Latest Trends in FPUs

Transcription

1 Latest Trends in FPUs Electromagnetic waves in the 700-MHz band ( MHz) have good transmission characteristics, and they have been used since the advent of analog field-pickup units (FPU) with FM modulation for mobile coverage of events such as road races. However, in response to the rapid increase in communications traffic from mobile phones and other devices, the Ministry of Internal Affairs and Communications (MIC) has indicated its intention to reorganize frequencies as to their uses, allocating the 700-MHz band to mobile phones and the 1.2-GHz and 2.3-GHz bands to FPUs. Accordingly, the frequencies used by FPUs will be migrated from the 700-MHz band to higher frequencies, and new mobile relay systems will need to be built in accordance with the more severe transmission conditions that can be expected at these frequencies. We have been involved in R&D on a multipleinput multiple-output (MIMO) transmission system since FY2006 with the objectives of increasing mobile transmission capacity and reliability. In this article, we describe this MIMO transmission system using spacetime trellis coding as well as prototype transmission equipment that will be used to demonstrate the frequency migration. 1. Introduction According to the MIC Frequency Reorganization Action Plan 1) published in September, 2011, FPUs using the 700-MHz band are candidates for migration to the 1.2-GHz and 2.3-GHz bands, and the technical conditions affecting their use are to be studied. The 700-MHz band has good propagation characteristics in mobile environments, so it has been used by broadcasters for program contribution requiring mobile coverage, such as road races and golf. The frequency reorganization policy calls for FPUs frequencies to be migrated from this band to ones from 1.5 to 3 times higher (Figure 1). This effort will require advanced FPU transmission technology, including MIMO technology and better error correction coding, to deal with the harsher propagation characteristics of the new frequencies. In light of this, the Information and Communications Council summarized the technical requirements that will have to be met by advanced FPUs in order to increase the capacity and reliability of mobile transmission. In this article, we describe space-time trellis coding, which is being studied as a MIMO technology, and give an overview of the architecture and prototype equipment for an advanced FPU system. 2. Requirements for Mobile Relay FPUs Broadcasters use FPUs to transmit video and audio contributions from outside to the station. These contributions are then used in news, informational, sports, and other programs. FPUs operating in the 700 MHz band, in particular, have often been used to cover road races such as marathons and relays and have been essential for winter sports coverage. As such, there is a need to ensure the same level of transmission and operational performance in migrating from the old band to the new ones. The propagation conditions for mobile relays are also more stringent than those for fixed relays, so modulation and error correction schemes were selected that are less prone to error than those used for fixed relays. As a result, the transmittable bit rates are lower and the quality of video deteriorates relative to that possible with fixed relays. Thus, to keep video quality comparable to the current level of fixed relay transmission, there is also a need to use high-efficiency video coding. The requirements for mobile-relay FPUs operating in the 1.2-GHz and 2.3-GHz bands are as follows. (1) Transmission * Mobile relay beyond line-of-site transmission * Accurate transmission of materials, even when the transmitting antenna is not pointed exactly at the Current FPU New frequency (1.2 GHz band) New freq. (2.3 GHz band) Frequency MHz Frequency Migration Carriers Figure 1: New frequency arrangements for FPUs 2

