ATSC 3.0 Backward Compatible SFN In-Band Distribution Link and In-Band Inter-Tower Communications Network for Backhaul, IoT and Datacasting

Yiyan Wu, Liang Zhang, Wei Li, Sébastien Laflèche Communications Research Centre Canada, Ottawa, Canada

Sung-Ik Park, Jae-young Lee, Heung-Mook Kim, Namho Hur Electronics and Telecommunications Research Institute, Daejeon, Korea

S. Merrill Weiss Merrill Weiss Group LLC, Metuchen, NJ, USA

Eneko Iradier, Pablo Angueira, Jon Montalban University of Basque Country, Bilbao, Spain

Abstract – In a previous paper [1][1], we proposed an approach for providing ATSC 3.0 Studio-to- Transmitter Link (STL) data delivery for SFN operation using In-band Distribution Link (IDL – a.k.a. in- band backhaul) over the air in a 6 MHz broadcast channel. In this paper, we present detailed analysis of the backhaul issues and implementation considerations. We also exhibit field measurement results that support viability of the IDL system implementation using full-duplex transmission. We further present a full-duplex Inter-Tower Communication (ITC) system – a scalable and re-configurable wireless network for SFN , in-band inter-tower communications, and IoT/datacasting applications – that is backward compatible with ATSC 3.0. The method uses Layered Division Multiplexing (LDM) transmission to carry STL data alongside broadcast data intended for public reception. IDL is able to offer better performance and more robust operation than on-channel repeater (OCR) technologies. It offers the possibility of delivering backhaul data for future applications – such as for IoT and data delivery to connected vehicles – over DTV infrastructure. IDL and ITC are enabling technologies for achieving convergence of broadcast services with and other wireless services on the DTV spectrum.

Introduction Deployment of the ATSC 3.0 Digital TV (DTV) broadcasting system, a.k.a., NextGenTV, is beginning in North America. In addition to delivering Ultra-High-Definition (UHD) TV broadcasting services to fixed receivers with rooftop, or even indoor, antennas, ATSC 3.0 also is capable of distributing robust mobile services to portable and handheld devices and of supporting localized datacasting services. To achieve these results, ATSC 3.0 is designed with the latest transmission technologies such as Layered- Division-Multiplexing (LDM) [2], Low-Density-Parity-Check (LDPC) coding, Non-Uniform Constellation (NUC) modulation, and Multiple-Input-Multiple-Output (MIMO) transmission [3].

Single-Frequency Networks (SFNs) [4] have been shown to be an effective deployment solution for achieving good Mobile Broadcast Service (MBS) distribution. SFNs also contribute to greatly improved signal strength and uniformity for Fixed Broadcast Service (FBS) distribution, enabling delivery of much higher data rates, with consequent improvements in spectrum utilization efficiency. In ATSC 3.0, a wide range of operation modes are defined with different Cyclic Prefix (CP) lengths, which allow flexible SFN deployments to provide broadcast services meeting particular quality requirements over specific geographic regions. For example, SFN deployment could begin with an existing single transmitter and gradually deploy additional SFN transmitters at locations that provide the best improvements in service coverage and quality. For each new SFN transmitter, a studio-to-transmitter link (STL) connection would be needed to transfer data from the network’s Broadcast Gateway (BGW) to the transmitter. As more SFN transmitters are deployed, the number of required STL connections will grow correspondingly.

Consequently, in deploying an SFN, a capable, scalable, and cost-effective STL solution plays a significant role in the real success of such a system. Current solutions, which depend on fiber links or dedicated microwave links, suffer not only from potential accessibility limitations but also from high infrastructure and operational costs. An effective alternative, possible only with ATSC 3.0 or a similar system, is to use In-band Distribution Link (IDL), a.k.a. in-band backhaul, technology, which transmits STL data via wireless connections from the BGW to the SFN transmitters using the same spectrum as carries broadcast services to the public. This is a spectrum reuse method that makes more efficient use of the spectrum.

The In-band Distribution Link (IDL) Approach An approach for providing ATSC 3.0 STL data delivery for SFN operation using in-band distribution over the air in the 6 MHz broadcast channel was introduced in a previous paper [5]. The basic concept, shown in Figure 1, includes a hub transmitter, Tx-A, which presumably is an existing, relatively high- power transmitter, having a dedicated, conventional STL connection from the BGW. Tx-A receives over the STL, on an IDL data stream delivered as an ordinary PLP (Physical Layer Pipe) stream using the STL Transport Protocol (STPTP), the STL data for the other transmitters in the SFN. The other

