Abstract :
In high capacity digital radio-relay systems (DRRSs) it is necessary to make use of complex modulation schemes (e.g. 32-, 64-, 128-, 256- or even 512-QAM) to achieve desired spectral efficiency. These modulation schemes are particularly sensitive to many types of impairments and their use requires sophisticated techniques to deal with some of them. The article describes two countermeasures used to improve performances and to face problems related to noise and nonlinear distortions, respectively. It refers to a SDH DRRS, which uses 128-QAM modulation to carry a STM-1 payload in frequency bands with 28 to 30 MHz channel spacing.
