The Promise of WiMAX: Complexity and Opportunity

By Antonio Policek, Tektronix

Mobile WiMAX (known in the industry as IEEE 802.16e) is gaining attention among carriers and network equipment manufacturers due to its potential for success as a step toward 4G mobile technology. WiMax offers a cost-effective path toward mass-market adoption of wireless broadband services.

Mobile WiMAX combines the promise of a true wireless broadband experience with the benefit of mobility. It offers both fixed and mobile carriers a range of options for services that complement their business strategies and technology plans. 

The use in WiMAX of technologies such as Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple-Input Multiple-Output (MIMO) allows higher transmission efficiency per available spectrum. These technologies also support more powerful and effective resource management:  depending to the requirements of the current user application, bundles of sub-channels are assigned to a data stream to ensure the desired QoS level.

The cost of WiMAX network deployment is relatively low compared to the cost of 3G and 3.5G infrastructure deployment. And the number and scale of industry players now investing in the standard also works in favor of WiMAX. At this writing, WiMax appears poised to take off.

New Technology Brings New Deployment Challenges

IEEE 802.16e poses many of the usual challenges found in the development and adoption of new standards. These range from business and technology risk management to investment protection.

Business risk and investment protection issues can be addressed by following some of the same strategies that led to the successful implementation of 3G and 3.5G networks. But dealing with the technology risks of WiMAX may call for extra attention and a “tailored” approach.  Why?

First, Mobile WiMAX architecture transfers more and more of the “intelligence” to the edge of the network, following a trend that began with the UMTS-UTRAN specification. This makes the base station more critical than ever, not only because it provides network coverage but also because it controls key processes such as dynamic radio resource management.   

The IEEE 802.16e Media Access Control (MAC) protocol in the R1 Interface exemplifies the increased complexity incorporated in the design of new Mobile WiMAX Base Stations. Figure 1 depicts the network architecture at this level. The MAC protocol handles several jobs: it supports the delivery of very high bandwidth (full duplex) transmissions; it runs various applications including data services as well as VoIP and other IP-based applications; it manages demands for both constant bit rate traffic and bursty traffic. And of course, it must be able to support the variable QoS levels required by user applications.

Fig. 1 Mobile WiMAX Network Architecture

Fig. 2 : Main Control Plane Protocols

With all these duties, it's no wonder that full access and visibility on the R1 Interface is critical. It is the only way to ensure control of the interoperability between the base station and the mobile (MS/SS), and it is the key to maintaining the performance of the base station and troubleshooting MAC-layer functionality.

But access and visibility on the R1 Interface is neither simple nor straightforward, especially when access to the air interface is taken into account. As Figure 2 reveals, some of the critical protocols in the R3 and R6 Interfaces are not yet standardized. Incidentally, the R6 Interface poses yet another technical challenge to developers as they  need to test the BS, including testing the interoperability between the base station and Access Network Gateways (ASN-GW), and testing Quality of Service (QoS) and Mobility management. 

Need another challenge? Consider security and its implementation within the IEEE 802.16e framework. The need for security is increasing, as is subscribers' demand for it.  Security solutions emerging among new cellular standards are increasingly complex.

Ciphering is used at the air interface (R1 Interface connecting base station to the mobile station (MS) with a combination of RSA Public Key and EAP ciphering algorithms. This scheme is very secure and hard to decipher.  On the backhaul at the R3 Interface (connecting the ASN to the CSN) IP-Sec is used. This too is difficult to decipher. On balance these facts are good news for end-users but very bad news for developers. Any process that makes it harder to decipher also makes it more difficult to test and troubleshoot the network.

Of course, WiMAX technology needs to inter-work flawlessly with other technologies including 3G and its evolutions. This adds yet another dimension to the development test puzzle.

Lastly, the network frequently will need to grant high data rates to subscribers attempting to run throughput-intensive applications. Developers must have a means to guarantee data capture and data processing performance up to the maximum WiMAX base station throughput per sector:  28 Mbps for the uplink and 63 Mbps for the downlink[1].   

Pragmatic Test Strategies Can Help Conquer WiMAX Challenges.

With all these tough challenges, what steps can a developer take to meet WiMAX head-on?

One approach, already adopted by leading vendors involved in WiMAX development, is to incorporate a dedicated test port in the base station design. This of course must be planned from the outset. The test port simply mirrors the traffic exchanged over the air between the base station and the SS/MS, providing easy direct access to the complex R1 interface. The content can be sent to a passive protocol monitoring device that accesses and decodes MAC protocol data units as well as all of the  messages passing between the MAC and PHY layers. The value of the test port is that it minimizes the need for RF interface tests and their associated disruptions.

Of course, any effective test or monitoring solution must be able to tap into many other physical links, including the R3 (ASN-GW to ASN-GW) and R6 Interfaces (ASN to CSN).  Furthermore it must be able to capture Protocol Data Units (PDUs) on both Control and User Plane, decode them, and ultimately present them in a human-readable format.

The test/monitoring solution for WiMAX network elements should also include these essential capabilities:

· Time-synchronized capture of multiple Rx interfaces (R1, R3, R6, etc.)

· Real-time and off-line decoding of captured traffic

· Reassembly of fragmented PDUs

· Filters for Protocol messages and information messages.

If the test/monitoring platform will be used for troubleshooting, several more advanced capabilities will be needed:

· Automatic, online correlation of all messages belonging to the same call/procedure across multiple interfaces (Multi-Interface Call Trace)

· Triggers on filters matching specified conditions

· Calculation of statistical information on protocol messages, procedures and payload

· Generation of Call Detail Records

In keeping with the WiMAX security implementation issues discussed earlier in this article, all of these test-related activities must be accompanied by deciphering features that can  grant reliable and effective access for design verification and troubleshooting. 

Lastly, the tool's data capture and processing performance should be sufficient to meet the demanding throughput requirements of WiMAX.

Conclusion

The high performance, flexibility, and security promised by Mobile WiMAX make the new standard a good candidate to succeed in the marketplace. From the carriers' standpoint, WiMAX provides valuable new business opportunities and more wireless broadband options that can be profitably offered to subscribers.

The price of the new technology must be paid in added complexity during development and deployment. Protocol implementation in the base station and ASN will be more complicated, as will the new ciphering and authentication procedures. Developers will encounter a more challenging design verification and troubleshooting process than they have experienced with earlier protocols.

Tektronix' G35-WiMAX, based on the proven K1297-G35 platform, is a protocol analyzer that developers can use for monitoring and in-depth decoding of R1 and R6 interfaces. It can assist in testing functionality, performance, conformance, and interoperability, and in the installation of base stations

The K1297-G35 solution is today in wide use by network equipment manufacturers and mobile operators to test 3G and HSPA network elements. Its unsurpassed functional test capability gives technicians in the labs a very powerful platform to support their efforts in behalf of investment protection and technology risk reduction.

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[1] Mobile WiMAX Forum – Mobile WiMAX Perfomance Evaluation, August 2006