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

Processing Techniques
There exist several flavors of receiver antenna diversity, as described herein:

  • Switching – In a switching scheme, the signal from only one antenna is fed to the receiver for as long as the quality of that signal remains above some prescribed threshold. If and when this signal degrades, another antenna is switched in. Switching is the easiest and least power consuming of the antenna diversity schemes, as it requires only a single receiver, but does not yield a significant improvement in reception.
  • Selecting – As with switching, selection processing presents only one antenna’s signal to the receiver at any given time. The antenna chosen, however, is based on the best signal-to-noise ratio (SNR) among the received signals. This requires that a pre-measurement takes place and that all antennas have established connections (at least during the SNR measurement), leading to a higher power requirement. The actual selection process can take place in between received packets of information. This ensures that a single antenna connection is maintained as much as possible. Switching can then take place on a symbol-by-symbol basis if necessary, with typical switching time periods of several milliseconds.
  • Combining – In this scheme, all of the antennas continuously provide signal to the receiver, at all times. The signals are then “combined” and, depending on the sophistication of the system, can either be added directly (equal-gain combination – EGC) or weighted and added coherently (maximal ratio combination – MRC). Such a system provides the greatest resistance to fading, and thus the best performance, but since all the receive paths must remain energized; it also consumes the most power as it requires all receiving paths to be active all of the time.

Antenna diversity optimized for DVB-T reception
The scheme yielding the highest performance and therefore the most adequate for DVB-T, is MRC. In multi-carrier systems such as DVB-T, this scheme can even be further optimized as the weighted signal combination is performed on a carrier-by-carrier basis. For optimal performance each carrier is weighted according to its own signal to noise ratio (SNR). It can be shown that in such a system, assuming that, after channel equalization, the noise received in each of the paths is uncorrelated, the SNR in each of the combined carriers is the sum of the SNRs on each of the carriers from the different antennas.
In other words, if the SNR of carrier k in the receiver that is connected to the antenna is denoted as, then after weighted maximum ratio combining, the SNR at carrier k would be:

null

(Summation over all the antennas)

There are several schemes to ensure non-correlation condition, i.e., to ensure that the noise, after channel equalization at the different paths is uncorrelated:

  • Spatial Diversity – Spatial diversity employs multiple antennas, usually with identical characteristics, that are physically separated from one another. Depending upon the expected incidence of the incoming signal, usually a space at a fraction of a wavelength is sufficient (rule of thumb states that distance should be more than a quarter of the wavelength). Note that, most MDTV systems use a UHF carrier frequency, as low as 470MHz. This implies that the effective distance between every two antenna in a diversity array must be at least 20-30cm to produce an effective solution. This scheme is most suited for MDTV technologies that use S-band (such as CMMB) where the wavelength is about 10 cm, and one can fit several antennas a fraction of a wavelength apart.
  • Pattern Diversity – Pattern diversity consists of two or more co-located antennas with different radiation patterns. This type of diversity makes use of directive antennas that are typically physically separated by a short distance. Collectively they are capable of discriminating a large portion of angle space and can provide a higher gain versus a single omni directional antenna.
  • Polarization DiversityPolarization diversity combines pairs of antennas with orthogonal polarizations (i.e. horizontal/vertical, ± slant 45°, Left-hand/Right-hand CP etc). Reflected signals can undergo polarization changes depending on the media. By pairing two complementary polarizations, this scheme can immunize a system from polarization mismatches that would otherwise cause signal fading.

Obviously, these schemes can be mixed. For example, two antennas can be located far away from each other, thus applying Spatial Diversity, but also positioned at a 900 angle from each other, thus applying Polarization Diversity as well.

NEXT: Weak signals, Blind Spots and Mobile Reception

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