Some broadcasters will be looking up to new technology, like this MISO antenna, to support their conversion to ATSC 3.0.
When a significant power increase is not an option, adding Multiple Input Single Output (MISO) diversity offers an attractive path to a stronger signal.
MISO antenna ready for helicopter installation.
The new ATSC 3.0 standard will provide broadcasters with new options and increased flexibility to best serve populations with defined, high data rate services. To increase the probability that indoor, pedestrian, and mobile users will receive reliable service, the ATSC 3.0 network will need to saturate the intended coverage area with a signal level above the required target level.
In previous work, boosting the signal strength with the addition of high null fill in the main antenna, as well as adding a single frequency network (SFN), have been investigated. Various hypothetical situations were used to analyze the impact of performance of these methods on different services. Not previously discussed was the use of diversity at transmitting locations to boost signal strength and understanding the best mode of polarization diversity in a mobile environment.
There are four basic methods of boosting the signal strength in selected areas within the defined FCC contour: An increase in transmitter power; increase in null fill or beam tilt; adding an SFN; or providing diversity gain through MISO, or Multiple Input Single Output. Assuming that a transmitter power increase up to 10 times is off the table, the benefits of increasing the main antennas null fill as well as adding an SFN have shown to produce the necessary signal strengths required for ATSC data intensive services.
The Value Proposition of MISO and ATSC 3.0
ATSC 3.0 has adopted a Multiple-Input Single Output (MISO) antenna scheme to improve the overall performance in a SFN, known as Transmit Diversity Code Filter Sets (TDCFS).
TDCFS is similar to the MISO scheme adapted in DVB-T2, which is based on Alamouti coding. However, TDCFS removes the need to double the pilot overhead, and it can be extended to more than two transmitters. Alamouti argued that the only way to cost-effectively achieve the requirements of next-generation wireless systems was to increase the transmitter complexity instead of the receiver allowing the user devices to have only one antenna, keeping them small and affordable to promote public acceptance.
The maximum diversity gain, Gmax, is based on the total number of independent signal paths that exist between the transmitter and the receiver. For M transmit antennas and N receive antennas, the diversity gain can be bracketed by:
This simply translates to an expected 3 dB improvement in apparent signal strength that can be achieved when a MISO diversity technique is applied.
Implementation of polarization diversity depends on spreading the power evenly between different polarizations. Discrepancy between the pair of signals results in reduced diversity gain. This occurs when one of the pairs of signals cannot be adequately resolved. The power imbalance between the paired transmitted signals is caused by the nature of electromagnetic propagation, including fading, attenuation, and scatter with constructive and destructive interference.
Since diversity gain is directly dependent on the power imbalance between paired signals, the figure of merit is cross polarization discrimination (XPD). XPD is defined as the ratio between the available power in the vertical polarization and the horizontal polarization. For optimal diversity performance, the XPD=0dB.
〈|R|^2 〉is the expected value of the powers in each polarization. To compare and evaluate polarization diversity techniques, the first step is to understand their static response to cross polarization discrimination (XPD). The second step is then to compare the techniques’ performance characteristics in real mobile environments when a linearly polarized receiver is in motion.
Using static receiver channel modeling of crossed dipoles at arbitrary amplitudes, phases and orientations, it can be shown that the XPD is defined as:
The impact of slant linear and circularly polarized antennas transmitting MISO to a linearly polarized receive antenna in an ATSC 3.0 network can be examined with performance comparisons. The different types of polarization diversity can described by the coefficients A and B.
Table 1. Slant right / slant left linear polarization MISO compared to right and left hand circular polarization MISO.
Because the A and B coefficients are the same in all six cases, the equations that describe the XPD, are identical in all six cases. Therefore, it can be said that slant linear and circularly polarized antennas transmit the same average performance in a static MISO based system. The expected diversity gain of slant left / slant right and right hand / left hand circular polarization are on average the same. This analysis assumes the channel characteristics react the same for a linearly polarized transmitted and circularly polarized transmitted signal which makes this analysis independent of any margin improvement that is observed by a linearly polarized receive antenna while in motion.
Circular Polarization Benefits
Over the last decade, extensive testing to quantify the benefits of transmitting circular polarization to a linearly polarized receiver in motion has been conducted. To quantify this benefit, margin improvement (MI) is defined as the reduction in signal strength variability when the receiver is in motion, changing both its location and orientation.
A decade of testing in both controlled and real world environments and basing measurements on both received signal strength (RSS) and bit error rate (BER) have confirmed that transmitting circular polarization to a linearly polarized receiver in motion in a heavy scatter environment provides 5 to 7 dB of margin improvement (MI) over transmitting a linearly polarized signal to the same receiver.
As defined, diversity gain (Gd) and margin improvement (MI) are not mutually exclusive but can be considered independent processes in which their benefits are additive. This is due to the fact that the channel characteristics are not the same for transmitted linearly and transmitted circularly polarized signals. This can be explained by understanding that circular polarization helps mitigate the effects of small scale fading which is present both indoors and outdoors.
The total system gain of duel right hand / left hand circularly polarized diversity MISO system transmitting to a mobile linearly polarized receiver in motion in a heavy scatter environment is given by:
Polarization diversity depends on the ratio of power in both polarizations. For linear polarization, this depends strongly on the environment.
Cross Polarization Isolation
Imperfect antennas that couple energy from one polarization to the other increase the correlation and thus affect the maximum achievable diversity gain. The non-correlation between the polarizations, either RH / LH circular or SL / SR linear signals, is ensured by polarization isolation. For SL / SR linear antennas, the isolation is simply dictated by the amount of cross polarization radiated into their orthogonal component.
For RH / LH circular it is dictated by the axial ratio of each polarization which in turns defines the purity of the signals. Studies have shown that a cross-pol pattern isolation of 17 dB is sufficient to reach within 1% of the final desired data rate for a fixed port to port isolation of 30dB. A typical specification for cross–pol pattern isolation in today’s wireless products is 20dB. The isolation between polarizations for a circularly polarized antenna is given by:
Where AR is the axial ratio.
From this analysis, an axial ratio specification of 1.2dB should be placed on the circularly polarized antenna used for MISO diversity in order to provide a cross-pol isolation specification of 20dB.
ATSC 3.0 services will require a new definition of received signal strengths. In addition to increasing null fill in the main antenna and the addition of signal frequency network sites, the use of MISO is considered. In order to provide diversity gain throughout the coverage area, a co-located MISO system must be employed. This comes at the expense of doubling the number of transmitters in the network. It has been shown that in an equivalent propagation channel with no power imbalance between polarizations, the use of dual circular polarization diversity provides the same gain benefits as a dual linear diversity system. When considering a mobile, heavy scatter environment where the receiver is in motion, it has been shown that circular polarization provides 5 to 7dB of extra margin improvement over linear transmission. This margin improvement is an added benefit to the diversity gain. Dual circular polarization diversity also provides constant, optimal diversity gain by alleviating the branch power imbalance degradation seen by dual linear systems. Finally, in order to provide the same cross polarization isolation specification used in typical dual linear diversity systems, an axial ratio specification of 1.2dB must be applied to a circularly polarized system.
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