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Joint source and channel coding (JSCC) has been widely reported for its superb capability in mitigating the legacy channel/user diversity problem in the wireless multicast/broadcast scenario, where the successive refined source information is manipulatively mapped with superimposed channel symbols via superposition coding (SPC). However, the use of SPC involves in additional hardware in the physical layer, which may not be in compliance in most standard equipments and protocols. This paper introduces a novel framework of logical superposition coded (L-SPC) modulation for multicast/broadcast of successive refined source via the JSCC approach. The framework is characterized by completely software-based logical SPC modulation at the sender, where the modulated signal is generated by mapping successively refined information bits onto a single signal constellation. At the receiving end, the proposed framework only needs simple modifications in the MAC layer software, while the required demodulation schemes are completely compatible with the standard ones with uniform constellation. Generalized formulations on symbol error rate are derived for performance evaluation and comparisons with the conventional hardwarebased approach for both 2-layer and 3-layer cases.

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This paper investigates the validity of Shannon's separation theorem in the finite block length regime. Under optimal tradeoffs between source rate and channel block error probability obtained from finite block length analysis, noisy channel quantizers based on joint source-channel coding principles are shown to outperform the separate quantizer designed via Lloyd-Max in terms of end-to-end distortion. Numerical results for the scalar case under the binary symmetric channel and discreteinput memoryless channel demonstrate that the separation of source and channel coding no longer holds in the finite block length regime, but the advantages of joint designs may be large or small depending on the system configuration.Index Terms-joint source-channel coding, finite block length regime, lossy source coding, end-to-end distortion.