User Device EL fixed services + STL

CL mobile services Tx-B Relay Relay Rcvr Exciter EL fixed services ONLY

CL mobile services

User Device

Tx-C

Relay Relay Rcvr Exciter

Broadcast ATSC 3.0 Gateway STL Exciter w/IDL Stream User Tx-A Device

Tx-D

Relay Relay Rcvr Exciter User Device

FIGURE 1: USING LDM TO IMPLEMENT IN-BAND DISTRIBUTION LINKS

2 transmitters are Tx-B, Tx-C, and Tx-D – also termed Relay Locations – each comprising a Relay Receiver (RL-Rx) and a Relay Transmitter (RL-Tx) in this discussion. Tx-A modulates the STL data for the other transmitters onto a PLP for ATSC 3.0 over-the-air transmission. This yields an in-band solution since the STL data distribution to the Relay Locations shares the same spectrum as the broadcast services from all transmitters in the SFN. LDM is preferred for combining STL Transport Protocol (STLTP) data and broadcast-service data within one RF TV channel. In the simplest case, Tx-A, transmits a two-layer LDM signal in which the Core Layer (CL) delivers robust mobile services and part of the Enhanced Layer (EL) capacity delivers STLTP data for the RL-Tx’s. At each RL-Tx, an associated RL-Rx is implemented to decode the STLTP data from the received EL signal. The decoded STLTP data then is fed to an ATSC 3.0 Exciter to generate the SFN broadcast-service signal for emission to the public and for potential further relaying of the STLTP data stream.

Backward-Compatible In-band Distribution Link (IDL) Implementation Considerations The proposed IDL is fully backward compatible and has no impact on existing services for consumer receivers. Figure 2 shows a block diagram of the STL signal reception at Tx-B. The received signal from Tx-A contains both the SFN service components (MBS+FBS) and the STL component. The signal from Tx-A is referenced as the Forward Signal (FWS). At Tx-B, the RL-Rx recovers the STLTP data and feeds it into the RL-Tx exciter for generation of the SFN service signal and its emission. To achieve a high-SNR condition for STL data recovery in the RL-Rx, a high-gain, directional receiving antenna is installed at Tx-B.

Figure 2 also depicts a design challenge at the RL-Rx for STL recovery. The receiving antenna collects not only the FWS from Tx-A but also the emitted signal from the RL-Tx transmitting antenna, which is called a loopback signal (LBS) at the RL-Rx input. Since the transmitting antenna and the receiving antenna both are usually installed on the same tower and both at high elevations, they are closely coupled. This results in a very high LBS signal power received at the RL-Rx input relative to the FWS.

Tx-A MBS + FBS

STL Signal Isolation RL-Rx

RL-Tx

Tx-B

FIGURE 2: IN-BAND DISTRIBUTION LINK SIGNAL RECOVERY AT RL-RX

3 In Figure 3, structures of the FWS and LBS received at the RL-Rx input are illustrated for an ATSC 3.0 SFN system with IDL using STL-TDM. In the FWS, MBS and FBS are delivered in a two-layer LDM configuration in a specific time slot, while the STLTP data for both services are delivered in a different time slot.

CL CL MBS STLTP MBS Training (MBS+FBS) Sequence

EL FBS EL FBS

(a) (b) FIGURE 3: SIGNAL STRUCTURE MODEL AT RL-RX INPUT; (A) FWS; (B) LBS

During the time slot allocated for broadcast services, the LBS waveform is the same as the FWS, although possibly slightly offset in time, because Tx-A and the Tx-B RL-Tx deliver synchronized SFN services. During the STLTP time slot, however, the FWS and LBS are different since the STLTP data has been delayed at Tx-B from the advanced data that was received in the FWS at its input. Therefore, the LBS becomes a strong interferor to STLTP recovery in the RL-Rx. This interference is also called self-interference in the in-band, full-duplex relay for LTE/5G [6].

IDL implementation requires consideration of the following design characteristics: Transmitter Timing Control for SFN Operation In an SFN of the sort shown in Figure 1, all the transmitters must deliver the same (MBS + FBS) service signals, and the waveform emissions from the different transmitters must be time-synchronized. When using IDLs, this requires that the STL data embedded in the Tx-A transmission must have a time advance relative to the service data transmitted by Tx-A. The time advance is needed to allow each RL-Rx to receive and recover the STLTP data and each RL-Tx to generate the service waveform for emission. Consequently, a timing control mechanism must be included in the Broadcast Gateway of an SFN employing IDLs to synchronize the relative timing between the STLTP data for relay by Tx-A and the STLTP data for service emission by Tx-A to align the operations of the different transmitters.

A simple timing control method for SFNs employing IDLs is illustrated in Figure 4. The figure assumes that multiple tiers of Relay Locations can be cascaded in a network and is explained as follows: • In the transmission signal from the nth hop Tx-A, the STLTP data, X(t−nT), is transmitted with a time advance of nT relative to the service data, X(t). • The loopback signal, carrying X(t−nT+T) data, is relatively much higher in signal strength at the RL-Rx input than is the STL FWS signal carrying X(t−nT) data. • The SFN time-synchronized stronger LBS signal and the weaker FWS signal, carrying the X(t−nT+T) data and the X(t−nT) data, respectively, behave like a two layer LDM signal. • Since the LBS carrying X(t-nT+T) data is a known signal to the RL-Rx, it can be successfully cancelled to recover the X(t−nT) data from the FWS.

