Ken Izatt and Stanton Zeff, PE

MICROWAVE LINKS are traditionally designed with adequate fade margin to protect the radio payload against harsh channel conditions. This additional fade margin may be thought of as a reserve to be used when adverse weather conditions occur. Adaptive modulation makes use of this additional fade margin for achieving higher capacity transport when available.

Adaptive modulation techniques strive to maintain an expected level of service at all times. When changing to a lower modulation state, link continuity is maintained and high-priority traffic is not affected. Similarly, existing traffic is unaffected when the link reverts to a higher modulation state.

Adaptive modulation is used to dynamically switch modulation schemes according to the prevailing channel conditions. The use of higher-order, quadrature amplitude modulation (QAM) schemes (to 256QAM) allows a greater amount of data to pass through a radio channel. The use of QAM improves the spectral efficiency, allowing for longer link distances or greater throughput, or where frequencies are limited and narrower-channel widths must be used.

Adaptive modulation allows microwave links to be engineered with a guaranteed minimum level of service while bursting to higher rates when conditions allow. Instead of being designed for the worst-case scenario, links are deployed according to a less-stringent criteria: The link operates at a higher capacity most of the time, but still provides a guaranteed level of service at a lower capacity when radio conditions are poor (e.g., during fading conditions). Traffic prioritization allows delay-sensitive traffic such as Voice-over-Internet Protocol (VoIP) and circuitemulated time-division-multiplexed (TDM) operation to be supported normally. In practice, the relative usefulness of adaptive modulation depends on a number of different factors, including the transmission distance, the weather conditions, the available transmit power, the modulation range, the capacity, and operating frequency.

Instead of using a fixed modulation rate under all path conditions, adaptive modulation provides guaranteed capacity and service availability by increasing the modulation rate, and hence the capacity. On a typical link, this means higher capacity is available more than 99.9% of the time. Typical modulation schemes selected for adaptive modulation are 4QAM, 16QAM, 64QAM, 128QAM, and 256QAM. For a given RF channel bandwidth of 5, 10, or 30 MHz, there is a twofold improvement in data throughput realized with an increase from 4QAM to 16QAM. Another twofold improvement is realized when increasing from 16QAM to 256QAM. Adaptive coding capability provides options to increase throughput based on modulation type. As an example, a 10-MHz channel in standard 4QAM with 99.999% availability can deliver approximately a 16-Mb/s data rate. However, using adaptive modulation in the same 10-MHz channel, the modulation can run at 64QAM with an average throughput of 45 Mb/s.

Adaptive modulation provides many benefits, not only to service providers but to communications systems customers. Dynamic adjustment of the radio transmitter modulation scheme ensures maximum data throughput, providing the best data integrity at any given time. Less robust but more efficient modulation schemes transform available fade margin (to 80 dB depending on link design) into increased data throughput. Traffic prioritization with quality of service (QoS) allows high-priority traffic to be handled normally during poor path conditions and low-priority, best-effort data to be discarded or delayed.

The figure shows a microwave link budget over time. It shows that the link can operate at higher modulations most of the time, with occasional reductions to lower speeds. Note that the amount of time spent using lower- or higher-order modulation varies with location.

As with packet-based microwave radios, adaptive modulation plays a key role in third-generation (3G) and fourth-generation (4G) radio access networks (RAN), as well. While microwave radios utilize only one dimension of the radio transmitter's characteristics (modulation), 3G and 4G networks manipulate multiple parameters by dynamically managing transmit power, coding rate, and modulation scheme simultaneously to address the harsh conditions of the mobile radio channel.

The following two examples demonstrate the power of adaptive modulation and highlight the flexibility that engineers have when designing microwave links. Table 1 is based on an existing 6 GHz, 3xDS3 capacity path (a digital interface with three 44.736-Mb/s lines) that is 19.7 miles long. The following steps were taken to ascertain the availability of the microwave radio link:

1. Determining whether the radio can meet "the five nines" of availability (99.999%) for 3xDS3 capacity. At 64QAM, the link meets this level of capacity and availability. Based on path loss calculations, the actual availability is 99.99952% (outage time of 151.76 s). Note that this radio link can be configured as a DS1 radio (1.544 Mb/s) with DS1 lines instead of DS3 lines, which eliminates the need for M13 multiplexers.

2. Determining what the availability is at 128QAM. Path loss calculations show the availability is 99.99862% (435 outage seconds). The outage time is increased by approximately 284 seconds but the data throughput is improved to 160 Mb/s. At 256QAM the availability increases to 99.99389% (1945 outage seconds) but there is now 183 Mb/s of peak throughput.

Table 2 demonstrates how adaptive modulation allows the use of smaller antennas while maintaining high capacity under changing channel conditions. Assume a requirement to backhaul 8 DS1 lines of fixed traffic and 12 Mb/s of variable Ethernet traffic from a cell site. Also assume this path is designed for five 9's of availability in the 23-GHz band with a 10-MHz channel. This requires the use of 3-ft. antennas to meet five 9's of availability. However, by using adaptive modulation, 2-ft. antennas can be used instead. A 4QAM link carries the 8 DS1 lines with greater than five 9's of availability. Nonetheless, during good channel conditions the radio will run at a 16QAM rate with greater than four 9's of availability.

These two examples highlight how adaptive modulation provides an improvement in system gain that enables higher channel throughput or smaller antennas sizes. In addition, the improvement in system gain allows the use of lower transmit power or the ability to travel longer distances.

Adaptive modulation is a powerful tool for service providers to cost effectively address the growth of data traffic in their microwave networks. Adapting the modulation scheme to the channel conditions and prioritizing traffic during fading conditions offers service providers the ability to fully exploit the microwave radio channel and drive down the cost per bit of transport. This technology will be a key feature in the next generation of microwave packet radios, which will feature gigabit throughput and integrated Ethernet switches. All-IP network transformation has expanded to the world of microwave radios.