Field Tests of Digital Audio Broadcasting in the FM Band Based on Continuous Phase Modulation

We propose a new broadcasting scheme for the transmission of digital audio signals simultaneously with existing analog frequency modulation radio (88-108 MHz) in adjacent channels, denoted as in-band-adjacent channel system (IBAC) . This broadcasting scheme enables the transmission of digital audio data in CD quality with a data rate of up to 156 kb/s within an FM 200 kHz channel. The digital transmission is based on continuous phase modulation (CPM) and a proper reduced-state sequence estimator. CPM is the digital equivalent of the analog FM and has a constant envelope, too. This is a great advantage, because on the one hand existing nonlinear amplifiers can be reused to achieve good efficiency and low out-of-band radiation, on the other hand it enables the broadcasting companies to reuse the existing infrastructure (amplifiers, antennas, frequency bands) which reduces the investment costs and improves the chances for an introduction of CPM as a new broadcasting standard. With the proposed method, the power level and the symbol rate of the transmitter signal is determined in a manner that the interference the CPM signal poses for the analog FM signal in adjacent channels remains below a level according to the radio frequency emission mask defined by international rules. Due to the multipath propagation of the transmitted signal, the transmission behavior of typical radio channels is characterized by high dispersion of up to 85 . For the parameterization of this demonstrator, the channel memory amounts 14 bits. Due to the implementation of the hardware, the symbol duration is different in this case from the one in . Since Viterbi detection is not feasible due to the number of channel states, detection is performed by a reduced-state sequence estimator that is able to eliminate the complete channel interferences by decision feedback . We will describe in this paper the underlying algorithms of synchronization and channel estimation, since they are important components of a real receiver. To underline the suitability of the CPM based broadcasting scheme, we have conducted field tests in the city area of Hanover with a real broadcasting evaluation system based on the proposed CPM broadcasting scheme with support of the northern German broadcast service (NDR) in Hanover with encouraging results.