Mobile TV Broadcasts
There are a lot of broadcast and delivery methods in use or in development. You can broadcast live TV to cell phones via satellite, terrestrial towers or WiFi networks. Here's a look at the basic techniques involved in each approach.
Sling Media's Slingbox uses this approach with a slight twist. Instead of broadcasting the TV signals directly from the content provider, the Slingbox hardware "placeshifts" the TV signals delivered to your home TV, streaming them via your home Internet connection to a mobile receiver like a Web-enabled cell phone or laptop.
Land-based broadcasting methods send out analog or digital TV signals over the air from terrestrial base stations. A phone with a TV antenna and an analog or digital TV tuner (receiver) can pick up the signals.
There are a bunch of mobile-TV versions that utilize land broadcast, including analog broadcast TV, digital broadcast TV and 3G-network broadcasting. Standards like T-DMB (Terrestrial Digital Multimedia Broadcast), MBMS (Multimedia Broadcast and Multicast Services), MediaFLO (a proprietary Qualcomm technology) and DVB-H all utilize aspects of 3G technology.
DVB-H, or Digital Video Broadcasting - Handheld, is an adaptation of the DVB-Terrestrial standard used to broadcast over-the-air DTV to homes in Europe. DVB-H uses orthogonal frequency division multiplexing (OFDM) to make efficient use of available bandwidth. OFDM lets providers transmit more than one signal in one bandwidth space and spread data streams over multiple channels. It may sound like a clutter of data, but the system modulates different signals at different frequencies so the receiver can figure out which it's supposed to listen to and which it should ignore and can put together related signals coming from different channels. In the DVB-H setup, a content provider sends live video and audio streams through an encoder (it's typically H.264 encoding for video and AAC for audio), and the encoder forwards them to a 3G streaming server. The server sends the data to multiple broadcast towers that deliver the content to the coverage areas. The system uses the previously mentioned time slicing technique to reduce power requirements. The typical maximum transfer rate for a DVB-H system is 15 Mbps.
Some standards rely on satellite broadcasting to deliver live TV to cell phones. They can broadcast from satellite to phone, from satellite to base station to phone or use both methods simultaneously.
Two systems that employ this approach are MBSAT and S-DMB. In the S-DMB (Satellite Digital Multimedia Broadcasting) system, a content server sends the live TV feed through an encoder (typically MPEG-4 for video and AAC for audio) and transmits the data to an S-DMB satellite in the frequency range of 13.824 to 13.883 GHz. The geostationary satellite rebroadcasts the signals directly to terrestrial repeaters at 12.214 to 12.239 GHz and directly to cell phones on the S-band, 2.630 to 2.655 GHz. The terrestrial repeaters fill in the gaps where satellite signals get disrupted, like in a city surrounded by tall buildings or in the subway. The dual broadcasts are coordinated so that if a subscriber happens to be within range of the satellite and a tower at the same time, he'll receive both broadcasts and end up with a stronger signal. An S-DMB system can reach data rates of 128 Kbps.
WiFi broadcasting is in use everywhere, and the S-DMB service has been up and running in Korea since mid-2005. DVB-H had its first commercial launch in June 2006 in Italy and is currently in trials around the world. In the next section, we'll check out some of the cell phones that are compatible with mobile-TV systems.