ATSC 3 . 0 – Boosting the Signal Strength-MISO

The new ATSC 3.0 broadcast standard will provide new transmission capabilities. Broadcasters will have options and flexibility to best serve populations with defined, high data rate services. In order 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 situation were used to analysis the impact of performance of these methods on different services. In this paper, the use of diversity at the transit locations will be considered. Specifically, different modes of polarization diversity will be compared. BOOSTING THE SIGNAL STRENTH As discussed in recent papers [1], [2], there are four basic methods of boosting the signal strength in selected areas within the defined FCC 41 dBu contour. 1. Increase transmitter power. 2. Increase null fill or beam tilt. 3. Add a single frequency network (SFN). 4. Provide diversity gain though MISO. Assuming increasing the transmitter power up to 10 times is not an option, 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 focus of this paper will be to analyze the benefit of adding MISO (Multiple Input Single Output) diversity to a ATSC 3.0 network. THE USE OF MISO IN AN ATSC 3.0 NETWORK 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) [5], [6]. TDCFS is similar to the MISO scheme adapted in DVB-T2, which is based on Alamouti coding, but with TDCFS there is no need to double the pilot overhead and it can be extended to more than two transmitters [6]. Alamouti argued that the only way of achieving the requirements of next generation wireless systems in a cost effective way was to increase the transmitter complexity and not that of the receiver [3]. In an ATSC 3.0 network, both the main antenna and the accompanying SFN sites will serve hundreds of thousands of receivers. It is therefore more economical to add equipment at the transmit site rather than the remote units, allowing the user devices to have only one antenna, keeping them small and affordable to promote public acceptance. MISO diversity techniques such as TDCFS and Alamouti coding can be deployed in either a co-located or distributed configuration. Figure 1: Co-located vs. distributed MISO. The benefit to co-located MISO is that diversity gain is observed throughout the coverage area and not just in the overlap areas as in the case of distributed MISO. The advantage of distributed MISO is that is does not require any new RF equipment within an existing SFN, where colocation of the MISO system requires the doubling of transmitters. 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: 1 ≤ Gd ≤ Gmax = M ∗ N (1) This simply translates to an expected 3 dB improvement in apparent signal strength that can be achieved when a MISO diversity technique is applied. DIVERSITY IMPLEMENTATION Traditionally, both spatial and polarization diversity have been employed for improving connectivity in the wireless industry. For maximum performance using spatial diversity, antenna separations on the order of 10 to 30 wavelengths are needed. Due to the broadcast operating frequencies and subsequent tower space limitations, this may be difficult or impossible to implement. Polarization diversity utilizes the antenna elements that are co-located and orthogonal in antenna polarization, making this a more attractive alternative in terms of aperture space. The most common type of polarization diversity is the transmission of two independent slant linear (+/45°) signals. A slant linear antenna element excites both slant components equally in amplitude with no relative phase assigned to either element. Another alternative is to use a pair of circularly polarized antennas. Again, the antenna element uses crossed dipoles and excites both equally in amplitude but the relative phase difference is set to 90 degrees. A dual circularly polarized antenna can be created from a dual slant linear antenna simply by adding a 90 degree hybrid at the input. Figure 2: Crossed dipoles used for slant linear and circularly polarized antenna configurations. MODES OF OPERATION – SLANT LINEAR / CP Crossed dipole configurations are commonly used to produce elliptically polarized transmission in the broadcast industry. Typically for high power applications, they are in the form of an array of individual dual input panels with each input feeding one of the crossed dipoles. The array of panels are then connected by a corporate feed system leading to two main inputs. Figure 3: Two basic modes of polarization diversity operation into a dual input broadcast antenna array. Slant right / slant left vs. Right hand circular polarization / left hand circular polarization. POLARIZATION DIVERSITY – FIGURE OF