Effect of adjacent channel interference and frequency-selective fading on outage of digital radio

Abstract

Digital radio is affected by frequency-selective fading (FSF), which causes intersymbol interference (ISI), and by adjacent channel interference (ACI), which in its turn is also affected by the same FSF. Using a two-ray model for the FSF, the transfer function of the channel is H(f) = a[1 − b exp ( − j2πτ(f − f0))]. Outage in digital radio occurs when the bit error probability exceeds a critical value of 10−3, or when an equivalent mean-square error exceeds a critical value σ2ec, which depends on the modulation used. In the paper we compute the critical mean-square error for M-QAM and M-PSK modulation where M = 4, 8, 16, 64 is the number of symbols. In the absence of FSF, the system response for the signal in the main channel is a raised cosine with excess bandwidth β, and the receiver filter is matched to the transmitter filter. The signal in the adjacent channel has the same modulation as the signal in the main channel, although they differ in carrier frequency, amplitude, phase and symbol timing. We derive an expression for the mean-square error in the presence of FSF and ACI. The outage probability is a product of two terms, one of which is independent of the modulation methods, while the second is proportional to the area under a signature. The signature is a plot of the minimum value of 1 – bc as a function of f0, where bc is a critical value of b such that, if b ≥ bc, the critical mean-square error is exceeded. We compute the signatures and the area under the signatures for M-QAM and M-PSK (M = 4, 8, 16, 64) with a bit rate of 140 Mbit/s and frequency separation between channels of 40 MHz. We conclude that, for low values of ACI, 4-QAM is the best system, while for large values of ACI, 64-QAM is the best system. 16-QAM is always better than 8-PSK.