Diversity Technique Employing Digitized Radio over Fiber Technology for Wide-Area Ubiquitous Network S. Kuwano, Y. Suzuki, Y. Yamada, Y. Fujino, T. Fujita, D. Uchida, K. Watanabe NTT Network Innovation Laboratories, NTT Corporation 1-1 Hikari-no-oka, Yokosuka, Kanagawa, 239-0847, JAPAN E-mail:
[email protected]
Abstract— Wide area ubiquitous networks should have the capability to handle more than 10,000 wireless terminals in a large wireless cell with the radius of several kilometers. However, the functions of wireless terminal are severely limited due to low power consumption and low cost requirements. Therefore, there is a need for a sophisticated wireless access system that employs smart wireless signal processing such as the site diversity technique with maximal-ratio-combining at the wireless access point. This paper presents the site diversity technique that employs digitized radio over fiber technology and field test that confirms the effect of site diversity. Digitized radio over fiber, maximal ratio combining, site diversity, wide area ubiquitous network
I.
INTRODUCTION
Embedded ubiquitous service employing small sensors and actuators will provide many applications, and a large market for machine-to-machine (M2M) networks will arise in the future. To achieve the M2M network, the cost and capability of the terminals should be low. A promising approach to implement ubiquitous connectivity is to use ad-hoc local networks established autonomously using terminals via gateways. However, if there are a huge number of lowcapability terminals that are distributed pervasively, this approach does not seem to work well particularly when such devices are mobile. The reasons for this are given hereafter. First, for low-capability terminals, constructing the network themselves is a heavy burden. Second, it is difficult to make a reliable ad-hoc network using many terminals that move often. Third, an appropriate gateway may not exist. Therefore, in such a situation, we propose a network infrastructure called the wide area ubiquitous network (WAUN) [1]. Wireless technologies are employed in the access system of the WAUN, and are key to establishing the WAUN. Since the WAUN is a new type of network, the conventional wireless access systems cannot fully meet the requirements such as scalability regarding the number of terminals, terminal mobility, and support for low-capability terminals. Therefore, new techniques should be developed to establish a wireless access system for WAUNs. Fundamental wireless link analysis [1] predicts that the availability of a 5km cell radius can be used with outdoor wireless terminals (WTs) that have 10 mW transmission power (license-free limit in Japan). In this case, the reception power is extremely low, and it is
difficult to achieve a wireless access system using the current wireless technologies. Therefore, to achieve these specifications, a sophisticated technology that combines wireless physical layer techniques such as modulation / demodulation, error correction and diversity should be developed. In particular, wireless access points (APs) should have a sophisticated configuration and support complex operations since WTs should have a simple configuration, simple operation, and a low power consumption level. The site diversity technique [2] with maximal-ratio-combining (MRC) using antennas separated by several kilometers is a very attractive to achieve high availability in a wireless cell. To deliver RF signals for site diversity, we developed a digitized radio over fiber (DROF) subsystem [3]. The DROF subsystem uses an Ethernet based network as the feeder lines to the separated antennas. In the following sections, we describe the diversity technique employing the DROF technology, and introduce field test to confirm the effect of site diversity using a prototype of wireless access system. II.
WIRELESS ACCESS SYSTEM FOR WAUN
A. System Architecture A schematic of the wireless access system for WAUN is shown in Fig. 1, and examples of the specifications are shown in Table I. The wireless access system consists of two network elements, an AP and WTs. The AP is connected to a WAUN IP backbone network via a radio access network server (RANS), and the WT is connected to a ubiquitous sensor or an actuator. TABLE I UBIQUITOUS WIRELESS NETWORK SPECIFICATIONS
Cell size (radius) Number of WTs per cell RF band Output power of AP Output power of WT Data rate Modulation format Demodulation scheme Coding rate Access control
~ 5 km > 10,000 VHF or UHF 100 mW 10 mW 9,600 bit/s π/4 shifted QPSK Coherent (Uplink) Incoherent (Downlink) 1/2 TDMA/TDD
978-1-4244-2324-8/08/$25.00 © 2008 IEEE. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.
