IJCNC 07

DYNAMIC INTERFERENCE SUPPRESSION FOR TV
WHITE SPACE: THE CASE OF THAILAND

Prathana Thaopanya 1 and Teerapat Sanguankotchakorn2

1Thai Public Broadcasting Service (ThaiPBS), Bangkok, Thailand

2School of Engineering and Technology, Asian Institute of Technology, Thailand

ABSTRACT

In this work, we study the problem of co-existence between the LTE (Long Term Evolution) and the Digita Terrestrial Television Broadcasting (DTTB) channel. There are three scenarios: co-channel, upper and lower adjacent channels. We use the broadcasted signal from channel 3 called ThaiPBS, the actual case ofThailand for our study where the standard of digital terrestrial television broadcasting is DVB-T2 adopting8 MHz bandwidth, while the 5 MHz bandwidth of LTE is considered as the interference. We propose a dynamic interference suppression method for increasing spectrum usage by optimizing TV white spaceutilization and minimizing interference. This method adopts the protection ratio concept to suppress the LTE interference on TV receiver. We implement our proposed algorithm as an adaptive interference controller using a Radio Frequency (RF) attenuator and a Raspberry Pi board for our testbed hardware. We illustrate the effectiveness of our proposed algorithm by doing experiment using our testbed and assessing the quality of the received TV signal by adopting the Quality of Experience (QoE) assessment. In our testbed hardware, a Log Periodic antenna is used for receiving the DTTB signal, while an RF digital transmitter is used for generating a 5MHz bandwidth of LTE signal, an ultra-high frequency (UHF) mixer is used to combine both signals, then a field strength meter is used to monitor video picture quality and to analyze the spectrum. According to the experiment, our proposed method can reduce the perceived video distortion by at least 62.5% for co-channel and 87.5% for adjacent channel, while the spectrum usage is increased by 100%.

KEYWORDS
Adjacent Channel, Co-channel, Protection Ratio, Quality of Experience, Radio Frequency Attenuator, Raspberry Pi, TV White Space, Visibility Threshold

1.INTRODUCTION
In digital terrestrial television broadcasting system, the VHF (Very High Frequency) and UHF (Ultra High Frequency) frequency bands are assigned by regulators for television broadcasting service of licensed users. However, there are many in-band frequencies that are unused in each area. That is, there are frequencies available for secondary or unlicensed users. In addition, the
need of frequency spectrum for wireless and mobile communication services, i.e., wireless broadband Internet access, IoT network, etc,. has been significantly increasing in recent years [24]. One good example is the reassignment of 700MHz frequency band for mobile communications [19]. Since not all the designed channels have been deployed in terrestrial television broadcasting in any given region, therefore, the channel called TV White Spaces (TVWS) in which the channels that are not used for broadcasting may be available for the other purposes. In general, the TV White Space spectrum ranges from 470 MHz to 790 MHz, but it may be different on any region [18]. For instance, in Thailand and some countries, the frequency spectrum of 510 MHz to 790 MHz has been assigned for Digital Terrestrial Television Broadcasting [15].

