802.16 Enables Versatile Broadband Wireless Systems – Flexibility & Performance Key for Worldwide Deployment

By Matthias Feulner, Texas Instruments, Business Development Manager, Wireless Infrastructure

802.16 Outline

As a complement to existing state of the art wireless technologies like WCDMA , 802.11 WLAN, UWB and others the promise of the IEEE 802.16 standard and its industry support group, the WiMAX Forum, is not only to deliver cost-efficient wireless broadband data services of up to 70 Mbit/s over distances up to 50 km distance. It is beyond that aimed at catering to a wide range of wireless link applications, ranging from backhaul for cellular base-stations or WiFi hot spots over fixed wireless access, often labeled as a DSL replacement feature for rural and last-mile urban areas, to nomadicity, for example a laptop on the move, and eventually full mobility. Initial deployments following the 802.16d standard are expected to target fixed wireless access and backhaul, where first trials and contract awards have already been announced. More advanced features like nomadicity and mobility according to 802.16e are to follow from 2006 onwards.

Frequency Bands Considered for Use

The IEEE 802.16 specification sets a very general outline for systems that comply with the standard, defining several modes of operation based on different definitions of the medium access control (MAC) layer, power class, physical (PHY) layer and RF channelization profiles. Initial work focused on systems in the 10-66GHZ frequency range which are requiring line-of-sight (LOS) operation, whereas the later inclusion of frequency bands from 2-11GHz for non-line-of-sight (NLOS) systems has opened opportunities to address a much broader application range. Right now most attention is on the licensed bands in the 2.5GHz (allocated in south and middle American countries, some southeast Asian countries and the US, also 2.3GHz particularly in Korea) and 3.5GHz (allocated in most countries, except the US) range as well as on the unlicensed band in the 5.8GHz range (available in many countries, but release pending) which together allow to cover major regions of the world where at least one of these frequencies is available for use. Below graphic illustrates the available bands and possible interference with other systems already occupying part of the spectrum.

Figure 1: Spectrum allocation in WiMAX frequency bands

OFDM Modulation Scheme

Since operation in NLOS environments is a crucial requirement for 802.16 systems, the impact of multi path interference (i.e. multiple reflections of the transmitted signal being incident at the receiver) needs to be addressed and therefore OFDM (Orthogonal Frequency Division Multiplexing) technology, which is already employed by 802.11 WLAN systems, has been adopted using a 256 points FFT (Fast Fourier Transform) coding scheme instead of a single carrier. ‘Orthogonal' means that the OFDM frequencies used are orthogonal or perpendicular to each other, allowing sub channels to overlap without interfering with each other. With that, at the receiver the sub channels can be separated from each other, thus maximizing spectral efficiency.

System Configurations for WW coverage – Flexibility Required

Even though recent standardization has narrowed down the choice of frequencies over which WiMAX is to be delivered by defining a number of usage cases (so-called profiles), regulatory requirements and the presence of other already existing systems in certain bands eventually will require any WiMAX equipment (be it Infrastructure or CPE) to adapt easily to the three main frequency bands to allow for worldwide operability. On top of this a wide range of possible signal bandwidths must be supported, the standard specifies optionally multiples of 1.25MHz, 1.5MHz and 1.75MHz up to a total bandwidth of 20MHz.

Similar applies to further options in system design like for example the choice of the duplex method, i.e. implementation of Time Division Duplex (TDD) or Frequency Division Duplex (FDD) architectures.

Systems in TDD mode can alternatively transmit OR receive at a given time, sharing a good portion of the transceiver chain components including the antenna and thus allowing for more integrated and compact system implementation. This will result in benefits including savings in components, board space, power consumption and eventually cost, but at the same time limit capacity and similarly the maximum number of users. At the same time requirements on isolating transmitter and receiver from each other are much less critical since only Tx or Rx are active alternatively and thus no self-jamming can occur. TDD based systems are most likely to be found in unlicensed spectrum applications where there is no cost for obtaining a license and thus the drive for lowest overall cost favors use of TDD.

On the other hand FDD implementations obviously do allow for more capacity / users to be served in both the base transceiver station (BTS) as well as customer premises equipment (CPE), thus satisfying needs for higher performance systems. But in turn more bandwidth is required since Tx and Rx operate simultaneously in separate frequency bands plus an additional gap or guard band is required to account for the fact that no ideal pass band filters do exist. Beyond that Tx / Rx isolation needs special attention to avoid jamming of the Rx by the large Tx power of the local transmitter. Consequently FDD based systems are most often deployed in licensed bands where obtaining a license may be costly and thus higher performance systems may be desirable to get the most return on investment by supporting the maximum capacity / number of users.

RF Front-end Transceiver Implementations

How to meet time-to-market requirements and still maintain flexibility? Using a highly integrated RF front-end chipset can help. With a complete set of highly integrated RF front-end chipsets available for the full range of frequency bands including 2.5GHz, 3.5GHz and 5.8GHz from Texas Instruments, a vast variety of implementations based on above considerations can be supported. A clear advantage given the relatively loose definition of 802.16 based systems that ultimately mandates flexible implementations of the RF transceiver chain.

The block diagram system view representative both of the 2.5GHz and 3.5GHz FDD transceiver is shown in picture below, demonstrating an implementation with only a minimum number of additional external components required. The synthesizers for generating the local oscillator as well as flexible automatic gain control (AGC) functions are already integrated with the chipset. With the Tx and Rx paths separated this chipset architecture readily supports FDD architectures (as well as TDD of course).

Figure 2: 2.5GHz & 3.5GHz WiMAX transceiver solution

Below picture shows the reference design board for the 2.5 GHz & 3.5 GHz RF chipset, demonstrating the compactness of the total solution with only very few external components necessary.

Figure 3: 2.5GHz / 3.5GHz WiMAX RF front-end reference board

An additional chipset for the 5.8GHz band is a very highly integrated heterodyne TDD architecture. Again the LO synthesizers as well as AGC functions are already integrated with chipset, minimizing external component count. As discussed before the TDD architecture requires fewer components and thus is ideally suited for use in cost-optimized deployments in unlicensed signal bands.

Figure 4: 5.8GHz WiMAX transceiver solution

Performance will be key and meeting the Tx and Rx signal chain specifications set by the standard is mandatory for labeling equipment WiMAX compliant. The use of super heterodyne architectures allows to better cope with the filtering of unwanted blocker signals at the receiver and more easily comply with spurs requirements at the PA output, even at high output power, allowing for more robust operation in environments where presence of other users that can interfere must be expected.. Additionally the difficulties of compensating for IQ imbalance with direct conversion architectures are avoided. In conclusion, choices made today for flexible, high performance RF chipsets backed by leading process technologies, manufacturing capabilities and systems expertise will be the fundament for solutions that optimize cost, power and space in the future.