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The ins and outs of MIMO in HSPA+

With a few exceptions, more is always better — and antennas are no exception! Realizing this, HSPA+ evolution incorporates a slew of advanced multi-antenna techniques such as MIMO (Multiple Input Multiple Output), Beamforming/Transmit Diversity, and others. MIMO is one of the main components for enabling HSPA+’s famed, successively higher peak data rates – starting from 28 Mbps in Rel. 7 to 168 Mbps in Rel. 10, and even higher in Rel.  11 and beyond, also called HSPA+ Advanced.

Thanks to the popularity of 3G, the demand for data services is growing at an epic rate. Faced with this growth, operators are looking at opportunities to cost-effectively increase capacity. MIMO is one of the potent tools in their arsenal, as it significantly improves network data rates, capacity and user experience within existing spectrum through an upgrade to existing infrastructure.

It’s no wonder that many operators have already started on this path. According to the GSA, as of July  2011, there were 8 commercial HSPA+ networks supporting 2x2 MIMO, showing the way forward for the remaining HSPA/+ operators in the world (400+). There are already about 10 commercial MIMO devices in the market, and virtually all infrastructure vendors support MIMO. 

In this series or articles, I attempt to briefly explain MIMO and its enormous benefits.

What is MIMO?

MIMO, as the name suggests, involves leveraging multiple transmit and receive antennas available at the BTS and the device in many ways to increase data rates, overall capacity and the user experience. Essentially, the MIMO system leverages antennas and “processing” at both the transmitter and the receiver side of the equation to create multiple, uncorrelated radio links (streams) between the transmitter and the receiver. (Uncorrelated radio links have different fading characteristics.)

These streams use the same time and frequency resources, which means that within the same amount of spectrum, one can increase the capacity by multiples. To sum it up, the more antennas we can deploy, the more streams there are between them. And that means more information being transferred, at higher data rates. All of this is possible, while using the same amount of spectrum and transmit power. MIMO deployment may require a hardware upgrade at the BTS, and antenna system changes, depending on the existing infrastructure.

Every order of MIMO doubles the peak rates, for example, HSPA+ Rel.7 2x2 MIMO doubles the peak rate over the same system with 1x1 antenna configuration. For MIMO to effectively create these uncorrelated streams, devices have to be in a good cover coverage area, and with no line of sight (LoS) to the BTS (rich scattering and multipath), which is typically the case in urban environment. In most instances, that’s exactly where the higher capacity is desired.

In the next article I will talk about how MIMO improves data rates for all, and increases capacity. So, stay tuned.

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