May 24, 2006

How to speed up application development of WiMAX-ready subscriber stations

Software for configuration and control, network monitoring and management, does have to take months to develop.

By Ali Zeeshan, Fujitsu Microelectronics

Within the WiMAX systems development community, the delivery of subscriber station equipment for carrier deployment is considered by many to be the primary revenue opportunity.

Typically deployed at a ratio of dozens or hundreds of subscriber units to each base station installed, they represent the majority of mass market sales available to systems companies.

In order to speed deployment, Fujitsu Microelectronics America recently developed a WiMAX reference kit that can be used for either the popular Time Division Duplexing (TDD) scheme or the prevalent legacy Half Duplex Frequency Division Duplexing (HDX-FDD) scheme.

The kit consists of a development board and complete MAC-layer software, also includes Fujitsu’s MB87M3400 802.16-2004 WiMAX SoC and all other hardware resources required for a typical subscriber station.

This article provides a how-to overview of the steps involved in leveraging the reference kit to create self-configurable indoor or outdoor subscriber stations that operate in licensed or license-exempt bands from 2 to 11 GHz.

The reference kit is provided with the mandatory MAC and PHY functions as outlined in IEEE802.16-d standard. If the developer plans to roll out product with the mandatory features included in the kit, time-to-market can be reduced from one year to as little as three months, while the margin of error is substantially reduced and ease of implementation greatly improved.

TDD or HDX-FDD reference design
As shown in Figure 1, the WiMAX reference design board provides a radio module, a baseband daughter-board incorporating the Fujitsu WiMAX SoC, Ethernet and serial ports, power module and clock-generation circuitry. The SoC integrates MAC and PHY blocks that comply with the fully ratified IEEE 802.16-2004 standard.

Click here for Figure 1
Figure 1: Subscriber Station Reference Design in TDD Mode.

The PHY implements Orthogonal Frequency Division Modulation (OFDM) with 256 carriers. Under OFDM, the PHY supports various modulation schemes (including BPSK, QPSK, QAM16 and QAM64) in Time Division Duplex and Frequency Division Duplex modes using all channel bandwidths as specified by the WiMAX forum.

The reference kit’s complete subscriber station MAC-layer software consists of upper and lower MAC layers (UMAC and LMAC, respectively). This software runs on the SoC’s two embedded processors, but can also run on external processors under multiple operating systems.

The upper MAC software implements the MAC management layer, service-specific convergence sub-layers, and multiple service classes to differentiate the service quality. The UMAC also provides common-part sub-layer, privacy, authentication and key-management services.

Running on another of the SoC’s embedded processors, the lower MAC firmware implements time-critical lower-level 802.16 MAC functions, including PHY setup and control. Key blocks of this UMAC firmware include PHY Service Access Point (PSAP) management, PSAP scheduler, data control processor and OFDM PHY driver.

To assist in application development and customization of the upper MAC software, Fujitsu’s reference kit provides detailed documentation on the application program interfaces (APIs) to the upper and lower MAC software as well as the PSAP block.

Application design
Application software for the host processor must manage and monitor wireless-centric network parameters, including:

• Channel selection
• Network association
• Data rate
• Number of Ethernet packets and PHY frames received/transmitted
• Receiver Sensitivity Signal Strength (RSSI)
• Bit Error Rate (BER)
• Site survey (local as well as remote to minimize maintenance costs)

Click here for Figure 2
Figure 2: Indoor CPE design.

These resources can help implement a complete range of customer premises equipment (CPE) capabilities, including device configuration, network entry and other network operations. The host PC can communicate with the subscriber station via either a serial interface (RS-232) or Ethernet port for remote monitoring and management.

VoIP
Voice over IP (VoIP) applications can take advantage of the 10/100 Ethernet MAC interface provided by the Fujitsu WiMAX SoC. Most VOIP chipsets available today contain a three-port Ethernet switch with two ports (i.e., MII or PHY ports) external to the switch.

An analog phone can use one of these ports via a subscriber-line interface controller (SLIC) chip, and a WiFi-enabled router can use the other, as shown in Figure 2. The third port, configured for LAN, connects to the Ethernet port of the WiMAX reference board. Developers can also use an analog codec to interface the subscriber station with an analog telephone.

A Power PC board is also available, to provide a Time Division Multiplexing (TDM) interface to support T1/E1 interfaces for business-class CPE applications. Figure 3 shows this board with the WiMAX development board.

Click here for Figure 3
Figure 3: External processor board with subscriber station reference design.

Indoor and outdoor CPE configuration
Fujitsu’s WiMAX SoC enables an indoor subscriber station to use several different types of antennas, such as fixed broad-beam or fixed narrow-beam antennas. The equipment can also 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 combine 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.

Narrow-beam antennas send radiated power to one receiving location, or receive power from one transmitting location. 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 lies in the large gains in link transmission capacity that they can achieve within a given channel bandwidth. 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. Similarly, the power supply module of an indoor CPE can be either part of the station or an external unit.

In contrast to indoor subscriber stations, the antennas of outdoor stations are usually installed away from the station box and the power supply. Antennas may be mounted on roof tops, towers and 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. The application layer or configuration manager of the MAC will provide a proof of concept, along with the radio drivers.

In actual systems, different types of antennas are required based on requirements, so the developer will need to write the specific application accordingly.

Self-configurable subscriber stations
Self-configurable subscriber stations perform a variety of functions with minimal input from users, including system initialization and configuration, negotiating network-entry parameters and burst profiles with the base station. Application software negotiates and sets the required parameters, as well as quality-of-service (QoS) profiles for services such as VoIP, video and audio streaming, video surveillance and high-speed internet access.

Each QoS profile contains metrics (sometimes as simple 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). The MAC-layer software included in the reference kit uses the mechanisms, formats and protocols outlined in the IEEE 802.16-2004 standard to implement mandatory functions.

To support QoS arrangements, the Fujitsu WiMAX SoC enables CPE to simultaneously use as many as 16 physical transmit sub-channels and 1 sub-channelized receive link for communication with the base station.

Conclusion
The Fujitsu WiMAX reference design board serves as an enabling platform for developing indoor and outdoor TDD or HDX-FDD subscriber stations. System software deliverables include the MAC-layer software and device drivers for various operating systems. Using these resources, OEMs can rapidly develop application software for configuration and control, network monitoring and management, and multiple QoS profiles for applications such as VoIP, video surveillance, and audio and video streaming.

About the author
Ali Zeeshan is a Senior Applications Engineer with Fujitsu Microelectronics America. He has been with Fujitsu Microelectronics in applications engineering since July 2001. He received his Bachelors of Science in Electrical Engineering from the University of Engineering and Technology at Lahore, Pakistan. He can be reached at azeeshan@fma.fujitsu.com.