WHDI was defined so as to fulfill two main requirements: (i) it would be optimized for video transmission, and (ii) transmitted video contents would not be compressed.

In general, communication between video sources, players and displays (both in home or production environments) is based on the transmission of uncompressed video. This way it is possible to guarantee a maximum image quality, as well as diminishing latency due to codec selection and decoding processes, while avoiding illegitimate signal captures and enabling the implementation of content protection strategies (e.g. HDCP).

Because of the high bandwidth requirements of uncompressed video transmission, these connections have been wired so far. Traditional wireless networks support, in their best performance, data rates of only around a few hundred Mbps. Besides, these networks are in general packet transmission oriented, and use retransmissions and other link-level strategies in order to overcome the unreliability of the wireless channel.

Therefore, today the most extended approach for the distribution of digital video is the compression, then packetization of video contents for their transmission through wireless data-oriented networks. In this scenario, optimization consists in the application of strategies at different levels, aiming to mitigate the severe impact that packet losses and delays have for the perceived video quality.

On the contrary, WHDI solution proposed by the company AMIMOM takes a completely different approach. WHDI starts up from the premise that distributed video will not be compressed, and then adopts different optimization strategies from there, aiming to add capacity and robustness to the wireless transmission.

WHDI modulators analyze the structure of the uncompressed digital video inputs and split them in separate fragments, depending on their relative importance for video quality in the receiver. This way it is possible to transmit the most significant bits in a more robust manner, and devoting less channel resources to that information with minor importance. This additional robustness is obtained by tuning parameters such as power levels, spectrum assignment or channel coding. Hence, WHDI exploits aprioristical knowledge about what type of content is being distributed and applies Joint Source Channel Coding (JSCC) techniques in order to achieve the most efficient transmission for the most significant information, then boosting perceived quality in noisy environments.

At the physical level, WHDI shows many common points with other popular wireless technologies. Like Wi-Fi systems, WHDI is based on OFDM modulation, operates in the license-free 5 GHz band spectrum, takes benefit from MIMO (Multiple Input Multiple Output) antenna schemes and shares many functional blocks with 802.11n. However, WHDI uses a specific modem optimized for video transmission instead of using a data modem. WHDI also standardizes mechanisms for the transmission of audio and control information, but no support is given for the transmission of other data types.

As a conclusion, the following additional considerations can be made:

• Once the standardization process (directed form WHDI Special Interest Group) is done, WHDI will appear in the market as one of the most interesting alternatives for the wireless transmission of video contents within the Home Network. In this way, some WHDI modulators and receivers are recently starting to be shown in international forums.
• When evaluating this technology, the way the unavoidable signal degradation in the wireless channel (due to losses, delays, fading…) affects the received quality would have to be carefully analyzed.
• WHDI aims to take a strong benefit from the possibilities of multicast transmission. Many WHDI receivers would be able to connect to the same source device, after following a simple search-and-associate process (in a similar way to Bluetooth systems) with no additional bandwidth requirements. However, performance in this scenario must be observed since, unlike IEEE 802.11 standards, WHDI does not support Medium Access Control schemes in order to avoid collisions among several devices which can be interchanging requests at time.
• Finally, the content protection technique that will be implemented in WHDI interfaces has not yet been defined, although this feature has been taken as a requirement for the WHDI standardization group.

WHDI technology complements other wireless and wired standards with a new class of connectivity within the home. WHDI enable robust wireless delivery of uncompressed HD video between CE devices taking out HDMI cables.

WHDI Inhome Scenary.

For all the above, WHDI is recommended according to:

• Sharing High Definition content among all devices in the home.
• Eliminating unsightly cables.
• Hassle-free, low-cost installation.