2 Feature receiving antenna * Accurate transmission of materials in multi-path environments such as urban areas (2) Propagation distance * Guaranteed mobile relay transmission over distances from 0.1 to 10 km (3) Image quality * Ability to transmit high-quality, high-definition television (HDTV) with video bit rates of 35 Mbit/s for mobile relay (4) Number of simultaneously usable channels * At least four, as in the current 700-MHz band Conditions (1) and (2), regarding transmission and propagation distances, are general requirements for mobile relays, but to satisfy them under the more stringent transmission conditions, the MIMO technology has to use propagation path diversity effectively to make transmissions more robust against interruptions. Condition (3), regarding image quality, assumes the use of high-efficiency coding, but transmitting a 35-Mbps video signal from a mobile relay without interruption is difficult using conventional methods 2). Here, MIMO technology can also be used to increase the transmission capacity. Condition (4), regarding the number of channels, can be met by increasing the available bandwidth, as shown in Figure 1, so the channel bandwidth was increased to 18 MHz, double that of the channels used in the 700-MHz band (9 MHz). This makes it possible to use more than four channels simultaneously. Section 3 below discusses the introduction of MIMO technology in detail. 3. Use of MIMO Transmission Scheme Stable FPU transmission of video for mobile coverage of road races, etc., can be achieved using orthogonal frequency division multiplexing (OFDM) 2). However, it is difficult to ensure uninterrupted radio transmission of a video signal with the 35-Mbps 3) bit rate by using a single transmitter and a single receiver antenna (single-input single-output, or SISO), even when using advanced video coding/h.264 (AVC/H264). To resolve this issue, 2x2 MIMO transmission was studied, using two transmitting and two receiving antennas to increase reliability and double transmission capacity. A comparison of SISO and transmission schemes is shown in Figure 2; SISO uses only one transmission path, while 2x2 MIMO uses four, due to their being two transmitter and two receiver antennas. Because of the multiple transmission paths, the signal reaches the receiver more easily than with SISO transmission, even if there is an obstruction such as a sign, foot-bridge, or building on the transmission path. Spatial multiplexing is usually used to increase the capacity of MIMO transmission, but for FPU mobile transmission in a severe transmission environment, reliability of the connection, rather than capacity, is paramount. To achieve a reliable connection, we used space-time trellis coded MIMO (STTC-MIMO), which provides error correction effects in both the time and space directions. 4. 2x2 STTC-MIMO Transmission System 4.1 Overall System Architecture The system architecture of a road-race relay system using 2x2 STTC-MIMO transmission 4)5) is shown in Figure 3. The relay truck is equipped with two sets of transmitting antennas and power amplifiers (PA), transmitter high-frequency components incorporating an up-converter for each set, and an STTC-MIMO- OFDM modulator. The architecture and transmission characteristics of the MIMO transmission system are different from those of earlier systems, so it becomes necessary to keep the following in mind when building an effective relay system. * The distance between transmitting antennas should be as large as possible * The distance between receiving antennas should be as large as possible * Cable losses between the PAs and the antennas should be minimized By increasing the distance between the antennas, the correlation between the propagation path responses is minimized, thereby improving the transmission characteristics. Transmitting Ant. Receiving Ant. Transmitting Ant. Receiving Ant. Transmission path Transmission path (a) SISO Transmission (b) 2x2 MIMO Transmission Figure 2: Comparison of SISO and MIMO Transmission 3

3 Receiving Antenna Transmitting Antenna Rx. High Freq. Comp. Rx. High Freq. Comp. PA1 PA2 Tx. High Freq. Comp. Camera STTC-MIMO- OFDM Demod. Camera video STTC-MIMO- OFDM Modulator Relay truck Figure 3: Road-race relay system architecture Receiving station 4.2 Modem The architectures of the 2x2 STTC-MIMO-OFDM modulator and demodulator are shown in Figure 4. The basic OFDM parameters are the same as the 1k full mode of current FPUs (1,024-point FFT), and the modulation methods used on each transmission path have been extended to support quadrature phase shift keying (QPSK), 8PSK, and 16 quadrature amplitude modulation (16QAM) STTC. Also, in addition to the conventional Reed-Solomon (RS) (204,188) outer coding, RS(204,166) can be selected, thereby increasing the bytes that can be corrected by approximately 2.5 times and improving the carrier-to-noise ratio (C/N) needed for pseudo-error-free transmission by 1 db. Camera H.264 Video Encoder <Two-transmission STTC-MIMO-OFDM Modulator> Energy Dispersal RS* Coding Byte Interleaving STTC coding OFDM Mod.1 OFDM Mod.2 Transmitter1 Transmitter2 Two-transmitter STTC-MIMO-OFDM Modulator RS 204,166 code or RS 204,188 code is selectable Convolution coder 1 Convolution coder 2 <Two-transmission STTC-MIMO-OFDM Demodulator> Receiver 1 Receiver 2 OFDM Demod. 1 OFDM Demod. 1 Spatial Vitterbi Decod. Byte Deinterleaving RS Decode Energy Reverse Dispersal H.264 Video Decoder Transmitted Video Rx. Signal replica Metrics calculation Rx. Signal replica Metrics calculation Trellis diagram branch selection & decode Reed Solomon Two-recevier STTC-MIMO-OFDM Demodulator Figure 4: 2x2 STTC-MIMO-OFDM modulator/demodulator architecture 4

