Broadband connections are difficult to come by in remote or sparselypopulated locations because the customer base is too small to justifythe expense of installing wired networks. Satellite service may beavailable in these areas, but it has the significant disadvantage ofrequiring “line-of-site” access for reliable transmission.
Trees, buildings, or even the weather may cause interference. WiMAXis a wireless technology that solves this problem by providing “lastmile” broadband connections using radios instead of cables or telephonelines.
Unlike satellite connections, WiMAX does not need directline-of-sight access to provide wide area broadband access. A singleWiMAX basestation can provide broadband speed service to thousands ofcustomers within a three kilometer distance or backhaul functions atdistances to thirty kilometers, as shown in Figure 1, below.
|Figure1. Mesh networks using WiMax in the embedded infrastructure backbonewith local WiFi hotspots to complete the network|
There is a wide variety of IEEE802.x standards that include WiFi (802.11), ZigBee(802.15.4) and WiMAX (802.16), amongothers. These standards define wireless connectivity within certain RFbands. The most well known example of this is the 802.11x standards forWiFi, used in wireless home networks and cybercafés. However,setting a standard for radio transmission and reception does not meanthat the equipment made by competing vendors will work together.Multiple factors pose obstacles to interoperability, including physicallayer (PHY) performance, media access controller (MAC) protocols, layermessaging, and encryption methodologies, to name a few.
A broadband network must be accessible to all notebooks, PDAs andother equipment, from every vendor, so interoperability becomes a verybig issue. This is where the WiMAX Forum steps in. The WiMAX Forum isan industry-led, nonprofit corporation formed to promote and certifycompatibility and interoperability of broadband wireless products thatoperate on the 802.16 standard.
Member companies support the industry-wide acceptance of IEEE 802.16and European Telecommunications Standards Institute (ETSI) HiperMANwireless metropolitan area network (MAN) standards. The WiMAX forumcurrently has a membership in excess of 300 companies worldwide. WiMAXlabeled products must complete a certification process, specified bythe WiMAX Forum, that demonstrates their ability to interoperate. Tofacilitate this process the forum holds periodic sessions calledinterops, during which vendors test the interoperability of theirproducts with competing vendors.
Multiple WiMAX standards
The Worldwide Interoperability for Microwave Access (WiMAX) standard,based on IEEE Standard 802.16, is intended for use in both stationaryequipment, such as desktop PCs and mobile equipment that includesnotebook computers, mobile phones, personal media players (PMPs) andPDAs.
Since there are substantial differences in the characteristics ofradio signals being transmitted and received by stationary versusmobile devices, WiMAX profiles are based on multiple IEEE 802.16standards: one for stationary equipment and another for mobileequipment that may be in motion while receiving or transmittingsignals.
To get the standard deployed as quickly as possible and also supportvarious degrees of mobility the WiMAX forum has described five stagesfor the implementation of the standard: fixed, nomadic, portable,simple mobility, and full mobility as shown in Figure 2 below .
|Figure2. The WiMax Roadmap|
Fixed deployments are defined as stationary access to a singlebasestation. An example of this type of deployment would be videoconferencing for a convention center. In this example DVD quality videocould be wirelessly broadcast to monitors in a convention center. Witha sufficiently powered processing system these monitors could bestarted, restarted, and stopped independently without affecting thequality of the video signal. Fixed deployments could also be used forwireless broadband backhaul that connects multiple WiFi networks in amesh network, replacing the optical lines currently used for thispurpose.
Nomadic deployments are defined as being stationary, but movable,access to a single basestation. This deployment is similar to thecyber-café concept where the user can connect from anywherewithin the range of a basestation. Fixed and nomadic WiMAX deploymentsare governed by IEEEStandard 802.16-2004. With a bandwidth will beable to simultaneously support hundreds of businesses with T-1 speedconnectivity or thousands of residences with DSL speed connectivity.
