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Base stations are defined as low-power, multichannel two-way radios that are set up in fixed locations. They are used by low-power, single-channel, two-way radios such as mobile and portable phones, as well as wireless routers. When a consumer talks on a mobile phone, for example, signals are relayed back and forth between the base station and the mobile phone, as well as between the base station and the caller to which the consumer has called — either on another mobile phone or a land line. Understandably, base stations play a crucial role in the proliferation and widespread acceptance of today's mobile communications industry. For this reason, attention is continually focused on how to further improve base station performance and to cut costs, while enabling it to accommodate the ever-evolving set of new and existing wireless standards. One such standard is worldwide interoperability for microwave access (WiMAX); a standards-based wireless technology that provides high throughput broadband connections over long distances.

It is now possible to design a single or multisector base station using a WiMAX system-on-chip (SoC) reference board, which can be configured to support TDD or FDD mode, depending on the radio selection. The reference board can use either the internal (on-chip) processor or an external processor (Figure 1). Usually, a carrier-class base station requires a high-performance external processor. Use of the reference board provides a cost-effective solution for both subscriber stations and base stations in the licensed or license-exempt bands from 2 GHz to 11GHz.

A TDD base station allows the use of a single frequency for both uplink and downlink in the communication channel. This single-frequency operation is achieved by time-scaling the channel fast enough so that the transmitters and receivers see a continuous flow of information. The main advantage of TDD mode is that the time occupancy of the channel can be adjusted to accommodate asymmetrical traffic volumes on either uplink or downlink. The reduction in channel idle time leads to a greater efficiency than with FDD, in which some channel idle time may occur with asymmetrical traffic volume.

The major blocks of the TDD base station reference design include the radio board, WiMAX SoC, Ethernet and serial ports, power module and clock-generation circuitry. The application software and medium access control (MAC) functions use the onboard flash and SDRAM. A DDS generates the sampling clock for the analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), although it is needed only if the design must support multiple channel bandwidths. If the channel bandwidth is fixed, a clock source can replace the DDS to reduce cost.

A TDD base station can also employ an external processor with a reference design. The external processor connects to the WiMAX SoC through the direct slave interface (DSI) bus. The external processor can be used to run most 802.16-2004 MAC sublayer functions, as well as network management, configuration, security management, fault and performance monitoring, and additional user applications for a complex carrier-class base station. The on-chip auxiliary processor runs a set of time-critical functions of the MAC sublayer (lower MAC). As shown in Figure 2, the external processor provides SPI, I2C, software debug port, DSI and Ethernet MAC blocks.

FDD mode is implemented by allocating distinct uplink and downlink frequencies for the communication channel. FDD may be more useful in systems expecting symmetric traffic. A full-duplex FDD base station can be implemented as shown in Figure 3. In full-duplex FDD base stations, two WiMAX SoCs are connected to the same DSI bus. The on-chip auxiliary processor in the transmit path executes transmit lower-MAC functions and the one in the receive path executes receive lower-MAC functions. Similarly, each physical layer (PHY) handles either transmit or receive functions. Two separate radios are required to transmit and receive, which increases the overall cost of the system.

A multisector base station is one that is able to serve more than one sector. A sector is essentially defined as an area that contains a specific number of users. Sectors can also be made based on the geographical and structural properties of a region. For example, highways in a region can be covered in one sector, whereas the region's buildings and offices can be covered in another.

A multisector base station usually consists of multiple circuit boards housed in a box or rack and is tightly synchronized with other multisector base stations. A synchronizing signal eliminates interference during transmissions for various sectors. Interference between the signals can affect the performance of a base station in many ways. Specifically, interference can result in slower communication speeds and partial or complete loss of data.

Figure 4 shows a four-sector FDD base station that can be realized using full-duplex FDD reference boards. Each reference board supports two radios and processes the traffic of a specific sector. The radios are connected to a fixed broad-beam antenna system, which can communicate with several specific remote-terminal customer-premise equipment (CPE) locations or provide services to an area where the locations of the CPEs are not known. Such antennas may have beamwidths that vary from 15° to 360°. Diversity antennas, adaptive antennas or multiple-input multiple-output (MIMO) antenna systems can also be used for a given system based on its efficiency, frequency of operation, bandwidth and directivity characteristics.

Each base station board transmits or receives at a specific carrier frequency and channel bandwidth fixed for the associated sector. Multiple base station units can be stacked to provide additional bandwidth using multiple channels per sector as the bandwidth and subscriber needs increase. GUI-based network management software can run on a host computer for configuration of individual base station boards, traffic monitoring and management.

In the multisector base station outlined previously, each base station board is equipped with a 10/100/1000 Base-T Ethernet interface that enables direct connection to a Gigabit- or Fast-Ethernet-based MAN. It can also enable connection to a variety of other broadband networks as a means of cellular backhaul. The T1/E1 interface provided by the external processor is also one of the major transport layers for cellular backhaul.

Various mechanisms can be implemented in software to dynamically allocate the channel bandwidth for upstream and downstream. This move enhances system efficiency by optimizing bandwidth use and packet sizes according to actual demand and service level. Similarly, algorithms can be implemented in software to dynamically select and adjust PHY and MAC layer parameters, including antenna diversity, modulation, transmit power, retransmission policy and frame size. The benefit of these algorithms is increased capacity and broader coverage that includes non-line-of-sight subscribers.

Various techniques and algorithms can be developed to provide different quality of service (QoS) profiles to the subscribers in the network based on their requirements. Each profile contains various QoS metrics (such as maximum and minimum bandwidth) based on class of service (CoS) requirements such as constant bit rate (CBR), committed information rate (CIR) or best effort (BE). Using the scheduling mechanisms, the software can enforce the metrics in each profile. The MAC layer software uses the mechanisms, formats and protocols outlined in the WiMAX IEEE 802.16-2004 standard to implement the mandatory functions.

A multisector base station can be realized by using multiple WiMAX-based TDD or FDD reference boards housed in a box or rack and tightly synchronized with other base stations to minimize signal interference. Possible antenna systems for this base station include fixed broad-beam antennas, diversity antennas, adaptive antennas or MIMO antenna systems based on efficiency, frequency of operation, bandwidth and directivity characteristics. Various algorithms can be implemented in software to provide different QoS profiles to the subscribers in the network. As a result, the system can implement various mechanisms for dynamic allocation of channel bandwidth and adjustment of various PHY and MAC layer parameters.

Ali Zeeshan is a design engineer in broadband wireless at Fujitsu Microelectronics America Inc.

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