The tough technical and deployment challenges of mobile WiMax means the industry needs to adopt pragmatic test strategies, says Antonio Policek
Mobile WiMAX (IEEE 802.16e) is gaining increasing interest from major mobile service providers, and the industry, as a potential 4G technology for mobile broadband telecommunication, as many consider it offers a cost effective route toward mass-market adoption of wireless broadband services.
Mobile WiMAX's proponents say it combines the promise of a true wireless broadband experience with the benefit of mobility. It also has the benefit of offering both fixed and mobile carriers a range of options for services that complement their business strategies and technology plans.
A number of factors are driving interest in Mobile WiMAX including:
- Interest from countries with low 3G network deployments, such as Eastern Europe as well as major Asian economies.
- Operators who missed out on the first round of 3G licences. Of 800 mobile operators worldwide, only around 200 have 3G licences.
- Existing 3G operators who are developing ‘hybrid' scenarios using, for example, 3G for voice traffic and WiMAX for data in metropolitan areas.
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 on the requirements of the current user application, bundles of sub-channels are assigned to a data stream to ensure the desired Quality of Service (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 favour of WiMAX. While Mobile WiMAX technology is still at the beginning of its lifecycle, it is certainly poised to take flight.
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, with digital RF technology accelerating the move to packet oriented network technologies like WiMAX, packet collisions and spurious signal detection becomes much more complex.
WiMAX services may be sharing spectrum with other services operating asynchronously making interference detection very difficult but critical to proper operation.
In addition, 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.
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); it is also 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 shows, 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 base station (BS), including testing the interoperability between the base station and Access Network Gateways (ASN-GW) and testing 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
With all these tough challenges, what steps can a developer take to meet WiMAX head-on?
First, engineers need a comprehensive test set to be able to more quickly detect, diagnose and resolve design issues for WiMAX end-user products, including consumer electronics, computers and handheld devices.
This test set must be able to perform spectrum and modulation measurements on OFDM and OFDMA signals and must enable the capture of intermittent or random events.
WiMAX conformance measurements and other critical PHY layer measurements will include:
- Error Vector Magnitude (EVM)
- Power Spectral Density (PSD)
- Complementary Cumulative Distribution Function (CCDF)
- 802.16e Mapping Configuration
In addition, an approach already adopted by leading vendors involved in WiMAX development for testing the base stations and ensuring interoperability with the devices 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 R4 (ASN-GW to ASN-GW) and R3 (ASN to CSN) Interfaces. 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 these test-related activities must be accompanied by deciphering features that can grant reliable and effective access for design verification and troubleshooting.
Last, the tool's data capture and processing performance should be sufficient to meet the demanding throughput requirements of WiMAX.
T&M's role in Implementation
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 cost of the new technology lies in its added complexity during development and deployment.
Engineering teams must be able more quickly to detect, diagnose and resolve WiMAX design errors; to do so they must perform spectrum and modulation measurements on OFDM and OFDMA signals in accordance with the Mobile WiMAX standards.
Protocol implementation in the base station and ASN will be more complicated, as will the new ciphering and authentication procedures. Developers will encounter more challenging design verification and troubleshooting processes than they have experienced with earlier protocols.
That is why the right combination of test tools will give service providers and manufacturers a competitive advantage.
About the author:
Antonio Policek is
Senior Marketing Manager, Network Diagnostics, Tektronix
Standards and interoperability biggest obstacles to overcome
Even now, WiMAX standards are still in flux. While the wireless interfaces are well-defined and vendors are moving ahead with the development of new equipment, core network specifications are still in the draft stages. Draft 3 of the WiMAX standard, released in February 2007, went a long way in terms of defining network elements but until the final specifications are ratified, equipment manufacturers will build proprietary implementations or will delay network system design until the specifications are complete.
Once this is resolved, the next big challenge will be to ensure that the interests of network equipment manufacturers and operators are aligned. Operators do not want to offer a service until the equipment is ready, while the equipment manufacturers are reluctant to develop systems until the operators are fully committed.
The next big hurdle for WiMAX is interoperability. There is always the chance that interoperability guidelines will be interpreted differently, so that devices and network equipment may not interoperate as planned. A key requirement will be to validate equipment interoperability in the lab prior to deployment, rather than trying to resolve technical issues in the field after service launch.
Beyond basic deployment, the next challenge is ensuring the overall performance of the network, including the air interface. The key market driver for WiMAX is increased data throughput. Service providers will need to ensure that the end-user experience matches expectations.
The first step is to set realistic expectations and then design the network for optimum throughput. The throughput of the network will be affected by several things including; the number of users, the types of data traffic and the amount of corrupted data on the network. Validating the network design in the lab under real-world load conditions will help carriers properly design and optimise network configurations prior to deployment.
On the air interface, the latest Wave 2 WiMAX products use Multiple Input, Multiple Output (MIMO) techniques to enable higher data rates and extend operating range. Again, validating MIMO implementations in the lab under real-world load conditions will help ensure that anticipated performance gains are achieved.
After deployment, service providers will have to prove that WiMAX has significant value over existing technologies. Wi-Fi and 3G cellular networks both have wide market acceptance and a solid technology base. Carriers will have to show that WiMAX has a major advantage to persuade customers to invest in new equipment and to convince them that they need yet another way to access the internet.
Nigel Wright, vp product marketing, Spirent Communications.