MetroPCS shows CDMA carriers' 4G dilemmas

Caroline Gabriel/Wireless Watch
26 Sep 2011

But Verizon will not prove the typical LTE rollout. Many operators will have no new spectrum for some time to come, or small allocations, or they are struggling with tight budgets. In the GSM world – despite AT&T‘s pleadings that it must have T-Mobile‘s spectrum to make its LTE plans feasible – there are plenty of flexible solutions in terms of HSPA+ expansion and multimode systems. For CDMA carriers, with a far more restricted migration path to 3G+ performance, the options are more limited, and in markets with multiple CDMA operators, such as the US and India, there will be a rush for frequencies and partnerships.

Until there is a robust solution to LTE voice, switching off CDMA too abruptly is not an option, yet without new spectrum, CDMA operators usually have very little bandwidth to play with. While CDMA itself is famously spectrally efficient and can work in very narrow bands, converting these to LTE will not deliver the kind of performance that can be achieved in wider channels.

Typically, a cellco might have 5MHz of spectrum, accommodating three CDMA carriers and a 625KHz guard band at each end. In introducing LTE, either CDMA can be turned off, devoting the full 5MHz to the new technology, or LTE can be run in one or more of the 1.4MHz carriers.

In neither case will the operator see anything like the speeds adopters are commonly expecting, which are usually promised on the basis of having 10MHz or even 20MHz. For that, an uncluttered band is needed – as Verizon and AT&T have with their shiny new 700MHz licences. Both are demonstrating strong data rates, admittedly on underloaded networks, even though 700MHz will soon run up against capacity constraints.

But at least they have clear spectrum for their initial rollouts, giving them breathing space to address the more complex issues of overlaying LTE in 3G bands, whether running simultaneous networks or turning the older one off.

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