A WiMAX system or network that can support portable operation addresses a much larger market need than a strictly "fixed" 802.16-2004 WiMAX implementation does that must be plugged in or bolted to walls. To date, it has been stated and accepted by the industry at large that portable operation must wait for the benefits of 802.16e before viable products can be delivered. However, when the requirements for portable operation are closely examined, it becomes clear that this, in fact, is not the case, and portable operation is possible with "fixed"WiMAX-compliant products.
By David Sumi
Glossary of Acronyms ASIC— Application-Specific Integrated Circuit
CPE— Customer Premise Equipment
MAC— Medium Access Layer
MIMO— Multiple Input, Multiple Output
PCMCIA— Personal Computer Memory Card International Association
PDA— Personal Digital Assistant
PHY— Physical Layer
SOFDMA— Scalable Orthogonal Frequency Division Multiplexing Access
WiMAX— Worldwide Interoperability for Microwave Access
The limitation to delivering portable operation lies not in the standard, but in the implementation of the standard, and specifically, in the ASICs that power WiMAX products. An 802.16-2004 WiMAX chip that meets the requirements for delivering Portable WiMAX offers operators advanced services and additional revenue opportunities before full broadband mobility arrives.
Portability What It Is and Where It Works
The portable mode of operation supports wireless broadband communication within a given service area while the end-user or device is either stationary or moving slowly at "pedestrian" speeds through the area. This means a user can connect to a WiMAX "fixed" network at home, take their WiMAX-enabled device (PDA, laptop modem, handset, etc.) to work or play and connect to a WiMAX "fixed" network at those locations as well. In addition, the user can maintain their broadband connection as they move around within the WiMAX "fixed" network coverage area. From a service-provider perspective, this means they can "own" the end user throughout this large coverage area, providing increased revenue opportunity on their 802.16 2004 WiMAX network. Figure 1 presents some of these scenarios graphically.
Portability Technical Requirements The Standard
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Figure 1. Some scenarios for portable WiMAX.
The primary requirements for portability lie in the fact that the devices used are portable, and as a result, are typically battery-powered. This is addressed in the 802.16-2004 standard by allowing an optional implementation called "sub-channelization." In this technique, a CPE or device does not have to transmit to the BTS on all the sub-carriers or tones on the waveform, thus conserving power.
In the 802.16e standard supporting SOFDMA, this same technique is used. The CPE, or end-user device, will only transmit on a small subset of tones. Although there are several other significant differences between the two standards, in terms of power conservation for the CPE device, this is the main feature. Thus, a product that supports 802.16-2004 and the optional sub-channelization feature will be as power efficient as a device based on 802.16e.
Portability Technical Requirements The Products
Since both the 802.16-2004 and 802.16e standards can support portable operation and portable devices, it is up to the products themselves to deliver portable broadband. The requirements for a portable device largely rely on the ASIC supporting MAC and PHY compliance as well as the RF chain. The requirements for the baseband/MAC ASIC are summarized as follows:
Power consumption (active receive) Battery life is a critical issue in portable devices. Therefore, it is important that the standard supports basic operation that, at a core technical level, can reduce power consumption. But the implementation of these features in the product itself must do more. The ASIC must be power efficient, consuming no more than 700 mW of power when in the active-receive state.
Power consumption (transmit) While most of the power consumed is typically in the receive state, it is equally important to conserve power in the transmit mode as well. This can be addressed somewhat with sub-channelization, an optional feature in WiMAX. It can be further enhanced when techniques such as transmit diversity combining are also supported from the CPE to the BTS.
Heat Portable devices demand small size and density. The chips for these devices will likely be built into very small spaces (consider a PCMCIA card) and hence the heat generated must be limited. Of course, heat and power are related, so the more power an ASIC consumes the more heat it generates.
Performance or range The greater the range of coverage for a portable device, the more "usable" it becomes. When a WiMAX CPE is implemented in a small device such as a PDA, the ability to include high-gain antennas for range extension is not possible because portable devices use smaller antennas. In order to compensate, the device must derive the necessary performance, or additional dB of link budget, with advanced signal-processing techniques such as MIMO and diversity combining. To take advantage of these techniques, two antennas are required.
Footprint Portable devices are, by definition, small and lightweight. For most chips, weight is not the issue. However, for some, footprint or space required is very much a problem. There are no hard limits to the size of a chip that can be used in a laptop or PCMCIA or even a handset, although smaller is better. When advanced techniques such as MIMO are implemented, dramatically improving performance even more, it is vital that the multi-antenna processing techniques that are used to extend the range or performance be supported in a single chip. A single baseband/MAC ASIC must be able to support two antenna inputs.
From a technical standpoint, the existing standard, IEEE 802.16-2004, if implemented properly, can support the portable mode of operation just as well as 802.16e, and it can do so sooner. With parity between the "fixed" and "mobile" 802.16 standards in terms of technical features supporting low-power operation, it is up to vendors to deliver 802.16-2004 chips that power the PDAs, laptops and handsets, meeting all the requirements for portable operation mentioned above for portability on today's WiMAX "fixed" networks.
Conclusion: Portable WiMAX A Giant Step Toward Broadband Ubiquity
The real value of wireless technologies lies in their ability to un-tether the end-user while communicating. WiMAX "fixed" can and will have widespread adoption, delivering voice and data communications to buildings and facilities around the world. But it is the capability of chips to deliver portable broadband that will greatly enhance the applicability of the IEEE 802.16-2004 standard.
The eventual goal of wireless is broadband ubiquity multi-megabit connectivity wherever and whenever you want it. By supporting portable operation under the IEEE 802.16-2004 standard and being fully WiMAX "fixed"-compliant, the addressable market for broadband wireless is significantly expanded, enabling operators around the world to take a huge step toward achieving broadband ubiquity, with attractive operator economics, without waiting for the "next best thing."