One of the biggest challenges currently facing wireless operators is the increasing demand for capacity.
Data intensive devices are driving wireless throughput requirements upwards at an alarming rate, consequently putting a massive strain on networks that don’t have the ability to cope with this increase in data traffic.
With traditional network infrastructure suited for less intense 2G services, wireless operators are constantly looking for ways to increase the spectral efficiency of their networks.
Multiple Input/Multiple Output (MIMO) technology is now gaining substantial momentum in wide area mobile wireless networks with the launch of 4G services and is a key technology that substantially improves network capacity and user throughput in 4G networks.
MIMO has proven itself in the Wi-Fi world with the introduction of 802.11n. Using multiple antennas at each end of the wireless connection, 802.11n exploits multipath diversity that improves the quality and reliability of the signal and typically increases throughput by 50% or more.
MIMO is especially important for improving in-building cellular coverage, which can often present challenges that traditional methods of providing coverage simply cannot address.
In order for MIMO to deliver 70% to 100% throughput and capacity gains versus a Single Input/Single Output (SISO) deployment, a rich RF scattering environment with a high signal-to-noise ratio is required. The rich scattering environment describes most in-building environments and in-building distributed antenna systems (DAS) can provide the high signal-to-noise ratio.
How MIMO works
MIMO is a method of using more than one antenna on radio transmitters and receivers to improve communications performance.
With the first large scale commercial MIMO applications implemented for Wi-Fi (IEEE 802.11n), this smart antenna technique has become interesting to mobile operators because it offers the potential to either increase the capacity of networks or to enlarge the footprint of a given cell without the need for additional frequency spectrum.
For our purposes, we will assume each transmitter and receiver has two antennas (known as 2x2 MIMO), which will be the most common MIMO configuration.
MIMO operates in two basic modes: diversity space time coding (shown in Figure 1) and spatial multiplexing (shown in Figure 2).
In diversity space time coding, 2x2 MIMO transmits the same data stream from two antennas but codes them differently, to improve coverage within a cell using Tx/Rx diversity processing.
Essentially, the overall signal being exchanged is nearly twice as strong as it would be in SISO mode, so the cell coverage area is correspondingly larger.
When spatial multiplexing is used, a different data stream is sent (or received) by each antenna, which has the effect of doubling the amount of data that can be exchanged at a given time.
These methods are important for mobile operators because 4G protocols such as LTE are expected to provide both better cell coverage and higher capacity for a given cell.
However, MIMO’s performance improvements depend on the type of environment where it is used.
In order to gain the maximum benefit from MIMO, each of the two transmit signals must bounce off of objects so that they are received with two distinctly different profiles at the receive antennas.
Interior offices with lots of walls and furniture will be better venues for MIMO than will large convention centres or other facilities that consist mainly of continuous open space without such obstructions.
Why MIMO matters
Given the realities of 4G service demands, carriers will need all the help they can get to ensure a high quality experience for every in-building user, and MIMO will be a critical part of making that happen.
Carriers face two problems with LTE:
- Ensuring that every subscriber who wants it can get multiple megabits of network throughput
- Maximising spectral efficiency so they can deliver LTE to as many customers as possible with the spectrum allocation they have paid for.
Traditionally there have been several ways for operators to increase capacity. For example, they can buy or use more spectrum, split cells, use higher-order modulation schemes, or use higher-efficiency protocols.
MIMO allows operators to increase capacity without requiring new spectrum or new sites – both of which can be extremely costly.
It can be used either to increase the size of a cell or to increase the capacity within a cell. In either case, it’s a benefit to mobile operators, particularly at a time when 4G network bandwidth is a key advantage in a service offering.
As the use of smartphones and tablets continues to rise, it has become crucial, perhaps now more than ever, for mobile operators to address their capacity needs.
As such, any technology that could improve network capacity is better than one that won’t, and MIMO definitely will improve capacity in the right environments.
John Spindler is v-p of product management at TE Connectivity