Commercially Available Levels of QAM
The following table displays the commercially available levels of QAM and the number of information bits that can be represented by the transmission of one symbol. For example, in 4 QAM, a symbol is able to represent two bits while in 256 QAM, a symbol is able to represent eight bits.
This table also indicates an important practical matter: as we create the equipment that can extend the QAM family of modulations, transmission systems get less benefit from each extension. For example, microwave systems that utilize 4 QAM can enjoy a 100% boost in capacity by going to 16 QAM. (In 4 QAM, one symbol represents two bits while in 16 QAM, one symbol represents four bits or twice as many.) To gain another 100% boost, these systems have to be upgraded to 256 QAM (from four bits per symbol to eight bits per symbol). To gain yet another 100% boost, well, we will require the creation of 65536 QAM to get there! Today, the highest commercially available level of QAM modulation is 1024. Doubling this level to 2048 QAM yields only a single-digit increase in capacity and might not be justified in many applications.
There are also a number of physical factors that limit steps up the ladder of capacity advances in QAM. As the number of symbols increases (higher QAM), transmission systems require a significantly higher signal-to-noise ratio (SNR) since the difference (distance in amplitude/phase) between the symbols is reduced and, consequently, the transmission becomes increasingly sensitive to noise. Transmitted symbols are affected by phase noise in oscillators and external noise like interference from other carriers effectively limiting the ability to detect correctly the symbol received. As such, the industry is approaching the point where it is not practical to implement a higher QAM in long-haul networks due to significant limitations in achievable path length, equipment complexity, etc.
1024 QAM in Long-Haul Microwave
Leading long-haul microwave equipment vendors are just now announcing the availability of dependable long-distance transmissions using 1024 QAM. Relative to the industry-standard 256 QAM, this represents a 25% increase in capacity (and up to double the capacity of legacy SDH links), all things being equal. Operators of long-haul microwave links will certainly enjoy the boost to their capacity with 1024 QAM, especially when these upgrades are relatively painless and generally require only a minor and quick swap of equipment and a software upgrade.
Ceragon’s Adaptive Coding and Modulation
Furthermore, leading microwave equipment vendors are able to keep their long-haul transmission links functional even in transient noisy conditions. For example, systems such as those of microwave provider, Ceragon Networks, sense the quality of the transmission link and can automatically decrease the modulation technique in case of degraded signal quality due to interference or other microwave propagation problems. So, if a microwave transmission is humming along at maximum capacity using 1024 QAM and suddenly encounters interference, a system such as the Ceragon microwave system automatically steps down the modulation to lower levels until the transmission network, although less capacious now, maintains the requisite level of reliability. As the transient problems disappear, the microwave system automatically re-applies more efficient modulation techniques to regain full capacity.
Advancements in modulation offer network operators an easy and inexpensive path to higher capacities to meet demand. The advanced modulation technique, 1024 QAM, is available today and is a very cost-effective way to boost capacity in long-haul microwave applications.
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Eirik Nesse, VP, is responsible for the global strategy for Ceragon
’s microwave product portfolio and associated products. Formerly, he was the Chief Technical Architect at Nera Networks where he worked in various positions and has more than 27 years of industry experience. Mr. Nesse holds a BSc degree in Electronics Engineering from University of Stavanger.