| Summary: | 碩士 === 大同大學 === 通訊工程研究所 === 97 === DRM systems are designed for AM signals with similar broadcast bands working below 30 MHz. Compared with conventional analogue AM broadcasts, they have made a great improvement in audio quality, including signal-to-noise ratio. Moreover, they can counter the effects of frequency selective fading and audible interference from other stations and analogue AM signal (DSB, VSB or SSB) and a DRM can be co-existed executed by simulcast transmission.
One of the most challenging issues for simulcast is that DRM signals cannot interfere with AM signals. Moreover, DRM signals are on an adjacent broadcast channel. An Orthogonal Frequency Division Multiplexing (OFDM) based DRM system has a relatively large out-of-band spectrum. As a result, we may have to take into account how to reduce out-of-band power radiation caused by high sidelobes of modulated subcarriers.
In this thesis, we adopted two methods derived from the Symbol Boundary Smoothing (SBS) concept and Insertion of Cancellation Carriers (ICC). In SBS, the system retained a position on both side of the null subcarriers. By adding subcarriers with parameters calculated, then the boundary of continuity between continuously sent OFDM symbols was maintained in order to achieve the effect of SBS in the time domain. By contrast, in the frequency domain, the sidelobes of signals were suppressed, thereby reducing out-of-band power.
In ICC, subcarriers of calculated complex weighted factors were inserted on both sides of the original data subcarriers. By applying the superposition property of signals in the frequency domain, the sidelobes of the original data subcarriers and the sidelobes of the cancellation carriers cancelled each other as both were positive and negative amplitudes, to suppress out-of-band power.
Besides, by adopting the current combination approach - combining the ICC approach with the Tail-tracking Approach or Zero-forcing Approach and employing the advantages of both minimizing the sidelobes for OFDM signals to be transmitted in the frequency domain and SBS in the time domain, a considerably great out-of-band power suppression effect was yielded on the adjacent channel and the alternate channel.
This simulation found that by adopting the Zero-forcing Approach, adding extra subcarriers position needed to adjust ±104 to ±105. This was to prevent the determinant of the matrix inverse becomes zero fails to solve the equation and failing to achieve the effect of 0 boundary smoothing. Compared with a WIMAX system, there was an similar power spectrum density trend. But as DRM subcarriers were more and the position inserted into subcarriers shifted a carrier space, out-of-band power suppression was 1 dB lower than WIMAX on average.
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