Long-haul microwave systems in telecommunication networks carry information across distances of more than 15 km and even more than 200 km in a single hop. They are deployed in a wide variety of applications such as mobile networks, carrier networks, offshore links, digital broadcasting and many more. The alternative, fiber cable, is often not available due to cost, deployment time, regulations, reliability, security, terrain difficulties or other limiting factors. Easy and fast-to-deploy, microwave is often the medium of choice for long-haul applications.
Microwave systems use digital modulation techniques to carry information in free space over an analog carrier signal. As use of mobile telephones, laptops and other computer devices increases along with demand for data-rich services like video, network operators are bombarded with requirements to increase capacity in the transport network. As microwave spectrum is limited and is often encumbered with expensive frequency fees, advances in modulation techniques can contribute significantly to microwave spectrum efficiency and thus to capacity improvements.
A modulation technique uses some number of distinct signals, symbols, to represent the digital information that is to be transmitted. Modulation techniques are considered to be more efficient when each symbol can represent a greater quantity of information—more bits. Modulation techniques are important considerations in the efficiency and capacity of a microwave system.
An analog carrier, like a microwave signal over the air, can be modulated to carry digital information in three ways: by amplitude, by frequency and/or by phase. Each modulation technique varies a parameter of the signal to represent the information that is being transmitted. Microwave links and other types of wireless transmission equipment employ various modulation techniques to boost the amount of information that can be transmitted in a given period of time, viz. capacity.
Each modulation technique has advantages and disadvantages. For example, amplitude modulation, in a common implementation called amplitude shift keying (ASK), is affected by noise and interference. Frequency shift keying (FSK) behaves better than ASK in the presence of interference, but is more expensive to implement. Phase shift keying (PSK) is a better performer than either ASK or FSK and is used frequently in digital communications.
Amplitude, Frequency and Phase Modulation Techniques
PSK also gives rise to a family of modulation techniques starting with binary-PSK, or BPSK. In BPSK, a single bit—a zero or one—is sent per symbol (change of phase). More advanced transmission systems make use of quadrature PSK, 8PSK, 16PSK, etc. In each case, PSK is able to increase the capacity of the signal by coding for more bits per symbol. For example, Quadrature PSK (QPSK) is effectively two independent BPSK modulations and provides twice the capacity.
Quadrature Amplitude Modulation (QAM)
QAM is a very advanced modulation technique and is today’s state of the art in microwave radio transmissions. In QAM, the transmitter splits the flow of bits (information) to be transmitted into two equal parts modulating two independent waves. A 90-degree phase is created between the waves. QAM represents information by modulating the amplitude of the two waves that are out of phase with each other to represent the signal. In effect, QAM is amplitude modulation and phase modulation combined.
Amplitude and Phase Modulation Combined
The advent of QAM has given rise to a series of increasingly efficient modulations. It is easy to understand the efficiencies. In QAM, a symbol can represent two or more bits of information. As with all modulation techniques, the greater the number of bits of information per symbol, the more capacious the transmission.