Adaptive Modulation (AM) is a technique now being introduced to point-to-point digital microwave radio systems to provide more user capacity over the air during good propagation conditions, where the modulation level of the radio link "adapts" dynamically to the conditions of the path.
In traditional point-to-point systems the modulation is fixed at a certain level, delivering a constant throughput for a defined channel bandwidth.
Adaptive techniques have been used in microwave radio systems, such as Automatic Transmit Power Control (ATPC), which lowers output power when conditions are good to reduce power consumption and network interference.
Under fading conditions the power is automatically increased to ensure the link continues to meet the required performance level. Adaptive Modulation takes ATPC a step further, by controlling both power output and modulation level dynamically, to adjust the link capacity to suit the propagation conditions.
Key benefits
The key benefit of AM is that it enables operators to easily multiply the available capacity of links, without requiring any hardware changes, without having to increase the antenna size, and without affecting license conditions.
Licensed microwave radio links typically designed to deliver system availability due to propagation induced outages of typically 99.99 percent, for example, meaning the link will be unavailable for about 50 minutes in a year.
However, for the rest of the time the fade margin (defined as the difference between the instantaneous receiver level and the equipment receiver threshold) is essentially unused. It is held in reserve.
This unused margin comes at a high price, requiring links to be shorter, antennas to be larger, or link capacity to be more limited than necessary. AM allows use of excess fade margins to dramatically increase the link capacity at little or no additional cost.
Using AM to introduce carrier Ethernet
Adaptive Modulation is particularly useful in the deployment of Carrier Ethernet services and is relevant for mobile network operators planning to migrate to all-IP their networks.
In this case, Carrier Ethernet is often implemented as an overlay on top of a legacy TDM transmission network that supports their existing 2G and 3G R99 base station infrastructure.
On TDM-only links throttling traffic flows as the modulation rate changed was problematic, since there was no easy way to prioritize traffic when the link capacity is reduced.
New Carrier Ethernet over Wireless systems, however, include embedded Layer 2 intelligence to enable traffic flows to be managed and adapted in circumstances where the link capacity varies.
When link capacity reduces by modulation level down-shifting to adapt to poor link propagation conditions, high priority traffic such as real-time TDM or IP voice or IP video services can be maintained, while lower priority traffic (non-real time data applications) are dropped.
Switching between modulation levels must be errorless so that maintained traffic is not affected. Some adaptive systems impose traffic interruptions of up to a few seconds for each modulation shift.
The Layer 2 Ethernet switch performs traffic shaping to control or police the output rate of the transport (airlink) channel, while a scheduler coordinates the traffic using an 8:4:2:1 Weighted Round Robin (WRR) algorithm to fit the available bandwidth.
