User manual MATLAB COMMUNICATIONS BLOCKSET 4

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[. . . ] The explicit mapping is described in the algorithm section on the reference page for the M-PSK Modulator Baseband block. Delays in Digital Modulation Digital modulation and demodulation blocks sometimes incur delays between their inputs and outputs, depending on their configuration and on properties of their signals. The following table lists sources of delay and the situations in which they occur. Delays Resulting from Digital Modulation or Demodulation Modulation or Demodulation Type FM demodulator All demodulators in CPM sublibrary Situation in Which Delay Occurs Amount of Delay One output period D+1 output periods Sample-based input Sample-based input, and the model uses a variable-step solver or a fixed-step solver with the Tasking Mode parameter set to SingleTasking D = Traceback length parameter Frame-based input, D = Traceback length parameter D output periods 1-103 1 Using the Libraries Delays Resulting from Digital Modulation or Demodulation (Continued) Modulation or Demodulation Type OQPSK demodulator Situation in Which Delay Occurs Amount of Delay One output period Two output periods Frame-based input Sample-based input, and the model uses a fixed-step solver with Tasking Mode parameter set to Auto or MultiTasking. Sample-based input, and the model uses a variable-step solver or the Tasking Mode parameter is set to SingleTasking. One output period All demodulators in TCM sublibrary Operation mode set to Continuous, Tr = Traceback depth parameter, and code rate k/n Tr*k output bits As a result of delays, data that enters a modulation or demodulation block at time T appears in the output at time T+delay. In particular, if your simulation computes error statistics or compares transmitted with received data, it must take the delay into account when performing such computations or comparisons. First Output Sample in DPSK Demodulation In addition to the delays mentioned above, the M-DPSK, DQPSK, and DBPSK demodulators produce output whose first sample is unrelated to the input. This is related to the differential modulation technique, not the particular implementation of it. Example: Delays from Demodulation Demodulation in the model below causes the demodulated signal to lag, compared to the unmodulated signal. When computing error statistics, the 1-104 Digital Modulation model accounts for the delay by setting the Error Rate Calculation block's Receive delay parameter to 0. If the Receive delay parameter had a different value, then the error rate showing at the top of the Display block would be close to 1/2. To open the completed model, click here in the MATLAB Help browser. To build the model, gather and configure these blocks: · Random Integer Generator, in the Random Data Sources sublibrary of the Comm Sources library - Set M-ary number to 4. Set Initial seed to any positive integer scalar. · OQPSK Modulator Baseband, in the PM sublibrary of the Digital Baseband sublibrary of Modulation · AWGN Channel, in the Channels library - Set Es/No to 6. · OQPSK Demodulator Baseband, in the PM sublibrary of the Digital Baseband sublibrary of Modulation · Error Rate Calculation, in the Comm Sinks library - Set Receive delay to 1. Drag the bottom edge of the icon to make the display big enough for three entries. · Display, in the Simulink Sinks library 1-105 1 Using the Libraries Connect the blocks as shown above. From the model window's Simulation, select Configuration parameters. Then run the model and observe the error rate at the top of the Display block's icon. Your error rate will vary depending on your Initial seed value in the Random Integer Generator block. Upsampled Signals and Rate Changes Some digital modulation blocks can output an upsampled version of the modulated signal, while their corresponding digital demodulation blocks can accept an upsampled version of the modulated signal as input. Each block's Samples per symbol parameter, S, is the upsampling factor in both cases. Depending on whether the signal is frame-based or sample-based, the block either changes the signal's vector size or its sample time, as the table below indicates. Only the OQPSK blocks deviate from the information in the table, in that S is replaced by 2S in the scaling factors. Processing of Upsampled Modulated Data (Except OQPSK Method) Computation Type Modulation Input Frame Status Frame-based Result Output vector length is S times the number of integers or binary words in the input vector. Output sample time is 1/S times the input sample time. Modulation Sample-based 1-106 Digital Modulation Processing of Upsampled Modulated Data (Except OQPSK Method) (Continued) Computation Type Demodulation Input Frame Status Frame-based Result Number of integers or binary words in the output vector is 1/S times the number of samples in the input vector. Furthermore, if S > 1 and the demodulator is from the AM, PM, or FM sublibrary, the demodulated signal is delayed by one output sample period. There is no delay if S = 1 or if the demodulator is from the CPM sublibrary. Demodulation Sample-based Illustrations of Size or Rate Changes The following schematics illustrate how a modulator (other than OQPSK) upsamples a triplet of frame-based and sample-based integers. In both cases, the Samples per symbol parameter is 2. 1-107 1 Using the Libraries Frame-Based Upsampling 1 0 4 2 1 -1 -1 j j Frame-based column vectors in input and output Sample-Based Upsampling Output sample time = Input sample time Output length = 2 x (number of input integers) Modulator 2 4 0 Modulator j j t=2 -1 -1 t=1 1 1 t=0 t=2 t=1 t=0 Sample-based scalars in input and output t=1/2 Output sample time = (Input sample time)/2 Output length = number of input integers The following schematics illustrate how a demodulator (other than OQPSK or one from the CPM sublibrary) processes three doubly sampled symbols using both frame-based and sample-based inputs. [. . . ] For synchronization purposes, the Upsample block oversamples the signal by a factor of 4. The Digital Filter block provides a GMSK pulse linearization, the main component in a Laurent decomposition of the GMSK modulation [3]. A helper function computes the filter coefficients and uses a direct-form FIR digital filter to create the pulse shaping effect. This block adds rotation to the signal, simulating a defect in the transmitter under test. [. . . ]