Build WiMAX base stations and subscriber stations

Designing for WiMAX requires an understanding of the newest MAC and PHY-layer features for fixed WiMAX systems and applications. Key areas include the MAC management layer, service-specific convergence sub-layers, multiple service classes to differentiate service quality, common-part sub-layer, privacy, authentication, key-management services, PHY setup and control, PHY Service Access Point (PSAP) management, the PSAP scheduler, data control processor, OFDM PHY driver, MIMO Wave 2, and available APIs.

This article provides guidelines and reference examples for creating base stations and self-configurable indoor or outdoor subscriber stations in 2 to 11 GHz bands. Fixed applications include wireless VPN, disaster recovery, backhaul, and broadcast-quality streaming media. Emerging mobile applications include full mobility, hi-speed vehicular data sharing, and consumer-in-motion/mobile appliances.

Today's fixed WiMAX system-on-chips (SoCs) can be used to design time-division duplexing (TDD) and frequency division duplexing (FDD) pico, micro, and multi-sector base stations in the range of 2 to 11 GHz, depending on the complexity of external components and software stack. Pico base stations are usually designed for smaller service areas with denser user bases, thus allowing a larger number of users to share the available bandwidth. A pico base station includes the radio interface, the baseband processing unit, the network interface unit, and the central control entity on one board.

In contrast, micro base stations are mainly used in sparsely populated areas where a full capacity station would be an overkill application. The micro base station is ideal for operators who need a cost effective, scalable WiMAX-ready base station solution for maximum return on investment from their network deployment, in low-density or rural areas. The representative WiMAX reference design operating in TDD or HDX-FDD, shown in Figure 1, can be used as a pico or a micro base station by developing the upper MAC and application layer software for two integrated on-chip RISC processors.

1. Fixed WiMAX Reference Design in TDD or HDX FDD Mode.

Major components of the reference design include a representative WiMAX SoC (e.g. here, the Fujitsu MB87M3550), SDRAM and FLASH memories, radio reference board, Ethernet PHY, VCTCXO, power regulator block and direct digital synthesizer (DDS). A 20-MHz VCTCXO provides the clock source for the SOC, sampling clocks for the analog-to-digital converters (ADC)/digital-to-analog converters (DACs), and reference clock for local oscillators in the RF circuit. A single clock source allows the synchronous operation of the entire system. The frequency adjustment of VCTCXO is done by the automatic frequency control (AFC) function using a 12-bit DAC. A DDS or programmable phase locked loop (PLL) is required to generate a sampling clock for the ADCs and DACs. DDS should be used only to support multiple channel bandwidths. If the channel bandwidth is fixed, a PLL can be used instead, for cost reduction of system.

A multi sector base station is able to serve more than one sector. Multi sector base stations are commonly used in service areas where large numbers of users are grouped into different sectors based on their locations. In order to implement complex upper MAC software for multi sector base stations, reference designs are available where the on-chip ARM processor and its subsystem are disabled and replaced by a more powerful external processor. PowerPC processors, based on the POWER QUICK III^TM architecture by Freescale, are shown in Figure 2.

2. TDD or HDX-FDD Reference Board System Using External Processor.

In order to connect an external powerPC to the direct slave interface (DSI) of the reference WiMAX SoC, the CMODE pin on the chip is set to 1. With DSI, the external processor is the master and the WiMAX SoC is the slave. The DSI connects to the Code RAM and LD/ST interfaces of the ARC processor, which run the time critical functions of lower MAC. In this application, the external processor can be used to run the upper MAC and user applications such as network and security management, fault detection, and system performance for a base station.

Fixed and Mobile CPE Configurations
Fixed Outdoor CPE
Fixed WiMAX SoCs are more suitable to design high-performance outdoor subscriber stations in internal or external processor modes, with minimal additional algorithms. The antennas of outdoor stations are usually installed away from the modem box and the power supply. Antennas may be mounted on roof tops, towers, or hill tops, depending on the type of terrain and desired coverage area. Software can adjust the antenna gain characteristics and the radiated transmit power to conform to local regulatory limits and reduce noise interference in large networks.