WiMAX is a means of increasing bandwidth for a variety of data-intensive applications, and is based on the IEEE 802.16 air interface standard. The WiMAX solution offers high data throughput, efficient data multiplexing, and low data latency, and is currently being deployed in two formats. The first, fixed WiMAX (802.16d), is primarily utilised 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 new technology, mobile WiMAX faces challenges in reducing size and power consumption to meet customer expectations for mobile devices. So far, functionality and interoperability have been the highest priorities for system designers. For future generations, smaller size and lower power consumption will be required to ensure market acceptance. Today’s solutions use modem processors that consume up to 500mW or more. Add the RF transceiver, with power as high as 350mW, and transmit power amplifiers (PA) that consume above 1.4W, and the task of designing mobile devices that aren’t hot to the touch is daunting.
Power Reduction
As specifications unify, one of the first ways to reduce power in any developing technology is 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 organisations 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.
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, allowing chip designers to optimise their devices. Limiting the channel bandwidth also improves efficiency, allowing designers to optimise filters and data converters to a set of bandwidths rather than scaling multiple octaves.
Although narrowing the scope of the 802.16 standard reduces some of its flexibility, it ensures that price, power, and size are reduced over time, while also ensuring functionality and interoperability. This improves the chance that mobile WiMAX will meet its overall market potential.
The second method of power reduction involves utilisation 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. Next generation processor designs will explore using smaller geometries (65nm and below), following the economies of the CMOS lithography progression and capitalising on the inherent power reduction provided by smaller gate sizes consuming lower currents.
Partitioning
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, 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 vice versa for use by RF transceiver chips.
Placing the data converters on the processor chip leads to an inefficient partition, as the smallest CMOS lithography will generally not be used. This forfeits 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 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 minimising software overhead between the transceiver and processor chip.
One of the largest consumers of power in the system is the PA. This is because it must be linear and low noise over the transmit power range. The good news is that the mobile WiMAX data link is generally asymmetric, downloading data approximately 70% time, versus 30% 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
Mobile WiMAX is a very flexible communications standard that includes features to support high data rates, high QoS, 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. 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 optimised version of OFDM. The mobile terminal uses sub-channelisation 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.
In summary, mobile WiMAX technology is in its relative infancy. A natural maturation will occur, as with any developing technology, and will include more efficient design philosophies as the specifications unify. As they develop, WiMAX solutions will also transfer to smaller, lower power IC processes and to newer system partitioning schemes. Additionally, the WiMAX standard is one of the most flexible the wireless industry has seen, with advanced features that will offer power saving attributes as they become available and mainstream.