Mobile WiMAX: The Evolution to Lower Power

By: Allen Hill, Analog Devices Inc.
( 1 Jul 2008 )

WiMAX (Worldwide Interoperability for Microwave Access) is the latest next generation wireless telecommunications standard making its way to the marketplace. Based on the IEEE 802.16 air interface standard, WiMAX is a broadband wireless solution offering high data throughput, efficient data multiplexing, and low data latency. The two versions of WiMAX that are being deployed are aimed at different market segments. The first, fixed WiMAX (802.16d), is primarily utilized in fixed locations. It competes with cable and DSL to provide the public with high speed Internet access. The second, mobile WiMAX (802.16e), is aimed towards mobile internet, which provides users the convergence of broadband wireless connectivity in handheld devices.

WiMAX is based on OFDM (Orthogonal Frequency Division Multiplexing). High order modulation schemes and wide bandwidths allow high throughput, but require high linearity and low noise for proper operation. Present profiles support up to 64 QAM (Quadrature Amplitude Modulation) in bandwidths up to 10MHz, putting tremendous pressure on system designers to meet conformance specifications.

As a relatively infant technology, mobile WiMAX faces challenges in reducing size and power consumption to meet customer expectations for mobile devices. Mobile WiMAX is in its first generation. Functionality and interoperability have thus been the highest priorities for system designers. As future generation devices are contemplated, smaller size and lower power consumption will be required to ensure market acceptance. Today's solutions use modem processors that consume up to 1W or more. Add the RF transceiver, with power as high as 750mW, and transmit power amplifiers (PA) that consume above 1.5W, and the task of designing mobile devices that aren't hot to the touch is daunting. Figure 1 shows where the mobile WiMAX platforms stand in 2006/2007 with expected improvements for 2009/2010.

As specifications unify, one of the first ways to reduce power in any developing technology is to utilize more efficient design philosophies. WiMAX has started down this path as the tasks of functionality and interoperability are now accepted as givens. WiMAX profiles driven by industry organizations ensure acceptance of the technology within the marketplace, but provide only a small subset of what the full 802.16 standard allows. This enables both system and chip designers to narrow their focus to meet realistic overall requirements. As an example, 802.16 specifies channel bandwidths from 1.25MHz up to 28MHz in FDD (Frequency Division Duplex), HFDD (Half Frequency Division Duplex), and TDD (Time Division Duplex) modes. Wisely, the industry has narrowed the scope by limiting initial mobile WiMAX profiles to channel bandwidths in the range of 5MHz to 10MHz, and restricting modulation to TDD modes.

The implications of this narrowed scope are tremendous for WiMAX system and chip designers. The fact that all mobile profiles presently defined are TDD means that the device can either transmit or receive at any time, but not both. This allows chip designers to optimize their devices to handle only this condition. Limiting the channel bandwidth also improves efficiency, allowing designers to optimize filters and data converters to a set of bandwidths rather than having to scale multiple octaves.

Although narrowing the scope of the 802.16 standard reduces some of its flexibility, it ensures that the industry will follow a course where price, power, and size can be reduced over time, while also ensuring functionality and interoperability. This allows a better chance that mobile WiMAX will meet its overall market potential.

The second method of power reduction involves utilization of more optimum IC (integrated circuit) processes and system partitions. This has already begun with second and future generation of chips. Most existing WiMAX processor designs started with FPGA verification, migrating to single chip devices that are fabricated in stable and mature IC processes. All these devices are fabricated in CMOS technology with geometries ranging from about 90nm to 180nm. Obviously, next generation processor designs will explore using smaller geometries (65nm and below), following the economies of the CMOS lithography progression and capitalizing on the inherent power reduction provided by smaller gate sizes that consume lower currents. Some of these processors are now starting to appear in the market, with power supply current that is one half to one quarter of first generation devices.

System partitioning also plays a large part in power reduction. Most existing WiMAX processors contain an applications processor, DSP, fixed engines, and signal path data converters. The applications processor contains the MAC (media access control), as well as high-level software. The DSP and fixed engines, which perform the modem functions, include encoders, decoders, correction algorithms, and FFT blocks. The data converters are included as part of the signal path to convert digital signals to analog and analog signals to digital for use by RF transceiver chips.

