Channel Characteristics for Fixed and Portable DTV Reception in a Single Frequency Network

— This paper presents the propagation channel characteristics of a digital TV single-frequency network (SFN) obtained by carrying out field measurements using two synchronized transmitters. The measurements are performed at 31 reception points using both a directive reception antenna, which is typical of fixed reception scenarios, and an omnidirectional antenna, which is used to receive mobile signals. The characteristic parameters of the channel are obtained, including the average delay, the root mean square (RMS) delay spread, and the Rician K-factor, which are important for the design of SFN systems. An empirical expression is obtained for the prediction of the RMS delay spread as a function of the K factor and the distances to the transmission antennas. directional antenna at the same 31 measurement points, as well as the Rician K-factor. These The relation between the RDS and the K-factor was analyzed. The SFN fading signal followed a Rician distribution owing to the existence of a line-of-sight (LOS) component from either or both transmitting antennas at almost all reception points. Results show that the average delay and RDS clearly decrease as the K factor increases. In addition, a trend of the RDS to increase with the distances to the two transmitting antennas was observed. It was possible to derive a simple empirical expression to estimate the values of RDS based on these distances and the K factor. This estimator can be useful because it is easier to measure the K factor than the RDS, which needs to be obtained from the PDPs. However, additional measurements will be required to verify this expression.

I. INTRODUCTION Single-frequency network (SFN) transmission in digital terrestrial television systems is notably different from the traditional single-transmitter mode. Additional transmitters can improve the coverage. but also increase the multipath effect owing to the presence of reflected and signals that are transmitted from different sources reaching the receiver. The occurrence of severe multipath propagation at the receiver is known as the SFN effect and is particularly significant for portable systems that use omnidirectional reception antennas.
In a previous paper [1], field measurements in a dual-site SFN network were reported, and initial results that were presented include the cumulative distribution of the path loss and average values of the average excess delay and root mean square (RMS) delay spread (RDS) measured with directional antennas. In this paper, the data collected in that experiment are further analyzed to provide the RDS and the Rician K-factor for portable reception with an omnidirectional antenna, and fixed reception with a directional antenna at the same 31 measurement points, as well as the Rician K-factor. These

Channel Characteristics for Fixed and Portable DTV Reception in a Single
Frequency Network Maurício V. Guerra  parameters are used to characterize the multipath behavior in the SFN network, and an empirical expression was obtained for the relation between them.
Many past studies have analyzed SFNs and issues related to their implementation. In [2], principles and properties of SFNs in digital terrestrial broadcasting, where basic definitions and contextual relationships such as the guard interval, SFN area, and influence of the used modulation parameters are explained.
Additional works regarding the evolutionary state-of-art are proposed in [3][4][5][6][7]. In [3], the author presents a design performed for the DVB-T digital terrestrial television network in Greece. Optimal SFN network configurations for second generation digital terrestrial broadcast system (DVB-T2) are obtained in [4]. In [5], the authors estimate the reception quality under the SFN environment with the delay spread of two transmitters shorter than the guard interval. In [6], the SFN threshold reception for broadcasting is obtained by analyzing and evaluating the effects of the delay time between two SFN transmitters within the guard interval time. The minimum reception threshold in single-input-singleoutput (SISO) mode SFN broadcasting is analyzed in [7].
The flexibility and configuration options provided by the new DVB terrestrial standard have been proposed in [8], [9]. In the same way, measurements of simulated and real channel characteristics in the digital video broadcasting-terrestrial (DVB-T2) system were presented in [10], [11]. Compared with the SISO mode presented in [1], where significant destructive spectral interference is translated to higher bit error rate (BER) values, in these two works the DVB-T2 advanced multiple-input singleoutput (MISO) transmission technique has been shown to be a primary contributing factor associated with the actual digital television (DTV) platforms that fulfill modern technical requirements, and which meet user and market demands for HDTV services.
Section II of this paper describes the measurement campaign, which was carried out in the coverage area of a two-transmitter SFN network operating in the UHF band in the city of Rio de Janeiro, Brazil.
Section III gives a brief overview of wideband channel characterization, including the definition of the channel characteristic parameters.
Section IV includes the main contributions of the paper. The analysis of the behavior of the RDS for fixed and portable reception is presented. The characteristic parameters of the channel are obtained, (ISDB-T) provides important improvements. The key technology bandwidth segmented transmission orthogonal frequency-division multiplex (BST-OFDM) enables ISDB-T to support multiple services [12], [13] over the same channel, including portable and fixed reception. In addition, a longer interleave (guard-interval) is used to improve the mobile reception performance. The main system transmission parameters include the carrier modulation scheme, the coding rate of the inner error-correcting code, and the length of time interleaving, which can be set individually for each segment. The ISDB-T offers three transmission modes having different carrier intervals to deal with a variety of channel conditions, such as the multipath (mitigated with the variable guard interval as determined by the network configuration) and the Doppler shift, which occurs for mobile reception. Table I lists the basic parameters of each mode. The measurement setup uses OFDM modulation in the ISDBT-T system to allow the evaluation of the RDS parameter by processing signals received from regular transmissions. Two transmitters, one at the peak of the Sumaré mountain and the other on the top of the Pena hill, were used to broadcast the same signal [1]. The parameters of the transmitted OFDM signal used in this particular experiment are shown in Table II. The mobile unit and the receiver set-up are shown in Fig. 1. It includes a low noise amplifier (LNA) which is connected to a vector analyzer and the set top box used to display the received signal. The data acquisition module was also in the mobile unit in order to perform the filtering of the collected signals and the necessary processing. On the top of the mast of the mobile unit it is possible to see the two reception antennas used in the measurement campaigns.  The ANRITSU MS8901A network analyzer, which is capable of measuring the multipath power delay profile (PDP), was configured to be used as an ISDB-T receiver. Fig. 3 illustrates the measurement of a three-component multipath signal at the receiver.

