Point-to-point (PTP) microwave has been around for many decades, providing powerful and convenient backhaul solutions to the market. With the constant advancements in spectral efficiency (bits/s/Hz), PTP microwave has been able to keep up with ever-increasing traffic demands and is still the most often the chosen method of backhaul for networks (due to its flexibility and cost effectiveness). However, without diligence in the design phase, the implemented microwave link will most probably under-perform, or fail to meet performance expectations. This article considers the factors contributing to microwave performance and what you need to know when designing a microwave backhaul link.
Link availability plays a massive role in defining microwave performance. The link availability achieved is most dependent on whether LoS (Line-of-Sight) can be attained between base stations as well as the system fade margin: the difference between the Maximum Allowed Path Loss (MAPL) and the Receive Threshold of the microwave equipment. The margin accounts for the worst case scenario, where environmental conditions are poor due to rain and fog. The formula for calculating fade margin is defined below. By improving certain parameters such as transmit power and antenna gains, the margin can be increased.
Using the fade margin, the theoretical link availability can then be determined using various methods specified in the ITU standards. However; due to its complexity, it is recommended using modelling software such as ATDI’s ICS Designer to generate this value. Such software will also determine whether LoS coverage is attainable through path profiles.
There are many different factors that affect the throughput performance of microwave radio systems.
The most obvious way of increasing throughput capacity of a microwave link is to use a larger portion of bandwidth. This method does not improve the spectral efficiency, but instead increases the size of the “pipe” that the data can flow through. An increase of 10MHz will improve the capacity 10-20Mbps at low modulation schemes (QPSK), and upwards of 60Mbps for high modulation schemes (256QAM).
The spectral efficiency defines the number of bits that can be transmitted over a single second within a single Hertz (Bits/s/Hz). Increasing the modulation order forces more information into a fixed sized “pipe”, which raises the system capacity however in turn, decreases the margin for error. A general rule of thumb is for every bit coded onto the signal, the Rx Threshold is reduced by 3dB (See Fade Margin formula). Typical modulation orders range from QPSK (2 bits) to 256QAM (8 bits); however, manufacturers are developing microwave equipment with much higher levels of modulation. Certain microwave vendors have microwave radios capable of realising modulation rates as high as 2048QAM, with 4096QAM in the pipeline. These levels of modulation promise fibre-level transfer rates of over 1Gbps in a single stream under certain conditions.
Number of Spatial Streams
High level modulation schemes are reaching the limit of the amount of data that can be sent over a single stream. This means the only method of significantly increasing throughput is to increase the number of streams transmitted at any given time. This can be achieved by introducing additional microwave links but this is expensive and in most cases not feasible. That being said, there are approaches to send multiple streams across the same microwave bearer. These are polarization multiplexing and NxN Line-of-Sight (LoS) MIMO (Multiple-Input-Multiple-Output).
- Polarisation Multiplexing – Two single-carrier radios transmitting on the same frequency channel but with orthogonal polarisations (horizontal and vertical). Uses Cross-Polarisation Interference Cancellation (XPIC) to ensure signal quality.
- NxN LoS MIMO – Comprises ‘N’ separate antennas providing ‘N’ different spatial streams resulting in the capacity in the channel to increase by a factor of ‘N’. Spatial separation between antennas is required to generate a 90◦ phase difference between the streams at the receiver to achieve orthogonality.
Throughput can further be increased by utilising both these techniques at the same time. Power consumption and cost are the key limiting factors.
Radio Link Bonding or Link Aggregation (LAG) is the process of combining two or more microwave channels to create a higher capacity virtual link. This drastically improves performance for services such as TDM SDH as the service no longer requires a complementary sized channel, but instead can bundle channels together. This technique, along with header compression, can improve the efficiency of the throughput and thus increase system capacity.
The ever-growing demand for capacity is pushing the limits of networks on a global scale. Thanks to ongoing technical innovation, microwave backhaul has the means to support this demand, however places an even greater impetus on the importance of a rigorous design to ensure the desired performance is attained.