Applications that are portable or mobile (i.e. in motion duringreceive and transmit) are based on IEEE Standard 802.16e and provide 15Mbps of capacity within a cell radius of three kilometers (about 2miles). The key characteristic of this 802.16e systems is the abilityto hand-off a signal from one basestation to the next, thereby enablingthe creation of “metro zones” that seamlessly provide continuousportable outdoor broadband wireless access in large cities andmetropolitan areas, and allowing end-users to remain connected duringtheir travels.
Hand-off capability will be essential to the mobile user becauseWiMAX technology is expected to be incorporated into consumer premiseequipment (CPE) products in notebook computers and PDAs by 2007.
An inherent challenge to deploying any broadband network is gettingenough people to subscribe to it to make it useful to consumers andeconomically feasible for service providers. The best way to fosteradoption of the standard is to make it affordable. Toward this end,WiMAX CPE vendors have set a target bill of materials (BOM) cost of$100.
A major factor affecting the cost of any wireless system is thearchitecture of the radio. Radios that are not designed specificallyfor WiMAX applications may need hundreds of external components inorder to transmit and receive signals.
IEEE Standard802.16-2004 radios use orthogonal frequency duplex modulation (OFDM)to modulate the data. The OFDM technique splits raw data into differentfrequencies called channels. The number of available channels isdependent on the frequency band of the standard and the channelbandwidth being used in the application. This approach reduces theprocessing effort required to compensate for multi-path ininterference.
Since each sub-carrier operates at a relatively low bit rate, theduration of each symbol is relatively long. Synchronization of thesignal timing is much easier due to the low bit rate and long duration.The 802.16-2004 WiMAX band is split into three different radiofrequency bands, 2.5 and 3.5 GHz for licensed bands and 5.8 GHz forunlicensed, each of which has unique processing requirements that areincompatible with the other frequency bands.
The channel bandwidths in licensed bands are 1.75, 3.5, 7, 14, and28 MHz, and for unlicensed 5, 10, and 20MHz. The bandwidth for eachchannel is determined by the number of channels required for a givenapplication. For example, in the 3.5GHz band, 3.5MHz bandwidth allows1024 channels.
The huge number of possible combinations of frequency band andchannel bandwidth could lead to an equally huge number of WiMAXprofiles, significantly complicating the specification andcertification process within the WiMAX Forum. It could also lead tohigher cost equipment, since vendors might be forced to a providesolutions for every possibility.
To avoid this unnecessary complexity, the WiMAX Forum considers onlya small number of profiles for inclusion in the standards as they arefinalized. For example, the 802.16-2004 standard included only fiveprofiles when it was first certified, and two more were added later.The forum is in the process of determining which profiles will beincluded for certification in the newly ratified 802.16e standard.
However, even a small subset of profiles poses a problem in terms ofselecting a proper radio for WiMAX applications. To address the issueof multiple 802.16 bands, one can either select a radio that isreconfigurable across a large range of frequencies and bandwidths, orone can select a particular frequency band and bandwidth and use aradio that works just in that band. Radios with multiple frequencybands and multiple bandwidths provide the most flexibility. They areusually implemented in a “double conversion” architecture that requiresexpensive SAW filters to define each different bandwidth.
Supporting three bands immediately increases the BOM by about $30.Each frequency band also requires its own voltage controlled oscillator(VCO) to set the frequency band, and each VCO requires hundreds ofadditional external components to get a clean signal in all bands.Flexibility notwithstanding, the high $200+ system cost associated witha multiple bandwidth radio may make systems prohibitively expensive andseverely hamper market adoption.
Performance may be another issue with multiple bandwidth radios. TheWiMAX Forum vision for long distance communication and high throughputcan be demanding for transceiver output power and receive sensitivity.Changing the frequency and/or the bandwidth alters transmit and receiveperformance of the radio.
At the higher spectrum, it may cause transmit power or receivesensitivity to fall below what is needed for interoperability.Recommended transmit power at the antenna for a WiMAX CPE device is+30dB and receive sensitivity -80dB. This problem can be overcome byadding high performance low noise amplifiers (LNAs) and poweramplifiers (PAs) to get the system into the desired range for fullinteroperability.
