Improving the MeshAP Signal Reliability

Signal reliability has a direct impact on the quality of service experienced by Mesh clients and is a key technical challenge facing Mesh Operators. Antenna diversity offers Mesh client a way to improve signal reliability by minimizing signal loss and mitigating the effects of multi-path fading. 

Antenna diversity will:

Antenna diversity involves the use of multiple antennas to receive multiple instances of the same signal and then make use of the otherwise redundant data contained within these signals. This allows the system to be more robust against the many factors that degrade signal reliability. A single antenna may not be able to receive a signal for several reasons. 

The antenna type - a patch antenna, for example - could be inappropriate. The antenna may be oriented in the wrong direction or obstacles could block the signal. In all these cases a second antenna would clearly improve the probability of receiving the signal. 

The use of multiple antennas is not a new idea and most Wireless Access Points employ antenna-diversity techniques; however, few if any MeshAP use antenna diversity. There are two main reasons:

1.      It is difficult to implement multiple antennas from a single radio card, which increase the cost of the MeshAP. 

2.      The effects of antenna coupling will degrade the performance of the MeshAP antenna.

Antenna-diversity techniques generally fall into four categories. 

Spatial diversity involves the use of physically separated identical antennas. The phase centre of each antenna is also spatially separated. 

Pattern or beam diversity uses co-located antennas that are of different size, shape, orientation and material. These antennas have dissimilar radiation patterns and their signals are combined in phase due to their collocation.

Polarization diversity uses two antennas oriented at 90° to each other. The result is mutually orthogonal polarization states, such as horizontal and vertical; left-hand circular and right-hand circular; or ±45° slants. The antennas used in polarization diversity schemes are often identical. 

Receive and transmit diversity schemes employ separate antennas for transmit and receive functions, as a result frequency filtering is not needed. 

To illustrate the antenna-diversity it is useful to consider a node-to-node system. A dual diversity system could process two input signals, to create an improved signal that would reduce fading and co-channel interference. The signal improvement depends on the cross-correlation, which is a statistical value that defines the level of similarity between the voltages received at the two antennas, and the relative strengths of two received signals. The two antennas then improve data reception speeds by fourfold.

Another example allows dynamic selection between full diversity and no diversity. As the link quality deteriorates, the receiver transitions from a single-antenna connection to the full-diversity mode. The transition back to a single-antenna connection is triggered by an indication of significantly improved link quality. When going back to a single-antenna configuration, a comparison of the received-signal strengths in the two antenna paths can easily lead to a preferable antenna connection.

There are various receive-antenna diversity schemes are available to the mesh operator and extension to multiple antennas is straightforward.

No diversity (single-antenna mode)

This is an obvious case where there is only one receive antenna. It allows the simplest implementation and results in the lowest power consumption of all cases.

Switched diversity

Only one receives antenna is chosen at any given time during reception, based on some prescribed selection criterion. The antenna connection is switched when the perceived link quality falls below a certain prescribed threshold.

Selection diversity

One antenna is chosen whose receive path yields the larger signal-to-noise ratio. The signal-strength measurement can take place during a preamble period at the beginning of the received packet. So, a single antenna connection is maintained most times, but during the measurement of the SNR/signal strength, both antennas' connections need to be established. The actual selection/switching process can also take place in between packet receptions, and can be done on a packet-by-packet basis or can take place once in a number of receptions or prescribed time period.

Full diversity

Both antennas are connected at all times. Since received paths must be powered up, this mode consumes the largest amount of power, but it also offers the largest performance gain compared with other configurations, especially in severe fading environments with large delay spread. The digital front-end techniques—signal detection, frame synchronization and carrier frequency offset estimation/correction, for instance—can also benefit from the availability of multiple receive paths.

Summary

Antenna Diversity Strengthens may strengthen signal reception. Dynamic diversity is optimized for low power consumption and high performance. However, when used in a Mesh application, this technique requires close attention to link-quality on the receive side for an effective implementation.