Enabling LTE

Increasing microwave capacity will accommodate the mobile data boom.

Siphiwe Nelwamondo
By Siphiwe Nelwamondo, technical marketing manager with Aviat Networks.
Johannesburg, 13 May 2013

With the exponential growth the mobile industry is experiencing in terms of data traffic, many of the older microwave backhaul systems are being overwhelmed with data payload.

Legacy Time Division Multiplexing (TDM) radios cannot accommodate the IP-based long-term evolution (LTE) backhaul payload, because they are inefficient when it comes to transport of IP-based traffic and limited when it comes to capacity and the features required to support and manage IP.

As such, hybrid microwave backhaul solutions are changing the way the industry looks at capacity and is giving the industry new capabilities.

There are currently three ways in which network operators can accommodate rising mobile data traffic demands on their microwave backhaul:

* More spectrum - more hertz via larger channels, use of multiple channels aggregated together or use of higher frequency bands;
* More efficient use of available spectrum - by transmitting more bits per hertz over the available spectrum; and
* More effective throughput - by managing what data is actually passed, so effectively sending more data using the same number of bits.

More spectrum

All three of these strategies are achievable and can be useful, but more backhaul spectrum is the best option.

However, as spectrum allocation and access to the digital dividend remains a challenge in South Africa, and available spectrum is already overburdened, operators need to look to more effective throughput to meet current and future data traffic demands on LTE. But, if they can get spectrum, they must use it.

More spectral efficiency

In terms of making the available spectrum more efficient, traditional techniques have included adaptive coding and modulation (ACM), a widely accepted method of optimising the microwave signal to the current atmospheric conditions. In ideal transmission conditions, ACM enables a microwave radio to transmit at a high modulation with a high data capacity. However, in less than ideal conditions due to rain, for example, a radio with ACM will temporarily step down to a lower modulation to maintain the link, but at a lower data rate.

Recently, other techniques such as higher-order QAMs (quadrature amplitude modulation) and multiple-input/multiple-output (MIMO) antenna usage for line-of-sight microwave have come into existence.

These techniques are not universally accepted by the industry and may pose some technical, physical and procedural obstacles before they can be put into regular usage - and then only for situation-specific applications where other solutions have proved inadequate.

More effective throughput

Hybrid microwave backhaul technology can help boost throughput, but the industry's ability to take advantage of some of these techniques will require a change in the way networks are viewed from an engineering standpoint. A network operator's comfort level with enabling new functions will dictate the extent to which they can leverage new capabilities and take advantage of hybrid microwave technology, as it offers numerous tools. Some are useful and some are not, so operators need to know how to use them.

A network operator's comfort level with enabling new functions will dictate the extent to which they can leverage new capabilities.

LTE backhaul throughput can effectively be increased by sending only the data that is needed through to the radio, by using two common optimisation techniques, Ethernet and packet header suppression. Ethernet packets consist of headers of a constant size and data of a varying size - the smaller the packet, the larger the percentage of the packet is header data and the more gains an operator will see by reducing the size of the header.

Using header suppression techniques, the size of the packet header is reduced using smaller placeholders for unnecessary information, leaving more space for data packets travelling across the link. These headers are then recreated on the far end. Capacity gains vary greatly between 1% and 45%, depending mainly on average packet size and the type of protocol used.

Some microwave vendors are also employing payload compression techniques, also known as bandwidth acceleration, which replaces strings of repeated patterns of data with a placeholder. Consequently, promises of dramatic throughput improvement of up to 2.5 times with no additional spectrum have been made.

However, improvement is neither guaranteed nor predictable, since it is highly dependent on the traffic mix and type of protocol used. As such, the typical real-world improvement is minimal, and because this approach also increases link latency, this explains why payload compression has not been generally adopted in the industry.

Realistic backhaul capacity

Basically, operators need to be realistic about their actual capacity needs and the technology they employ to meet them. Most capacity improvements are incremental. For operators to meet growing capacity demands a fundamental rethink of network design is required.

Overestimating required bandwidth is a common but very costly error - more common in the all-TDM radio era. Operators should be moving from TDM capacity calculations to take into account the 'bursty' nature of IP traffic. The 'burstiness' of IP traffic allows operators to take advantage of statistical multiplexing gains and 'overbooking' factors when implementing hybrid or all-IP microwave technology to increase throughput without designing for peak loading.

With the right strategy and proper planning, hybrid microwave technology can be the first step toward an effective backhaul deployment for high-traffic LTE cellular networks. Moreover, with the right strategy, more capacity can be added as needed.