We have an insatiable appetite for information, in all its guises. Whether for business or personal, commercial or recreational, does not really matter. What matters is its availability. It seems we want to be able to access everything, at any time, from any place.
Which means there is an increased need to provide high bandwidths to mobile subscribers - and to do it wirelessly.
The most challenging part of network delivery is between the backbone and end user - the so called ‘last mile’ - and the broadband wireless technology expected to dominate here is WiMAX.
Providing and maintaining high bandwidths, over any medium, is subject to the medium’s signal to noise ratio (SNR). For wired networks this is manageable, but for wireless technologies, even in ideal conditions, physical obstructions represent a challenge. Objects in the signal’s path lead to multipath reflections, representing increased noise in most wireless technologies.
However, the multiple reflections that arrive – albeit unexpectedly – at the receiving end, still contain valid data, and if that data could be extracted it would effectively improve the SNR and potentially increase the maximum available bandwidth. It is this premise that makes WiMAX ideal for last mile broadband wireless delivery.
The current WiMAX specification, IEEE802.16e.2005, uses scaleable OFDMA. This makes it more applicable to non line of sight (NLOS) systems and also makes the technology more applicable to mobile subscribers.
In conjunction with incidental multiple paths, WiMAX employs multiple antennas to intentionally generate multiple transmission paths, which in optimal cases are complemented by multiple receiving antennas.
Using multiple antennas
The success of this transmission deluge is not left to chance. Broadly speaking, there are three methods for deploying multiple antenna systems - diversity schemes, smart antenna systems, and multiple input, multiple output (MIMO) systems - each with its own technical and commercial benefits.
These methods generally employ some or all of the following techniques:
- diversity gain - achieved using multiple paths between the transmitter and receiver
- array gain - a function of combining two signals coherently
- power gain - a logarithmic gain across multiple antennas
- interference reduction - resulting from intelligent steering and beam forming
- spatial multiplexing - where two or more data streams are resolved by one or more users, to enhance system capacity and spectral efficiency.
Diversity
For diversity schemes, three approaches are popular.
The first is space-time coding (STC) which operates in space and time, by sending information over two transmit antennas, across two consecutive transmissions in time.
Antenna switching is a simple approach which uses multiple antennas but selects the single antenna with the highest gain at any given time.
The third technique is maximum rate combining (MRC). This compensates for channel characteristics by weighting each antenna in accordance with its transmission efficiency in order to maximise the summed signal’s SNR.
In general, diversity schemes can return between 2-5dB gain, depending on the application. Typically this can be in the uplink (MRC), in the customer’s premises (antenna switching) or the basestation (STC).
Smart antenna systems
If higher gain improvement is desired then smart antenna systems can deliver 10-15dB improvement, relative to single antenna architectures. This operates by combining an antenna array with signal processing to implement beam forming.
When receiving a signal, beam forming can increase gain in the direction of wanted signals and decrease it in the direction of interference or noise. When transmitting, it can focus the signal in the right direction and direct nulls at users who might otherwise be interfered with.
This active interference management can push spectral efficiency into the 5bit/s/Hz range, providing significant capacity benefits, especially when used in conjunction with spatial multiplexing techniques. However, these benefits begin to diminish as subscriber mobility increases.
Multiple input, multiple output
The final technique is MIMO, which realistically means two or more antennas on the input channel, and two or more at the output channel.
The profile defined by the WiMAX Forum Mobility Task Group specifies two MIMO versions - Matrix A MIMO and Matrix B MIMO.
Matrix A implements STC delivering higher link robustness and reducing fade margin by up to 6dB. Unlike the smart antenna system, it is more robust against degradation for mobile subscribers.
For channels with a rich multi-path environment it is possible to increase the data rate by transmitting separate information streams on each antenna in the downlink direction.
If suitable receiver technology is used, both streams can be decoded providing up to twice the capacity of a single antenna system. This can be particularly useful in urban areas, where a long span is less important than high data rates at the end user device.
In WiMAX, this spatial multiplexing on the downlink is made possible using Matrix B MIMO. The theoretical upper band of increased capacity achieved is roughly proportional to the number of transmit/receive chains used.
Subscriber mobility is clearly on the increase. With WiMAX technology expected to be integrated into more portable consumer devices in the future, combining the relative merits of MIMO and beam forming technologies could give digital subscribers the benefits of fixed access in a truly mobile world.
Paul Trubridge is senior director product management at Airspan