Mobile WiMAX: The Evolution to Lower Power

Tag : Mobile Data Cable


WiMAX (Worldwide Interoperability for Microwave Access) is thelatest next generation wireless telecommunications standard makingits way to the marketplace. Based on the IEEE 802.16 air interfacestandard, WiMAX is a broadband wireless solution offering high datathroughput, efficient data multiplexing, and low data latency. Thetwo versions of WiMAX that are being deployed are aimed atdifferent market segments. The first, fixed WiMAX (802.16d), isprimarily utilized in fixed locations. It competes with cable andDSL to provide the public with high speed Internet access. Thesecond, mobile WiMAX (802.16e), is aimed towards mobile internet,which provides users the convergence of broadband wirelessconnectivity in handheld devices.

WiMAX is based on OFDM (Orthogonal Frequency DivisionMultiplexing). High order modulation schemes and wide bandwidthsallow high throughput, but require high linearity and low noise forproper operation. Present profiles support up to 64 QAM (QuadratureAmplitude Modulation) in bandwidths up to 10MHz, putting tremendouspressure on system designers to meet conformance specifications.

As a relatively infant technology, mobile WiMAX faces challenges inreducing size and power consumption to meet customer expectationsfor mobile devices. Mobile WiMAX is in its first generation.Functionality and interoperability have thus been the highestpriorities for system designers. As future generation devices arecontemplated, smaller size and lower power consumption will berequired to ensure market acceptance. Today's solutions use modemprocessors 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 devicesthat aren't hot to the touch is daunting. Figure 1 shows where themobile WiMAX platforms stand in 2006/2007 with expectedimprovements for 2009/2010.

As specifications unify, one of the first ways to reduce power inany developing technology is to utilize more efficient designphilosophies. WiMAX has started down this path as the tasks offunctionality and interoperability are now accepted as givens.WiMAX profiles driven by industry organizations ensure acceptanceof the technology within the marketplace, but provide only a smallsubset of what the full 802.16 standard allows. This enables bothsystem and chip designers to narrow their focus to meet realisticoverall requirements. As an example, 802.16 specifies channelbandwidths from 1.25MHz up to 28MHz in FDD (Frequency DivisionDuplex), HFDD (Half Frequency Division Duplex), and TDD (TimeDivision Duplex) modes. Wisely, the industry has narrowed the scopeby limiting initial mobile WiMAX profiles to channel bandwidths inthe range of 5MHz to 10MHz, and restricting modulation to TDDmodes.

The implications of this narrowed scope are tremendous for WiMAXsystem and chip designers. The fact that all mobile profilespresently defined are TDD means that the device can either transmitor receive at any time, but not both. This allows chip designers tooptimize their devices to handle only this condition. Limiting thechannel bandwidth also improves efficiency, allowing designers tooptimize filters and data converters to a set of bandwidths ratherthan having to scale multiple octaves.

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

The second method of power reduction involves utilization of moreoptimum 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 FPGAverification, migrating to single chip devices that are fabricatedin stable and mature IC processes. All these devices are fabricatedin CMOS technology with geometries ranging from about 90nm to180nm. Obviously, next generation processor designs will exploreusing smaller geometries (65nm and below), following the economiesof the CMOS lithography progression and capitalizing on theinherent power reduction provided by smaller gate sizes thatconsume lower currents. Some of these processors are now startingto appear in the market, with power supply current that is one halfto 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. Theapplications processor contains the MAC (media access control), aswell as high-level software. The DSP and fixed engines, whichperform the modem functions, include encoders, decoders, correctionalgorithms, and FFT blocks. The data converters are included aspart of the signal path to convert digital signals to analog andanalog signals to digital for use by RF transceiver chips.

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

A better partitioning choice is to place the data converters on theradio transceiver chip. This allows the processor to be designed inthe smallest digital CMOS process node, with no extra (expensive)process steps that may be required for linear circuits. An addedbenefit to this partition is that all interfaces are digital, so nosensitive analog signals are routed on PC boards. A JEDECspecification (JESD207) has been approved that aims to unify thedigital interface between the RF transceiver and the digitalprocessor for mobile WiMAX and other high data rate applications.Other advantages to placing the data converters on the RFtransceiver include allowing all real-time loops, such as AGC(automatic gain control) on Rx and power control on Tx, to beintegrated on one chip, thus minimizing software overhead betweenthe transceiver and processor chip. Figure 2 shows the differencesbetween 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 spacingutilized by mobile WiMAX, the PA has to be linear and low noiseover the transmit power range. To meet these attributes, the PAconsumes quite a bit of power. The good news is that the mobileWiMAX data link is generally asymmetric, downloading dataapproximately 70 percent time, versus percent transmitting with anactive PA. Additionally, PA designers are working to achieve therequired linearity and low noise with advanced processes (GaAsHBT), and design techniques such as linearization. Even with theefforts underway, the PA will be a big power consumer in mobileWiMAX systems for the next generation.

Mobile WiMAX is a very flexible communications standard thatincludes features to support high data rates, high QoS (Quality ofService), scalability and security. Additional advanced features ofmobile WiMAX aim to improve data link performance and reduce powerin the mobile handset. Most advanced features are not available infixed WiMAX, and are yet to be deployed in mobile WiMAX due totheir complexity, but the improvements gained by their use willdictate that they be added.

Mobile WiMAX supports a wide range of smart antenna technologies.All of these are aimed at enhancing system performance and reducingoverall system power. The smart antenna technologies includemultiple transmit and receive antenna paths. Beamforming is onesupported technology that uses multiple antennas to transmit andreceive signals. For receive, the device uses different algorithmsto combine multi-path versions of the received signal to increasesignal level and improve signal quality. For transmit, the powersavings is seen by running multiple power amplifiers at loweroutput levels, combining their signals by beamforming. Thistargeted approach consumes less power than using a single PA athigher output power levels. Also on the transmit side, mobile WiMAXuses OFDMA (Orthogonal Frequency Division Multiple Access), anoptimized version of OFDM. The mobile terminal usessub-channelization where a limited number of subcarriers can betransmitted and the RF energy is concentrated in a narrower band.This improves signal strength for a given RF power, allows lesspower to be transmitted in many cases, and reduces overall transmitpower.