4 Feature Figure 7: Prototype transmitting antennas (from left hand, 700MHz, 1.2GHz and 2.3GHz) 2.3-GHz-band transmitter high-frequency component Figure 5: Bit error rate characteristics for standard and earphone monitor microphone transmission modes (16QAM, Rayleigh fading environment) 1.2-GHz-band power amplifier Figure 6: Bit error rate characteristics for interference-tolerant microphone transmission mode (QPSK, Rayleigh fading environment) Broadcast Technology No.56, Spring 2014 C NHK STRL 560mm Figure 8: Prototype recieving antennas (from left hand, 700MHz, 1.2GHz and 2.3GHz) 5. Conclusions We have discussed a space-time trellis coding MIMO transmission system being studied for frequency migration of 700 MHz band FPUs, as well as prototype equipment that we have developed. MIMO transmission is able to increase transmission capacity by utilizing multiple transmitting and receiving antennas, while it increases channel reliability by making use of diversity effects. However, the characteristics of MIMO transmission are different than those of conventional SISO transmission and tend to be somewhat degraded in *1 2.3-GHz-band power amplifier 300mm 360mm 760mm 4.4 Antenna In the design and development of the antennas, we prototyped transmitting and receiving antennas for the 1.2 GHz and 2.3 GHz bands with characteristics equivalent to those used with current 700-MHz-band equipment. The transmitting antennas have a two-stage collinear*3 structure and the ability to withstand inputs of 25 W and 40 W signals in the respective bands. The prototype transmitting antennas are shown in Figure 7. The prototype receiving antennas have characteristics equivalent to the current system. Eight-element Yagi antennas are used for both bands. The prototype receiving antennas are shown in Figure mm 1.2-GHz-band transmitter high-frequency component laterally diffused MOS (LDMOS)*1 devices, has power consumption of 300 to 400 VA, weighs approximately 15 kg, and has a 3U-sized*2 rack-mount enclosure. The 2.3-GHz-band power amplifier uses GaN (Gallium Nitride) devices, has a power consumption of 600 to 700 VA, weights approximately 25 kg, and has a 4U-sized rack-mount enclosure. 765mm 4.3 Transmitter High-frequency Section and Power Amplifier (1) Transmitter high-frequency section The 1.2-GHz-band and 2.3-GHz-band high-frequency transmitter components are shown in Figure 5. These components up-convert from the intermediate frequency (IF) output of the modulator to radio frequency (RF). Both assume two-transmission MIMO, both frequency conversion components share a local oscillator, and the two transmitter frequency circuits are the same. Also, the center frequency can be varied in 4.5 MHz increments so that they can transmit at each frequency within the regulated bandwidth. Note that in a recent study, it has been proposed that center frequencies be varied in 1 MHz increments so that interference with other systems using these frequencies can be avoided. (2) Power amplifier The transmission system should have a coverage area and number of channels comparable to earlier FPUs and should not require more receiver stations than there were before the frequency migration. To achieve this, we assumed that the output would be higher than the 5 W output of the current 700-MHz-band OFDM FPUs, and we prototyped power amplifier systems for the 1.2-GHz and 2.3-GHz bands with mean outputs of 25 W and 40 W, respectively. The appearances of these amplifiers are shown in Figure 6. The one for the 1.2-GHz band uses *2 *3 A horizontal metal-oxide semiconductor (MOS) transistor with a structure that mitigates electric field strength between drain and gate, increasing high-voltage tolerance. U (Unit) is a unit of height for equipment installed in racks. 1 U = 1.75 in. (44.45 mm). An antenna with multiple 1/2 wavelength dipole antennas aligned in a straight line to increase gain. 5

5 line-of-site environments. To compensate for this, it was necessary to introduce RS coding, with approximately 2.5-times the error correction capability as a new transmission parameter and to devise operational ways to reduce correlations among the transmission signals. Generally speaking, MIMO transmission is susceptible to transmission path effects. Designing the transmission system based on a good understanding of these characteristics, has enabled us to utilize the benefit of having twice the capacity of conventional SISO methods. On July 24, 2013, the Information and Communications Council summarized the technical requirements for MIMO transmission. We look forward to steady progress in the future, in setting standards at the Association of Radio Industries and Businesses (ARIB) and in commercialization of the technology by manufacturers. This article has been revised and corrected based on the following article in the ITE Journal: Ikeda: Recent Technologies for Advanced FPU, ITE Journal, Vol. 67, No. 10, pp (2013) (Japanese). (Tetsuomi Ikeda) References 1) Ministry of Internal Affairs and Communications: Frequency Migration Action Plan (Sept Rev.), (Japanese). 2) Association of Radio Industries and Businesses, Portable OFDM Digital Transmission System for Television Program Contribution, ARIB STD-B33, Ver. 1.2, (2011) (Japanese). 3) ITU-R Rec. BT-1872, User Requirements for Digital Electronic News Gathering (2010). 4) Nakagawa, Mitsuyama, Kambara, Ikeda: Transmission Performance of Space-time Codes in Urban mobile environment, IEICE Tech. Rept., RCS , pp , (2009) (Japanese). 5) Nakagawa, Ikeda: Performance Improvement of 2x2 STTC-MIMO-OFDM System, ITE Tech. Rept., Vol. 36, No. 10, BTC , pp , (2012) (Japanese). 6