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Capacity comparisons and TDD vs FDD

Capacity comparisons and TDD vs FDD

At CBNL, we’re continuing to head the capacity leaderboard in multipoint microwave, as we’ve done for the last several years.

With the advent of small cell backhaul, we’re seeing new entrants to the backhaul market in general and to multipoint backhaul in particular.

This is no surprise given the powerful benefits in TCO and time-to-market that multipoint backhaul confers.

Different backhaul systems are designed in different ways, of course, and one of the most fundamental system parameters is the duplexing scheme.

The duplexing scheme

The duplexing scheme is used to separate transmissions in the upstream direction (from the node B to the core network in the case of 3G backhaul) from transmissions in the downstream direction.

At CBNL, we design frequency division duplex (FDD) systems. 

This means that upstream and downstream transmissions each occur full-time on different frequencies.

An alternative approach is time division duplex (TDD).

In this scheme, upstream and downstream transmissions occur on the same frequency, but at different times.

A number of new entrants to the multipoint backhaul space, such as Taqua, Blinq and RadWin offer TDD systems and quote capacities in the range of 82Mbps to 250Mbps.

For a TDD system with a capacity of 250Mbps, therefore, there are 250Mbps available for both upstream and downstream transmissions.

Traditionally we’ve always quoted the net Ethernet capacity of VectaStar Gigabit as being 150Mbps. 

But this makes for a misleading comparison with TDD systems, because VectaStar Gigabit can simultaneously transmit 150Mbps upstream and 150Mbps downstream. 

Therefore, effective immediately, we’re changing the way we describe our platforms so that a direct comparison with TDD systems can be made. 

Since there is a total of 300Mbps available for both upstream and downstream transmissions, that’s the figure we’ll now quote as the system capacity.

Describing the system capacity in this way helps to clarify the differences not just between FDD and TDD systems, but amongst different FDD systems as well. 

VectaStar Gigabit’s all-outdoor hub and remote terminal (RT) equipment operates at 256QAM in both the upstream and downstream directions. 

But some systems can operate at 256QAM only on the downstream and are limited to 64QAM upstream. 

So in these terms our nearest competitor in the multipoint microwave space only has a capacity of 285 Mbps.

Increasing capacity is the single biggest driver for our technology roadmap and at CBNL we intend to maintain our leadership.

There’s another key benefit to an FDD scheme: low latency.  I’ll talk about that more another time.

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