In typical service scenarios, a time-advance T of one ATSC 3.0 frame duration is sufficient for SFN operation using IDLs.

4

FIGURE 4: ATSC 3.0 SFN TIMING CONTROL

Loopback Signal Isolation (SI) The first step in easing the process of STLTP recovery from the FWS in the presence of a strong LBS is Signal Isolation (SI) between the RL-Tx transmitting antenna and the RL-Rx receiving antenna. The objective of SI is to minimize the LBS power arriving at the RL-Rx receiving antenna and the RL-Rx input. SI can be achieved through several methods:

1) Increasing antenna spacing: At a Relay Location, both the RL-Tx transmitting antenna and the RL-Rx receiving antenna are likely to be installed on the same tower at high elevations. The LBS propagation channel between these two antennas is fundamentally a line-of-sight (LOS) channel. The distance between the antennas is at most a few hundred meters. For such a distance, the propagation loss can be well modeled as free space path loss (FSPL), which is calculated as,

FSPL( dB) =20log( d) + 20log( f ) + 32.44 where d is the distance in km and f is the frequency in MHz.

Therefore, an antenna separation distance of 100 meters results in an LBS power 20 dB lower than that from a separation distance of 10 meters.

FIGURE 5: LOOPBACK SIGNAL ISOLATION METHODS BETWEEN RL-TX AND RL-RX ANTENNAS

5 2) Blocking the Loopback Signal: Either metal shielding or RF absorbent material can be installed above the lower antenna (presumably the RL-Rx receiving antenna), as shown in Figure 5, to obstruct the path between the two antennas. This can be especially useful for some antennas with small form factors, such as panel antennas using few panels, which may not have much signal suppression along the axis of the antenna. In [7][7], a metal mesh installed on top of a panel antenna is shown to be quite effective to block the loopback signal.

3) Increasing receiving antenna directivity: Modern antenna design techniques can be applied to the receiving antenna to increase its gain in the direction of Tx-A, thereby increasing the FWS strength, and to a null in its pattern in the direction of the associated RL-Tx transmitting antenna, further reducing the received LBS power. It should be noted, however, that this scheme may require more engineering effort, installation of multiple antenna components, and a larger space for the antenna on the tower.

Loopback Signal Cancellation Whatever LBS signal power has not been sufficiently reduced before reaching the input of the RL-Rx to enable reception of the FWS signal must be cancelled in the RL-Rx itself. To cancel the LBS, the RL-Rx first must estimate the loopback channel and its channel response. To reduce the proportion of channel capacity required for STLTP data delivery, it is desirable to use a high-throughput signal configuration to carry the STL data, i.e., high-order modulation, high coding rate, MIMO, and the like. Such a configuration requires high SNR to decode and puts a premium on loopback channel-estimation accuracy.

Because the RL-Rx can have as an input the signal being transmitted during the STLTP time slot, as shown in Figure 3, the loopback channel can be estimated using decision-directed channel-estimation (DD-CE) algorithms.

To perform DD-CE, the frequency-domain (FD) least square (LS) channel estimation is first obtained as,

YkRL ( ) HkLB ( ) = XkLB ( ) X( k) + H( k) N( k ) =+Hk( ) STL FWS 0 LB Xk LB ( )

Where XSTL(k)*HFWS(k) is the received FWS from Tx-A.

Filtering in two-dimensions (2D-Filt) can be used to enhance the channel estimation accuracy. A frequency-domain filter (FD-Filt) is first applied to the LBS estimates,

HHˆ = LB− FD F LB  where F could be a minimum mean square error (MMSE) filter [8], a singular value decomposition (SVD)-based filter[9], a DFT-Filter [10][10], a Wiener filter, or simply a smooth windowing function.

A time-domain (TD) subsequently is applied to further improve the accuracy of the channel estimate:

HHˆˆ= LB−−2 D T LB FD

6 where T usually is implemented using a Wiener filter or a smooth windowing function. Computer simulations were conducted to evaluate achievable LBS cancellation performance, assuming a low-complexity DD-CE with 2D-Filt, which consists of a FD Wiener filter followed by TD average windowing.

Since the RL-Rx receiving antenna and the RL-Tx transmitting antenna are assumed to be installed on the same tower and to be closely located, the loopback channel could well be approximated as an LOS channel. For cases in which there are obstacles surrounding the tower or reflections within the tower, however, the channel could be modeled as a multipath channel having very short delay spread. Therefore, two channel models were tested in simulations – a typical LOS channel and a rare multipath channel, which was modeled as a Typical Urban channel [11] with a mean delay spread (DS) of 0.1 sec, and a maximum DS of 0.7 sec.

An ATSC 3.0 system operating in the 16k transmission mode was used in simulations. It was assumed that the FWS has an SNR of 25 dB at the RL-Rx receiver. Four loopback signal/forward signal (LBS/FWS) power ratios – 0, 10, 20, and 30 dB – were tested to evaluate LBS cancellation performance under a wide range of operational conditions. Since the