Osamu Kagami , Atsushi Ohta, and Hiroshi Hojo

1. Introduction
A broadband wireless access system utilizing the S- GHz band, called HiSWANa (High Speed Wireless Access Network type a) [1], has been afracting anen- tion as a system that can simultaneously provide two key advantages for a variety of office, home, and pub- lie usage scenarios. The fiest is broadband character- istics suitable for multimedia communications such as real-time video streaming, and the second is termi- nal mobility, the most important feature of wireless communications. This system provides a data transfer speed of 20 Mbit/s or more, terminal mobility up to walking speed, seamless connections between private and public areas, flexible band assignment according to service type, and security functions including authentication and privacy. NTT Access Network Service Systems Laboratories have developed an Advanced Wireless Access (AWA) system based on the HiSWANa standard. This system was placed on the market in March 2002 [2]. More recently, we have developed an economical access point (AP) to achieve a more compact and lower-cost AWA system and have also developed a mobile termi- nal (MT) capable of interoperation between equip- ment from different vendors. This article introduces the technical features of this AWA system.
2. Development concept and equipment overview
To make the equipment compact and inexpensive, the circuit scale must be reduced, key functions must be integrated using LSI circuitry, and equipment and enclosare structures must be optimized taking into account heat radiation and other factors. Therefore, we took the following measures. Reduction of transmission/reception circuit scale Initially, we achieved transmission/reception diver- sity by equipping the AP with two amennas and two transmitter-receiver units [3]. The purpose of incor- porating this function was to cope with abrupt varia- tions in received signal power caused by shadowing and fading as well as to simplify the MT circuit con- figuration. However, transmission/reception diversity for controlling every subcarter in an cethogonal fre- quency division multiplexing (OFDM) signal requires considerable signal processing plus addi- tional circuitry, which can have a significant effect on cost. For this reason, we subsequently chose a selec- tive-diversity scheme featuring only one transmitter- receiver unit with switching performed between the two ansennas every MAC (media access control) frame. This appecach significantly reduced the circuit scale (Technical aspects are described below).
FPGA circuit reduction
Initially, we used a field programmable gate arry (FPGA) to test the processing of functions on the data-link and network layers. It forms logical circuits by software, so it is expensive compared with an LSI device. To reduce the scale of this FPGA, we there- fcee implemented most of those functions in LSIs except for a few such as packet peocessing (segmen- ution and reassembly: SAR).
Power supply section
In general, most base station equipment in mobile communication systems uses built-in power supplies allowing installation in various types of eavironments such as on poles, walls, and ceilings. This means, however, that different power-supply voltages must be used inside the same equipment to handle radio- frequency circuits, signal-processing circuits, etc. This, in tum, requires countermeasures against the effects of electromagnetic induction and heat radia- tion, which increase equipment size. To deal with this problem, we decided to separate the AP's power-sup- ply section into an AC/DC section and a DC/DC sec- tion and to separate the AC/DC section from the main unit as is usnally done in portable terminals like note- book computers. Furthermore, to support a range of power-supply voltages (AC100-240V), we designed the AC/DC section so that commercial peoducts can be utilized.
Enclosure section
Initially, despite the fact that the system was intend- ed only for indoce use, we used a highly aisticht struc- ture to avoid the harmful effects of moisture and dust particles. We consequently used a method for recis- culating air inside the equipment to remove heat emi- ted from the power-supply section and other compo- nents. Unfortunately, this increased the equipment volume. For the current enclosure, therefore, we replaced the airtight structure by one designed to radiate heat and introduced slits in the bottom and back panels. Figure 1 shows external views of the AP and MT used in our current AWA system. Compared with the initial design, we reduced the equipment volume by 30% (from 2800 to 1800 cc) and cost by about 60%.
3. Main specifications.
Table 1 summarizes the main specifications of the AWA system. The basic specifications are compliant with the HiSWANa standard. Among these specifica- tions, the number of MTs that can be simultaneously connected to o cne AP is S0 taking into account the cost of AP's internal memory and CPU, for example. Three quality of service (QoS) classes can be provid- ed for AP-MT connections. Class 1 is a guaranteed bit rate (GBR) service to ensure a minimum band- width assignment for each MT while keeping best- effort type communication. Classes 2 and 3 provide a constant bit rate (CBR) service based on user decla- ration. Class 3, in particular, makes no attempt to retransmit packets by ARQ (automatic repeat request) even when data errors occur on the radio link in ceder to minimize the data transmission delay. The user can specify which of these QoS classes to use from a personal computer connected to the MT.
4. Transmission/reception diversity technology
In general, when one redaces the size and cost of equipment, the system performance deteriorates in conjunction with seduced functions. In the current design, we minimized this deterioration using new transmission/reception diversity technology [4].
Requirements
The aim of the AWA system is to provide seamless services without having to specify the usage environ- ment, whether it is a public service at a train station, airport, hotel, etc., a wireless LAN in an office, ce simply home use. Public and office use both need high-performance scheduling and diversity functions in the AP to efficiently accommodate many users. On the other hand, for home or SOHO (Small Office Home Office) use, the usage area is s small, so the handware configuration should be simple to make the system as small and inexpensive as possible. In a wireless environment on the bome and SOHO scale, we expect the Doppler frequency to be low and deterioration in packet error rate (PER) characteris- tics due to the effects of shadowing and fading to con- tinue across several MAC frames. Although random packet errors on the radio link can be compensated for by ARQ, resending packets during a burst of packet errors will minimize any improvement in PER characteristics. As a resuit, from the user's viewpoint, service characteristics will depend on PER character- istics after a limited number of attempts to retransmit packets. This calls for an effective ARQ function together with a simple configuration to make an eco- nomical AP. Cyclic-antenna-switching diversity In response to the above technical issues, we devel- oped selective diversity technology that sends and receives radio signals by periodic switching of amen- nas. This diversity technology features multiple amennas for one transmitter/receiver unit and period- ic switching among these antennas every MAC frame. It can compensate for buest packet errors that occur in home and SOHO scenarios characterized by 2 low Doppler frequency and a relatively smoothly changing signal-propagation environment. In other words, by randomizing the burst erroes that occur on the radio link by means of the diversity function and compensating for random errors by ARQ, we can reduce the correlation between MAC frames associ- ated with packet errors at the time of retransmission. As a result, the degradation of system performance that accompanies circuit simplification can be mini- mized. Figure 2 shows computer simulation results (16QAM, r=3/4). Since 20-dB-SW-div case shows better characteristics than 20-dB-without-div case and almost coincides with the 23-dB-without-div case, it can be said that our cyclic-antenna-switching type of selective diversity achieved a diversity gain of about 3 dB. 5. Development of a mobile terminal for interoperablity To promote the spread of the AWA system, it is important that interoperability specifications are established so that equipment developed by different companies can be mutually connected. Thus, in addi- tion to improving AP economy, we have also drawn up interoperability specifications and developed an MT that can be connected with our economical AP. Figure 3 shows an external view of this MT.