However, it will further increase the system cost. A second optionis to use a radio architecture that operates in a single band of the802.16 band width. Using a single band clearly limits the radio's totalflexibility. However, the bandwidth limitation may not be as confiningas it appears. Initially, the business model for WiMAX is expected tobe similar to that of the mobile phone industry. End-customers willsubscribe to a carrier service that provides the WiMAX equipment andconnection, just as mobile phone companies provide the mobile phone andthe connection today.
Thus, any WiMAX consumer will need to communicate only within thenetwork to which he or she has subscribed, at whatever bandwidth thecarrier selects. Service providers and consumer end-users will not needmulti-band radios, in the same way that subscribers to AT&T's GSMphone service do not need CDMA radios. The service provider canselect a frequency band and allocate the bandwidth of that channel, asrequired, to meet end-use demand..
For example, in the 3.5 GHz band, carriers may operate at 1.75, 3.5,and 7 MHz. A 7 MHz channel bandwidth allows more data to be transmittedin each packet, but limits the subscriber's distance from anybasestation.
A 1.75MHz channel bandwidth allows less data to be transmitted ineach packet, but allows users to be farther away from the basestation.This mixture of different channel bandwidths provides more efficientcoverage for all subscribers
The IEEE802.16 standard maintains data throughput by adapting themodulation technique to the total area of coverage. The use of multiplebands and channel bandwidths combi- nations is unique to the 802.16standards and mandates careful evaluation of the fairly wide variety ofsingle-band radios architectures available. There are three basic radioarchitectures, double conversion, direct conversion zero-IF(intermediate frequency) and direct conversion low-IF These differentradio architectures will affect total system cost and performance indifferent ways.
Double conversion, also called super heterodyne, architectures usetwo intermediate frequencies to filter and amplify the incoming weak RFsignal. This method results in two image frequencies, which arefiltered to eliminate interference from the two images.
The advantage of dual conversion is that because the firstintermediate frequency is typically fixed, it is easier to compensatefor the local oscillator (LO) phase noise. This is a good solution forapplications in which high performance and good receive sensitivity areimportant. The disadvantage of this architecture is that, as previouslynoted, the required additional filters and external components mayresult in a system cost that puts WiMAX out of reach of the massmarket.
In fact, to meet the 802.16 specification, a double conversion radiowill require about 600 external components that result in a BOM of over$150. Making it less than ideal for highly integrated systems, as shownin Figure 3, below .
|Figure3. ” Typical double-conversion receive architecture converts theincoming RF signal in two steps. Excellent interference rejection. Butneeds many external components with high BOM costs associated.|
Direct conversion radios virtually eliminate sensitivity to imageinterference by offsetting the signal from the zero subband and thenusing a direct current (DC) offset correction to compensate for theoffset effect from the radio. The channel filtering and amplificationare done at the baseband frequency, allowing a large number ofcomponents to be integrated into the RF silicon. This feature makesdirect conversion radio architectures ideal for 802.11a and 802.11gWiFi and WLAN applications because modulation techniques required forODFM fit easily with the architecture.
In the case of 802.16, however, direct conversion may actuallycreate interference because the initial frequency difference between abasestation and a subscriber could be equivalent to one or more of thesubcarriers of the OFDM channel frequencies. This DC offset coulddisturb some subcarriers in the OFDM symbol around the zero subcarrier.
This deficiency can be resolved by adding a high-resolutiontemperature controlled, voltage controlled crystal oscillator (TCVCXCO)or a high-resolution synthesizer that tunes the radio frequency towithin 1% to 2% of the subcarrier frequency spacing (85 part perbillion (ppb) at the 3.7GHz and 3.5MHz bandwidth).
The AC coupling frequency of the offset correction must be less thana few kHz during this operation. The drawback to this approach is thatthe small frequency spacing may require settling time of as long as100us when switching the transceiver from TX to RX mode. A solution tothis issue is to use a frequency dynamic offset correction, whichoperates like an offset sample and hold.