The placement of the data converters on the processor chip leads to an inefficient partition. Mixed signal components, such as data converters, tend to be one or more lithography step sizes behind digital functions. This is based on the fact that linear circuits require much more process verification and modeling than digital. With data converters contained on the processor chip, the smallest CMOS lithography will generally not be used, thus forfeiting minimum die size and minimum power dissipation.

A better partitioning choice is to place the data converters on the radio transceiver chip. This allows the processor to be designed in the smallest digital CMOS process node, with no extra (expensive) process steps that may be required for linear circuits. An added benefit to this partition is that all interfaces are digital, so no sensitive analog signals are routed on PC boards. A JEDEC specification (JESD207) has been approved that aims to unify the digital interface between the RF transceiver and the digital processor for mobile WiMAX and other high data rate applications. Other advantages to placing the data converters on the RF transceiver include allowing all real-time loops, such as AGC (automatic gain control) on Rx and power control on Tx, to be integrated on one chip, thus minimizing software overhead between the transceiver and processor chip. Figure 2 shows the differences between the two partitions.

One of the largest consumers of power in the system is the PA. Based on the higher order modulation and narrow sub-carrier spacing utilized by mobile WiMAX, the PA has to be linear and low noise over the transmit power range. To meet these attributes, the PA consumes quite a bit of power. The good news is that the mobile WiMAX data link is generally asymmetric, downloading data approximately 70 percent time, versus percent transmitting with an active PA. Additionally, PA designers are working to achieve the required linearity and low noise with advanced processes (GaAs HBT), and design techniques such as linearization. Even with the efforts underway, the PA will be a big power consumer in mobile WiMAX systems for the next generation.

Mobile WiMAX is a very flexible communications standard that includes features to support high data rates, high QoS (Quality of Service), scalability and security. Additional advanced features of mobile WiMAX aim to improve data link performance and reduce power in the mobile handset. Most advanced features are not available in fixed WiMAX, and are yet to be deployed in mobile WiMAX due to their complexity, but the improvements gained by their use will dictate that they be added.

Mobile WiMAX supports a wide range of smart antenna technologies. All of these are aimed at enhancing system performance and reducing overall system power. The smart antenna technologies include multiple transmit and receive antenna paths. Beamforming is one supported technology that uses multiple antennas to transmit and receive signals. For receive, the device uses different algorithms to combine multi-path versions of the received signal to increase signal level and improve signal quality. For transmit, the power savings is seen by running multiple power amplifiers at lower output levels, combining their signals by beamforming. This targeted approach consumes less power than using a single PA at higher output power levels. Also on the transmit side, mobile WiMAX uses OFDMA (Orthogonal Frequency Division Multiple Access), an optimized version of OFDM. The mobile terminal uses sub-channelization where a limited number of subcarriers can be transmitted and the RF energy is concentrated in a narrower band. This improves signal strength for a given RF power, allows less power to be transmitted in many cases, and reduces overall transmit power.

About the Author
Allen Hill is an Applications Manager on the WiMAX Transceiver Team at Analog Devices Inc. Allen has been with ADI for 26 years with engineering roles in Test, Marketing, and Applications. He has a BS Degree from Guilford College, Greensboro, NC.


CAPTIONS:
Figure 1: Where the mobile WiMAX platforms stand in 2006/2007 with expected improvements for 2009/2010.
Figure 2: Differences between analog partition and the JEDEC JESD207 digital partition.


ROUNDTABLE: Industry Should Let WiMAX Mature Properly

Elvis Tucker, Director, Solutions and Alliances, Aperto Networks; Michael Murphy, Head of Technology, Asia-Pacific, Nokia Siemens Networks; and Dr. Mohammad Shakouri, Vice President of Marketing, WiMAX Forum, discussed with Wireless Design and Development Asia the major changes that have occurred in the WiMAX space during the past year, as well as the challenges facing WiMAX in the next 18 months. They also talked about the WiMAX ecosystem development in Asia, and how the region compares with what is happening worldwide.

Click here for the Roundtable excerpts.

Click here for the illustrations:

Figure 1, Figure 2