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Build WiMAX base stations, subscriber stations

Posted: 28 Jun 2007 ?? ?Print Version ?Bookmark and Share

Keywords:WiMAX design? micro base stations? pico base stations? mobile WiMAX SoC?

By Ali Zeeshan
Fujitsu Microelectronics America

Designing for WiMAX requires an understanding of the newest MAC and PHY features for fixed WiMAX systems and applications. Key areas include the MAC management layer, service-specific convergence sublayers, multiple service classes to differentiate service quality, common-part sublayer, 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 2-11GHz 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 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 2-11GHz, 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.

Figure 1: The representative WiMAX reference design operating in TDD or HDX-FDD can be used as a pico or a micro base station.

Major components of the reference design include a representative WiMAX SoC (for example here, the Fujitsu MB87M3550), SDRAM and flash memories, radio reference board, Ethernet PHY, VCTCXO, power regulator block and direct digital synthesizer (DDS). A 20MHz VCTCXO provides the clock source for the SoC, sampling clocks for the ADC/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 12bit DAC. A DDS or programmable 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 multisector base station is able to serve more than one sector. Multisector base stations are commonly used in service areas where large numbers of users are grouped into different sectors based on their locations. To implement complex upper MAC software for multisector 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 architecture by Freescale, are shown in Figure 2.

Figure 2: TDD or HDX-FDD reference board system using external processor.

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.

CPE configurations
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 rooftops, towers or hilltops, 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.

With the mobile WiMAX SoC, in addition to outdoor CPE, an indoor subscriber station can also be implemented which can take advantage of multiple-input/multiple-output (MIMO), multiple-input/single-output (MISO) or single-input/single-output (SISO) antenna systems. These antenna systems can be combined with techniques such as space/time coding (STC), convolutional turbo code (CTC) and automatic repeat request (ARQ) to improve the overall link budget for enhanced transmit and receive capability, while minimizing cost and power consumption.

The antenna of an indoor CPE is usually fixed to the subscriber station and can be either a built-in block or an external unit of the system. Self-configurable indoor subscriber stations perform a variety of functions with minimal input from users, including system initialization and configuration, negotiation of network-entry parameters, adjustment of radio power and burst profiles with the base station. Application software negotiates and sets the required parameters, as well as QoS profiles for services such as VoIP, A/V streaming, video surveillance and high-speed Internet access.

Advanced MAC features
For some of the available WiMAX SoCs, the MAC layer software is divided into upper (UMAC) and lower (LMAC) MAC. The LMAC isolates PHY complexity from the UMAC, offloads the burden from upper MAC by performing AES/DES data encryption and decryption, CRC, HCS generation and checking and CID based filtering. The lower MAC binary files are provided for both base station and subscriber station. An LMAC binary is saved in the load/store memory of the on-chip ARC processor, which loads and executes it, once released by the UMAC running on main processor. A high level block diagram of the components and architecture of the software solution is shown in Figure 3.

Figure 3: For some of the available WiMAX SoCs, the MAC layer software is divided into upper (UMAC) and lower (LMAC) MAC.

Both the board support package (BSP) and device drivers are provided for the supported operating systems, as well as the RTOS wrapper, simplifying the porting of the complete software suite to any OS supported by the SoC. Framing, ranging, statistics reporting, bandwidth allocation, support of QoS parameters and the uplink scheduling service are some of the key features of upper MAC. The bridge is a link layer which directs the Ethernet packets destined for air link to the 802.16 MAC service-specific convergence sublayer and vice versa. A full API and interface specifications may be provided to enable third parties to develop upper-layer MAC components, for configuration of the overall system, cryptography suite, service flows, basic capabilities, scheduler queues, time intervals, security features, message counters and radio.

For example, by using the system configuration API, the user can set and get the IP address, net mask, default gateway and MAC address on both the Ethernet and WiMAX interfaces. Similarly, the functions in the basic capabilities configuration allow the user to set and get the minimum and maximum transmit power as well as configure the bandwidth allocation and the gap between uplink and downlink bursts. The core MAC API includes the functions for system initialization and shutdown, system management and system traffic management. Network entry, MAC bridge initialization, security management and performance data management are some of the important functions performed by the core MAC API interface.

Fixed WiMAX system features
Automatic gain control (AGC), automatic frequency control (AFC) and channel encoding are some of the key features of the SoC's PHY system. Figure 4 shows important blocks of a representative PHY block in the fixed WiMAX SoC.

Figure 4: AGC, AFC and channel encoding are some of the key features of the SoC's PHY system.

AGC improves the link budget and greatly helps to reduce the BER during the synchronization and data transfer between base station and subscriber station. AFC is used to adjust the frequency of an external VCTCXO so that the transmit and receive local oscillator frequencies are aligned to those of base station.

An OFDM PHY is able to support the adaptive modulation schemes, including BPSK, QPSK, QAM16 and QAM64, using the channel bandwidths of 1.5-20MHz. Automatic Power Control is a feedback mechanism implemented in the fixed WiMAX SoC that monitors certain quality parameters, such as frame error rate or BER. It characterizes the received signal and instructs the transmitter to adjust its power up or down to maintain signal quality. The unused APC range or headroom (maximum transmit power minus the normal clear sky transmit power) can also be thought of as additional fade margin that can be used to respond to rain and other fading events.

Mobile WiMAX system features
Space/time coding (STC) is the technique of adding redundant symbols to the transmitted signal to improve the receiver's ability to correctly detect the information data and it is an important feature of the mobile WiMAX SoC. With STC, multiple transmit antennas are used to separate data streams made up of symbols that are organized to achieve performance improvement when detected at the receiver. MIMO antenna systems are similar to adaptive antennas and consist of multiple-element antennas connected to an optimizing array processor. The attraction of MIMO antenna systems is the large gain in link transmission capacity that they can achieve within a given channel bandwidth. The system circumstance in which MIMO offers the greatest potential is non-line of sight systems where a high degree of scattering leads to highly uncorrelated channels, even when the antennas have fractional wavelength spacings.

Beam forming is another signal processing technique implemented in mobile WiMAX systems. It is used with arrays of transmitting or receiving transducers that control the directionality of, or sensitivity to, a radiation pattern. When receiving a signal, beam forming can increase the receiver sensitivity in the direction of wanted signals and decrease the sensitivity in the direction of interference and noise. Beam forming takes advantage of interference to change the directionality of the array.

Indoor and outdoor customer end modems supporting the IEEE 802.16-2004 standard can provide fixed and nomadic broadband wireless access for high data-rate traffic, streaming multimedia content and VoIP. They can be deployed in areas with no wireline or telephone service, such as in remote villages or as a part of disaster recovery effort to restore communications quickly. A point-to-point WiMAX system with a conventional microwave radio can also be used to backhaul point-to-multipoint Wi-Fi or WiMAX network traffic to a fiber optic backbone.

One of the most promising applications of mobile WiMAX systems is enabling low-cost cellphones, together with high data-rate multimedia traffic and Internet access, over a single IP-based network. A mobile WiMAX system can also be used to more economically and efficiently backhaul cellular, T-1, DS-3 and 10/100 Ethernet traffic than conventional cellular networks, especially in areas with no existing wireline infrastructure.

Mobile WiMAX subscriber and base stations can also be used to share high-speed vehicular data and eventually backhaul that traffic to the Internet, if required. Multimode residential gateways and digital STBs are additional applications which can enable users to download multimedia content wirelessly, almost anywhere.

About the author
Ali Zeeshan
is a Senior Applications Engineer at Fujitsu Microelectronics America.

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