However, the zero-IF receive path requires coordinated control ofboth the frequency and offset correction that is extremely difficult tointegrate into the radio and equally difficult to manage between theradio and the baseband. As a result, the zero-IF radio subscriber willtake more time to get into synchronization with the basestation whichwill limit mobility whenever the subcarrier spacing is tight andhandover to another basestation requires fast switching, as shown in Figure 4, below .
|Figure4. Typical direct conversion, sero-IF receive architecture directlyconverts RF in to IF out. Advantageous for complex I/Q modulation usedwith ODFM. Allows more components to be integrate into RF silicon, buthas problems with DC offsets.|
The third single-channel radio option is a direct conversion low-IFradio architecture with a bandwidth programmable integrated channelfilter for receive and transmit paths and an offset cancellationcircuit that rejects the DC offsets inherent in the receive gain pathin a mobile radio. The settling time of this circuit is much fasterbecause the lowest signal subcarriers are far away from the DC offsetfrequency.
Low-IF radios are easier to integrate with other components thanother radios. They can include, on a single piece of silicon, a singlecompletely integrated synthesizer, digital gain settings for thereceive path that improve sensitivity and digital transmit powercontrol within a large control range, integrated image rejection, LOleakage digital control settings, and calibration detectors.
This solution minimizes the number of external components, to about250 or less, while still allowing the implementation of programmablechannel bandwidths for the different WiMAX profiles. The total BOM witha highly integrate low IF radio is less than $100 ” a 33% reductionwhen compared to other options ( Figure5, below ).
|Figure5. Typical direct conversion low IF receive architecture directlyconverts RF in to IF out as in the zero IF receiver. But it eliminatesDC offset issues by converting to low IF frequency. Advantageous forcomplex I/Q modulation used with ODFM and allows more components to beintegrated into the RF silicon.|
The synthesizer of a WiMAX radio is the other demanding component. The”30dB transmit error vector magnitude (EVM) certification limit forsubscriber stations must be split between the transmit componentsand the synthesizer. A 37dB EVM target for the synthesizer means thatit contributes 20% of the total EVM, allowing more headroom for thepower amplifier (PA) distortions and production margins.
Since the PA is a critical component and the largest consumer ofpower, it is extremely important to consider the ef- ficiency of thisblock when designing battery-powered mobile terminals. A higher EVMbudget improves the total power efficiency of the system. The best wayto get a better EVM is to implement the frequency correction in anintegrated programmable synthesizer with a frequency resolution up tothe required subcarrier accuracy. A synthesizer with a fast(10-50µs) settling time can support RX/ TX frequency switching inhybrid frequency division duplex (HFDD) systems.
The sub-channelization option of WiMAX requires a power controlrange of more than 50dB. In a low-IF radio this can be implemented withfull digital control and a resolution of less than 1dB. Instead ofcontributing to the TX-EVM budget, transmit path imperfections in a lowIF radio contribute to the TX emission mask. These masks, defined inEurope by ETSI for licensed frequency bands, are prone to leakage andimage imperfection.
However, this problem can be corrected easily using a calibrationalgorithm. The analog detectors that support the calibration can beintegrated into the low-IF radio, but must be controlled by thebaseband firmware.
The only real drawback of low-IF radios with integrated programmablesynthesizers is that they tend to be more expensive than other radioarchitectures. However, the added cost is typically more than offset bythe fact that they can significantly reduce the external componentcount and the BOM cost for consumer applications by $50 or more, whilestill allowing the implementation of programmable channel bandwidthsfor the different WiMAX profiles.
The unique characteristics of WiMAX IEEE 802.16 standards complicatethe task of choosing the appropriate radio architecture. Selecting amulti-band or single frequency band; or selecting one of thedual-conversion, zero-IF, or low-IF radios that are available willaffect the cost and influence performance of the application beingdeveloped. Because of the need to achieve early adoption by a largenumber of end-users, the external component count and total BOM costare critical. It is equally important that performance not besacrificed to cost considerations. In most cases, a low-IF radio withintegrated synthesizer will be the best option.
MichaelLivingston is a Product Manager with Atmel in Colorado Springs while Reiner Franke isPrincipal Senior RF Design Engineer at Atmel, Duisburg.
To read a PDF version of thisstory, go to Choosinga 802.16 radio for use in a WiMAX application.