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

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

The special challenge of indoor reception
Two main factors might degrade signal quality in indoor scenarios – weak signal and “blind spots”, and antenna diversity has significant influence in both cases:

A.     Weak signal – as current infrastructure does not employ gap-fillers inside buildings for DTV transmission, the signal level inside a building is often attenuated compared to the signal level outside the building. In this case the limiting factor is usually the intrinsic noise of the receiver. In a well-designed receiver this noise will be a few dBs higher than the thermal noise. In order to be able to show smooth video, the signal level at the antenna output should be stronger than the noise floor by the minimal SNR required to receive the transmission, the minimum signal level which is required in order to show smooth video – called sensitivity. When using several receive paths, since the noise at each path is uncorrelated to the others, MRC simply means averaging the signals from the antennas, reducing the noise variance by a factor of N, hence diversity will increase the overall sensitivity of the receiver by 10*log10(N)  [dB], where N is the number of antennas.

B.     Blind spots – inside a room, where the receiver seldom has a line-of-sight view of the transmitter, it actually receives the signal through numerous reflections from various reflecting objects (neon bulbs, steel plates, people, walls, etc.) placed fairly close to the receiver. Reflections combine – interfere with each other – constructively or destructively, depending heavily on the exact location of the receiver. Usually in a room environment one might receive a good signal at a certain spot, and when moving the receiver device by just a few centimeters the signal fades away. Another phenomenon is when the device is placed at a presumably ‘good’ spot, but when people are moving around reflections pattern changes and a ‘good’ spot suddenly becomes a blind spot.
Antenna diversity improves the reception quality as several antennas continuously receive the signal through different propagation paths, thus the chance of having a blind spot in all the antennas becomes negligible – usually two antennas will suffice to practically eliminate any blind spot.

Mobile Reception
Antenna diversity is not only necessary in addressing indoor MDTV challenges, but is a significant factor in enabling high-quality MDTV reception whilst traveling at speeds.

In addition to the problems mentioned above, there are several factors that might degrade the signal quality when the receiver is on the move:

A.     Selective fading – A mobile channel usually consists of several reflections spanning a few microseconds apart. When a few transmitters cover the area there are several clusters of these reflections, with the time gap between the clusters usually in the order of 10’s of microseconds. While on the move each reflection changes phase and gain rapidly. This creates a phenomenon that can be observed in the frequency domain as “notches”. The number of notches, their depth and their location changes from one symbol to another; the rate of change corresponds to the speed in which the receiver is traveling. Channel equalization compensates for the notches by introducing a gain on the attenuated carriers – by doing so it in effect increases the noise on those carriers. Thus, after channel equalization, the signal becomes flat across the different carriers, and the noise is at full range and has peaks in some specific frequencies.

Depending on the number of notches and their depths, the average SNR across the carriers might degrade to the point where bursts of errors are imminent. As mentioned above, when using diversity, the SNR in each carrier is a sum of the SNRs on each of the carriers across the different paths. Since most of the noise in the mobile channel is contributed by faded carriers, and the probability that the same carrier will be faded at more than one antenna is low, the average gain in SNR is large – much more than 3dB. In fact, experiments conducted both in the lab and in the field show that the SNR gain in a two-path diversity receiver is 6-8dB. This means that, in a mobile environment, a diversity receiver can still show clear video when noise is stronger by 6-8 dB’s relative to a single antenna receiver.

View full size
Figure 1: Combination of two paths with selective fading (mobile environment). Each path has some notches that lead to negative SNR, but after combination all the carriers have a positive SNR. Average SNR, which is mostly influenced by the carriers with the strongest noise, is improved dramatically.

NEXT: Shadow Fading, Mobility

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