Actually, there were several TVWS trials conducted by various countries [9][11][13]~[15] [18]. In 2011, the largest TVWS trials occurred in Cambridge, and there was an implementation of the TVWS to connect the city’s infrastructure in Wilmington, North Carolina. In 2013, the TVWS was used to power a “super Wi-Fi network” in West Virginia University. In 2014, NICT Japan
and partner confirmed a successful implementation of long-range wireless communication by using IEEE 802.22 and IEEE 802.11af based systems in Tono city, Japan [18]. In Thailand, NBTC also conducted a first TVWS trial to provide the wireless broadband access in a rural area [15]. Regarding the spectrum usage in Thailand nowadays, although the demand is rapidly increasing, it is not enough. During the last year, there was the reallocation of the 700 MHz band of frequency channel used for DTTB to the mobile broadband system [19]. Presently, only the bandwidth of 184 MHz is left for the DTTB. Since the digital terrestrial television broadcasting system in Thailand has been operating in both Single Frequency Network (SFN) and Multi-Frequency Network (MFN), this can save the frequency spectrum on UHF (Ultra-High Frequency) band and can avoid the interference as well. However, only five frequency channels are adopted in some areas. This results in many unused frequency spectrums and inefficient usage. In case of coexistence between TV broadcasting signal and wireless broadband signal such as LTE, the LTE causes interference in DTTB system. In [17], they found that this situation happens when the traffic load of LTE base station exceeds 80% of its capacity in rural and suburban scenarios. In ITU recommendation [7], the protection ratios for DVB- T2 being interfered by LTE base station and user equipment were analyzed. This shows that different traffic load can cause different protection ratio. A dynamic interference management has been proposed before in the literature [17]. However, it investigated only rural and suburban scenarios in Havana, Cuba, and Ghent, Belgium. Certainly, these scenarios are different from the scenario in Thailand due to different terrain, frequency band allocation and network topology. Additionally, the specifications of the broadcasting transmitter, the propagation environment and the modulation and coding schemes are different as well. Based on the rationale described above, we are interested in the flexible approach that can analyze the interference case-by-case and the appropriate analysis will lead to the high efficiency of the system. Furthermore, for future development of wireless communications operated on TVWSs, it also needs the optimization of spectrum usage efficiency without affecting the users’ Quality of Service (QoS) and Quality of Experience (QoE). In this work, the advantages of TVWS are thoroughly studied. The solution to the problem of operating TVWS in Thailand effectively is investigated. We propose a dynamic interference management for TVWS, especially in Thailand. In our system, we consider the scenario of DTTB signal being interfered by LTE signal. Then, we implement our proposed system using Raspberry Pi as a testbed. One of the inputs to our hardware testbed is the DTTB signal which is actually received from the Thai Public Broadcasting Service (ThaiPBS) network, which is the major public broadcasting network in Thailand, while the LTE signal is generated from the signal generator. We consider the interference of frequency spectrum of 3 scenarios: co-channel, upper and lower adjacent channels. The effectiveness of our proposed method is evaluated by QoE assessment method. The contribution of this work is the hardware devices which could be used to suppress the interference of LTE on DTTB signals. The rest of this paper is structured as follows: In section 2, we describe the relevant literature review. Section 3 presents the proposed method including the detailed parameters, algorithms, all of details of testbed and assessment method, while Section 4 describes the experimental results including program and testbed validations and assessment results. Finally, we conclude our work and future work in section 5.

