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November 11, 2010

WiMAX vs LTE: Does it Matter?

By TMCnet Special Guest
Dr. Alan Solheim, VP Corporate Development, DragonWave

There is a lot of debate about the future of WiMAX (News - Alert) and the ascendancy of LTE. The LTE camp is proclaiming the wide adoption of LTE and the demise of WiMAX. The WiMAX camp are arguing for the time to market advantage of WiMAX combined with ultimate convergence of LTE (News - Alert) and WiMAX in the IMT Advanced standards as reasons that continue to support the roll-out of WiMAX networks. Although this debate is relevant to the RAN equipment players and the service providers, as people who focus on the middle mile we have to ask ourselves what impact does this debate have on the solutions and services that we offer?

If we take a look at the evolution of both LTE and WiMAX we do see that that both have been adopted as proposals that meet the IMT advanced criteria for performance and spectral efficiency. Both enable the type of mobile broadband services that consumers are demanding. The underlying technologies are also very similar – based on OFDM radio techniques and IP – promising the possibility of handsets and base stations that can be configured via software to support one protocol or the other. So from a network requirements point of view, or from a user experience point of view, it is not clear that choosing one technology path or the other will have a significant impact. Indeed, one comes to the conclusion that this debate is more about marketing between base station vendors than about real differences in service levels.

So when we are designing backhaul networks, what do we need to care about? We need to look at average and peak capacity, latency and quality of service – all of which drives the cost per bit in the total cost of ownership. It is a safe assumption that is the RAN technologies deliver the services that people want, the capacity per base station will quickly be driven to the maximum possible. The iPhone (News - Alert) introduction has re-enforced the maxim that if you give users what they want they will consume the service to the limits imposed by the network. As most operators in the world have limited spectrum available for the RAN, the base station capacity will be limited by the spectral efficiency of the RAN technology. Here both 802.16m and LTE Advanced promise similar maximum numbers – both in terms of peak (16 to 18 bits per Hz) and average cell (3 to 5 bits per Hz per cell) spectral efficiency.

Although the headlines always tout the peak per user capacity, when designing networks, what matters is the average cell capacity. Here we see that with the typical RAN spectral allocation of 20 to 30 MHz available to carriers such as Verizon, and AT&T (News - Alert) in the US, or the 20 MHz blocks recently secured off in the Indian BWA auction, the average base station capacity, even with the maximal configuration of the next generation technology, will be on the order of 100 Mbps per base station. Even allowing for peak rates to exceed this by a factor of 2, the capacities required are well within the capabilities of today’s packet microwave solutions. Wholesale providers may need to provide 2 to 3 times this backhaul capacity as they may want to service multiple base stations per site, however even at these levels of 500 to 600 Mbps per link, today’s packet microwave solutions can deliver this kind of capacity at the edge.

As we move closer into the core of the network, we need to support traffic from multiple base stations, and thus multi-gigabit capacities. This is where fiber and packet microwave overlap, but even so packet microwave solutions delivering several gigabits per second are available today. The aggregation portion of the network is also where network design becomes an important factor in delivering the maximum network capacity as opposed to the maximum link capacity.

When we are aggregating the traffic from multiple sites we can take advantage of statistical multiplexing to reduce the total network capacity required. The larger the community of interest that we are averaging over, the more effective is this technique, but a statistical multiplexing factor of 2 to 3 is readily achievable. Spatial re-use (the ability to simultaneously send traffic along every path from a remote node to the hub node in a ring/mesh network) can also significantly reduce the normal state traffic load. For example, sending traffic both ways around a ring cuts the per link capacity required in half.

Under failure or rain fade conditions QoS becomes important to ensure the high priority traffic is always maintained. Using a combination of these techniques, we can see that even for the highest capacities promised by the next generation IMT Advanced RAN standard, we can aggregate up to 40 base stations using today’s packet microwave technology (200 Mbps peak capacity, 2:1 statistical multiplexing, 2x spatial re-use, 2 Gbps per link capacity). This delivers more than enough capacity given the fiber penetration ratios between 0 to 30% seen in most western carriers.

So does the choice of RAN technology matter? From a network design perspective perhaps not, but the reality is that customers need to stay healthy so they can continue to roll out the next generation networks. The uncertainty caused by the debate may benefit the short term goals of individual RAN equipment vendors; however it hurts the industry as a whole in that it delays investment decision and destabilizes the industry as it struggles to deal with the rapid evolution to high capacity, all IP networks. So in the end the debate is relevant and needs to get resolved as soon as possible, however as usual, not for the obvious reasons.

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Edited by Stefanie Mosca

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