A Day in the Life of a Mobile WiMAX™ Handset
by Michael Lawton, Agilent Technologies

The IEEE 802.16 standards group develops standards in support of the broadband wireless access market. This group has been active since 1999, during which time it has progressed from developing standards for point-to-point wireless infrastructure links, to point to multi-point fixed links for wireless broadband access, through to the latest specification (802.16e) which supports broadband connectivity to mobile clients. This progression brings with it a number of technical challenges, which have been addressed by the standards work. This article highlights mobility features in a lighthearted way with “A Day in the Life” description of a mobile’s typical day-- examining the different mobile features and the inter-related processes that broadband mobile connectivity requires.

A Day in the Life…
I like to wake early, am normally up, and ready to go by around 7am. If I am lucky, I will have gotten some good rest and re-charged my batteries overnight. I find I need this from time to time, or I cease to function. After a good rest, I leap into life and start my first tasks of the day. I know what frequency I last used, so I check this for any base station transmissions. If I find one, then I synchronize with the cell and learn its BS (base station) ID. If not, then I will start to scan cells in my frequency band. Depending on where I was the night before, I may or may not need to change frequency. After finding a suitable base station, I embark on the network entry process (see Figure 1).

The first step in the network entry process is to perform initial ranging (see Figure 2). I start by sending a low power signal to the base station. If the base station hears me and responds, I adjust my transmit power, timing, and frequency offsets as directed. If the base station does not respond, then I increase my power and try again. The goal is to set my power so that the base station can hear me without me causing undue interference to others.

This is helpful for the base station and serves to save some of my valuable energy. I like to perform ranging many times during the day as it helps me to feel connected and know I am being considerate to others.

All base stations have their little quirks, so it is important that during network entry I negotiate how we can communicate together, a process called Subscriber Station Basic Capabilities (SBC) negotiation. I send an SBC request message up to the base station where I advertise all the new features I can support. The base station then sends a response to confirm what is possible between us. I like to find a base station that supports MIMO (multiple input multiple output) and many HARQ (hybrid automatic repeat request) channels so that I can enjoy robust high data throughput-- but sometimes I do not get this and I have to settle for less.

The next step is for the base station to check that I am authorised to use the network.

Base stations need to trust that mobile stations are who they say they are (authentication), as well as check what they are authorized to support. The base station asks some security questions to check I am genuine before allowing me to register. During this exchange, I use an encryption technique in order to ensure that no one is listening in to my private information.
In the old days, registration was all about phone numbers, but things have moved on now and I look to the base station to give me an IP address. With that, I am good to go, and can look forward to a day full of web surfing, voice calls, and video streaming.

The last step of the network entry process is to establish an initial service flow. Mobile WiMAX is a new standard, written with the assumption that all data traffic is IP based, and that services are layered on top of this. Each of the services has different needs and priorities in terms of error rate requirements and real-time constraints, so I have to be able to provision service flows with different classes of QoS (Quality of Service); see Table 1. This helps manage the tasks required between the base station and me.

With network entry over, I can relax. I am ready to receive things and primed for action! At times like this, with not much happening, I like to go into idle mode (see Figure 3). I do this by signing up to a base station paging group. This allows me to switch most of my circuits off for long periods of time, and then only switch on periodically to listen to the base station and see if there is data or a call for me. In this mode, I can take a rest and save power. During the early morning, I will often come out of idle mode and do an expedited network entry in order to connect with the email server and check for messages, or even multi-task and check for voice messages at the same time.

During a voice call, I only generate about 32 kbps of the data on the uplink, which is not much for me, so I use my sleep mode facility. In sleep mode, (see Figure 4) I only transmit for a fraction of the time, which stops me from getting bored, and saves energy.

After breakfast, I will usually head for the office, which means looking out for new cells and performing handovers where necessary. Even though I am in idle mode, I need to listen to the MOB_NBR_ADV messages and when I see the signal strength drop on my serving cell, I send a MOB_SCN-REQ request to perform a scan of alternative cells (see Figure 5). Following the scan, I send a MOB-SCN-REP and if I think I need to change, I advise the serving base station that I wish to handover (MOB-MSHO-REQ) and await instruction. All being well, I will get the go-ahead (MOB_BSHO-RSP) message, and I will then perform network entry with the new base station. Sometimes my drive takes me out of coverage for one base station area only to drive back into coverage five minutes later-- if only I could find a better route!

Once I am at work, I am always busy. I like it though, as every day is different and I never know what might happen next. At least ten times a day I am alerted by the serving base station that I have an incoming call. If I can answer, I will establish a UGS (unsolicited grand service) or ertPS (extended real time polling service), (see Table 1) service flow for a VoIP call. If not, then I leave it to the network to answer the call and take any messages. Whilst I am good for voice, I am also great for data. I am often used for web surfing. Here I use a Best Effort (see Table 1) service flow to establish internet connectivity and provide full web access. I am so clever I can even manage multiple service flows, which allows me to talk and check the web for information at the same time. Increasingly, I also provide streaming video. I do this by setting up an ertPS, which is great for catching up on your favorite television show or the sports news.

The future is bright for me with many possibilities for doing new and exciting things. I am WiMAX™ -only right now, but I would like to do more, and eventually support handover to WiFi or 2G/3G cellular technologies.

Still the future is for another day, for now I am happy to provide high-speed connectivity with no wires. I do enjoy my life, as I provide a valuable service with lots of travel and opportunities to meet new base stations. I feel lucky compared to a friend of mine. Like all of us out here, he has to pass examinations before can earn his place in the working world. It should not take him long though, because just like me, he has been proving he can handle real world operations since he was first “assembled” by using the E6651A WiMAX test set to ensure every feature will operate well in a real network! He enjoys proving his capabilities, but can’t wait to join the work force.

E6651A Test Capabilities for Fast Device Development
Table 2 shows how your mobile WiMAX devices can enjoy all the features associated with a real network, without ever leaving the lab!

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