Mobile Digital TV Antenna Diversity for Indoor and In-Vehicle Reception

Understanding how diversity antenna systems work, and how Mobile Digital TV viewing can be improved through the use of multiple antennae — even for space-constrained portable devices, and for in-vehicle and deep-indoor reception.

By Roy Oren, CTO, Siano Mobile Silicon

Page 4 of 5
Video/Imaging DesignWire
(10/5/2009 4:00:47 AM)

Shadow fading – A frequent phenomenon in the mobile channel is that of shadow fading, which occurs when close reflections combine destructively. When reflections which are distant less than (1/bandwidth) apart are of opposite phase, the whole signal will completely fade. This happens quite often when passing under a bridge, even at low speeds. A reflection from the bridge is a very close reflection and as such a destructive combination of this reflection with the main signal completely eliminates the signal. In some other wireless transmissions this issue is resolved by introducing long interleavers which in effect creates time diversity, but this is lacking in DVB-T as the interleaver spans about one millisecond. When placing more than one antenna, the phases of the reflections change and while in one of the antennas the signal combines destructively, it is bound to combine constructively in some of the other antennas.

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Figure 2: Driving towards a bridge example, the direct path and the reflection often combine destructively in a single antenna receiver.

Mobility – When the receiver is traveling at higher speeds, channel variations during the symbol duration are no longer negligible. They result in Inter Carrier Interference (ICI), e.g., carriers are no longer orthogonal to each other, and energy from neighboring carriers leak to one another and creates noise. Another issue is the channel estimation which under these conditions becomes less accurate. These effects degrade the minimum SNR in which the receiver can still function. A diversity receiver that can still operate at lower SNR will not suffer from this effect, until it becomes very strong. 

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Figure 3: Min C/N vs Fdoppler qualitative graph

Lab and field testing show that a two way diversity receiver will be able to function at more than twice the speed relative to a single antenna receiver! This is illustrated in Figure 3 and Figure 4

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Figure 4: Reception while driving 120kmph, between Hannover and Braunschweig. Top picture shows reception quality when using single antenna receiver, bottom picture shows reception quality when using two antennas receiver.

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