A. Root-Mean-Square Delay Spread (RDS)
The RDS is the most important single parameter used to characterize the multipath effect in a radio channel. The RDS is defined as the square root of the second central moment of the PDP. It is given by where: For both reception points with PDPs depicted in Fig. 5, two clusters of multipath components can be clearly identified, corresponding to the signals from each transmitting antenna. Compared to the measured PDP received from a single transmitter, the measured PDPs received from two transmitters had greater spread, and the PDP has long delay echoes owing to the multiple SFN transmitters.

B. Rician K-Factor
The Rician K-factor [15] is defined as the ratio of the signal power in the dominant component and is also known as the line-of sight (LOS) component over the power of the scattered components.
where rs is the amplitude of the dominant component of the signal, and represents the variance of the multipath components. The K-factor is a parameter that is used to quantify the channel fading severity.
An accurate characterization of the K-factor is useful in link-budget calculations and in the design of adaptive receivers.

A. Average delay and RDS
The delay dispersion parameter was calculated for the 31 measurements points considering the threshold below which the multipath components are ignored. Table III shows   μs), in mode 2 with a guard interval of 1/32 (Tg = 15.75 μs), and in mode 3 with a guard interval of 1/32 (Tg = 31.5 μs), thus maximizing the spectral efficiency of the system [8]- [11].
The use of more than one transmitter, although improving the coverage [17], [18], can produce additional multipath at the receiver, and is known as the SFN effect. However, lower average and maximum values of RDS are observed for the omnidirectional antenna, as shown in Table III. This is due to the higher gain of the directional antenna, which will enhance multipath components, and will be negligible for the lower-gain omnidirectional antenna [19].  In addition, if the RDS is plotted against the distances to the main antenna and the auxiliary antenna of the SFN network, as shown in Fig. 11, a slight trend of the RDS to increase with distance can be observed. In this figure, d< corresponds to the smaller distance to an antenna, and d> corresponds to the largest distance to an antenna. Considering the limited number of measurement points and the specific configuration of the experiment, additional measurements will be required to verify this expression.

V. CONCLUSIONS AND FURTHER WORK
Delay spread parameters have been obtained from measured PDPs collected during a measurement campaign using a two-transmitter TV broadcast SFN that covers a suburban area of the city of Rio de Janeiro and its surroundings. Recommendation ITU-R REC. P.1407 definitions were used to calculate the average delay and the RDS. The method that was employed allows measurements to be carried out in operating networks and can be used in further measurement campaigns to be performed in the future.
Directional and omnidirectional antennas that are positioned 13.4 m above the ground were used in the measurements at the same 31 points inside the coverage area. The directional antennas can be used for fixed reception, whereas omnidirectional antennas are used for mobile reception.
The results include mean and maximum values of the average delay and RDS for different measurements thresholds, and the Rician K-factor, which is defined as the ratio of the signal power in the dominant component over the power of the scattered components. The maximum values of the average delay and RDS vary only slightly with the threshold, but their mean values decrease for lower thresholds as additional components are detected. Further, the RDS values are higher for the directional antenna case owing to its higher gain.
The relationship between the variation of the reception conditions and the SFN effect has also been considered. At some points, measurements were performed with the omnidirectional antenna positioned 4.1 m above the ground. Considering only these points, the average value of the RDS was slightly higher for the measurements with the lower antenna than with the higher antenna.