AP WT
Backbone NW
RANS
AP
AP AP Wireless Cell
AP: Access Point WT: Wireless Terminal RANS: Radio Access Network Server
Figure 1Schematic of wireless access system for WAUN Site diversity Space diversity AP-R
AP-R
Space diversity
DROF Sector Cell
Omni Cell
AP-R
: Base station with three-sector antenna
AP-M
Diversity Configuration
Figure 2 Wireless cell configuration An example of the cell architecture is shown in Fig. 2. Each cell has a hexagonal shape with the radius of several kilometers, and the cells comprise a honeycomb structure. There are two types of cell configurations, sector cell and omni cell. For the sector-cell configuration, three sector antennas are deployed at the center of the cell. RF signals are emitted toward each sector. For the omni cell configuration, sector antennas are deployed at the three apexes of each cell, and RF signals are emitted toward the inside of the cell. In the omni cell, since antennas are separated by several kilometers, the spatial correlation between antennas is much lower than that in the sector-cell, and the effect of shadowing can be reduced. Therefore, the site diversity technique can be achieved only in the omni cell, and high coverage availability in the cell can be achieved. B. Site Diversity Technique Implementation of the diversity techniques is achieved in the following manner. Since WT should have a low capability level and a simple configuration with a single antenna, the diversity circuit should be implemented at the AP. So, transmit diversity is used in the downlink, and the reception diversity is used in the uplink. To achieve the site diversity based on the MRC scheme in the uplink, RF signals received at separate AP antennas should be combined before decision. So, the function of the AP is physically divided into two types of modules: the
modulation / demodulation module and RF transmission / reception module. The first module is centralized as the AP-M (master equipment of the AP) and the second type is deployed with an antenna as AP-R (remote equipment for the AP). Diversity signal processing is implemented in the AP-M. The same downlink RF signals are delivered to the AP-Rs, and the uplink RF signals at the AP-Rs are collected at the AP-M. To transmit RF signals between the AP-M and AP-Rs, we employ the DROF subsystem deployed over Ethernet or a higher IP service. Three AP-Rs are deployed to achieve site diversity, and each AP-R has at least two space diversity branches. In the downlink, a digitally modulated baseband RF signal at the AP-M is transmitted to an AP-R over Ethernet, and at the APR the received signal is converted to an analog signal and upconverted to RF at the same time. At the WT, the power of the received RF signals from the AP-Rs are combined and demodulated. The carrier frequencies of each AP-R are slightly different (lower than the symbol rate), and the reception level at the WT is time varied with the beat frequency of the received carriers from the different antennas. Therefore, there is no null point in the cell. Errors that occurred in the faded period due to the interference of carriers can be corrected by forward error correction (FEC) and an interleaving technique [4]. Fast level change due to fading can be compensated by the space diversity technique. In the uplink, the received RF signal at the AP-R is downconverted to a baseband signal and digitized, and then transmitted to the APM and digitally demodulated. At the AP-M, collected received
978-1-4244-2324-8/08/$25.00 © 2008 IEEE. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.
Packet Network
AP-R
LNA Down Conv.
A/D
Packetize
SW
PHY Up Conv.
D/A
Depacketize
AP-M Data IF
Depacketize
HPA
Modulation/ Demodulation
PHY Packetize
AP-R
AP-R Figure 3 Wireless access point employing DROF
signals with the same sampling timing are combined using the symbol-by-symbol MRC scheme to achieve the highest performance. III.
DIGITIZED RADIO OVER FIBERSUBSYSTEM
A. System DescriptioP Fig. 3 shows a schematic of the DROF subsystem configuration. For the uplink, the received RF signal at the AP-R is downconverted to a baseband or low-IF signal, and sampled and quantized using an AD converter. The digital data stream that is output from the AD converter is packetized and transmitted via a packet network that is higher than Layer 2 (Ethernet or IP network). At the AP-M, the digital data stream is subtracted from the received packet and passed to the demodulation unit. For the downlink, the generated digitized baseband or low-IF signal data stream at the AP-M is transmitted to an AP-R via the packet network, and at the AP-M, the baseband or low-IF signal is regenerated using a DA converter and upconverted to an RF signal. The user datagram protocol (UDP) and realtime protocol (RTP) are used to carry DROF packets. The use of Ethernet or a higher IP service for DROF enables the use of low-cost commercially available network service and rapid network deployment. Since the BER of digital data transmissions on the DROF subsystem is very low (