2. RELATED WORKS
   In this section, we review the relevant literatures to our work. International Journal of Computer Networks & Communications (IJCNC) Vol.15, No.1, January 2023 104 Up until now, there are many research works proposing the methods to solve the interference problem in TVWS (TV White Space), such as interference occurring with the primary users; TV broadcasting users. The cognitive radio and dynamic spectrum access are the potential solutions, along with the geo-location database method [14]. Cognitive radio paradigms over different regulatory constraints have been proposed as a future solution for the increased radio spectrum demands. ESTI technical report analyzes the feasibility of Long-Term Evolution (LTE) Cognitive radio systems operating on TVWS, and it also evaluates the coexistence between LTE on TVWS and television broadcasting services [3]. In terms of the frequency allocation, the cognitive radio system has been used to identify the white spaces adopted in other networks using three methods, namely beacon signal (pilot channel), geolocation database, and spectrum sensing method [23]. There are two standards that provide rural connectivity such as IEEE 802.11af and IEEE 802.22, where both standards use a geo-location database to acquire the TV band [12]. The geolocation database is the preferred method for detecting available channels and for tackling the interference problems for TVWS. In Japan, NICT has successfully developed a white space database to be included in the OFCOM’s databases list [20]. In Thailand, a similar system was carried out [15], and it is a challenging topic nowadays. Although geolocation base method might lead to lower interference and it also works as a common spectrum distribution mechanism for different networks, it causes a lower usage efficiency [16]. The case-by-case analysis is not carried out, and the database is not updated in real time. In [17], it found that cognitive radio networks on TVWS in real scenarios might cause severe interference to the broadcasting services, even though ETSI determined specifications of the device in TVWS to ensure that they will not interfere to the licensed users [20]. In [14][19], they proposed technical point of view by using the geolocation database. Although the geolocation database is a solution for TVWS usage without interference, the database updating in non-real time is still the main disadvantage. In [17], a dynamic interference management algorithm operating in a centralized spectrum management architecture along with assessment from the users was proposed. The Quality of Experience (QoE) with Threshold of Visibility (TOV) and Subjective Failure Point (SFP) are the proposed assessment for evaluating the DTTB system in [6][8]. This technique can reduce at least 50% and 27.5% of the interference in rural and suburban areas, respectively, with only 8% increment of spectrum usage compared to traditional cognitive network. To protect the mutual interference from primary to secondary services, that are operated in the same or adjacent frequency and vice versa, the protection techniques such as cooperative sensing and advanced geo-location database were proposed [3]. These techniques use the combined interference monitoring and geo-location database to protect the incumbent system from harmful interference while expanding white space opportunities. The interference monitoring aims to enhance accuracy of carrier-to-interference ratio (C/I) of the incumbent receiver [4]. There are white space and gray space concepts in [3]. This concept concerns TV coverage areas where the broadcasting stations use the same frequency (SFN; Single Frequency Network). Based on only the location of TV broadcasting stations, this frequency can only be used by a Cognitive Radio (CR) device outside the area of the coverage areas. This means that the white space spectrum concentrates on protection of TV broadcasting stations. At TV receivers, a CR device can utilize the same frequency as the signal frequency transmitted by TV broadcasting station inside the coverage area. This means that the gray space spectrum focuses on protecting TV receivers. In practice, the technical standards of DTTB of each country are different, so it can cause different protection requirements such as the minimum protected field strength, the minimum required separation distance, and the percentage of location probability degradation [2]. In [22], the coexistence scenario of a DTTB transmitter and an 802.22 base station was performed using International Journal of Computer Networks & Communications (IJCNC) Vol.15, No.1, January 2023 105 indoor and outdoor Customer Premise Equipment (CPEs) in rural environment. The initial transmission power level of the IEEE 802.22 signal was set with the protection ratio to achieve the bit error rate (BER) of 2×10-4 In [11], protection of incumbent service in TVWS was considered. In [5], the significant impact of terrain and frequency on the protection distance was shown. The separation distance values depending on type of Cognitive Radio (CR) device, antenna height and co-channel or adjacent channel adoption. In the interference protection requirements of the FCC, portable CR devices should comply with the minimum co-channel separation distance of 4 km. For instance, in [21], they investigated the coexistence of the DVBT/T2 and LTE downlink services in co-channel, considered interfering LTE signals with different bandwidths. Moreover, the modulation error ratio (MER) is used to evaluate performances of DVB-T/T2 systems. From the study, in co-channel coexistence scenarios, unwanted narrowband interfering LTE signals have less impact on DVB-T/T2 performance than the broadband. In [10], the LTE radio planning pertaining to the maximum acceptable LTE radio interface load, up to which a targeted user data rate can be maintained, was studied. The model of LTE radio scheduler was given. It provides the optimum traffic balancing for two cells. The work of [11] was studied under pure LTE radio spectrum and was analyzed using the actual data from commercial LTE networks. Some works in the literature studied the impact of interference in the co-channel scenario [10],[25]. In [25], the transmitter power on co-channel interference was studied. However, the signal under consideration is WiFi. In [1], the study of the SFN which compares field measurement results with simulation results of propagation models using the PROGIRA software was carried out. It was found that the Okumura-Hata model provides the smallest average error for a suburban area and for a path with obstruction. In [17], the experiment was performed to find how frequent the protection ratio limits are exceeded. Both TOV (Threshold of Visibility; defined in Report ITU-R BT.2035-2/2008) and SFP (Subjective Failure Point; defined in Recommendation ITU-R BT.1368-13/2017) have been defined as a criterion to find a limit for a just error-free picture at TV screen for protection ratio measurements.

3. PROPOSED MODEL
   3.1. System Model and Parameters The block diagram of the proposed dynamic interference analysis system is shown in figure 1. In our work, we consider the scenario where DVB-T2, the digital terrestrial television broadcasting (DTTB) standard adopted in Thailand, is a primary service coexisting with mobile wireless communication (LTE) which is a secondary service. Both signals are the inputs of the dynamic interference suppression algorithm, which is implemented on Raspberry Pi, our programmable testbed. Then, we assess the effectiveness of our implemented algorithm in reducing the LTE interference on DTTB signal by the QoE assessment method. The detail of all these proceduresis described in the following subsections.