Chapter 9 Modulation and Demodulation

8.1 Introduction

1) Modulation and demodulation

2) Modulation types

Analog: AM, FM

Digital: ASK, FSK, PSK, BPSK, QPSK, 8-PSK, 16-PSK, MSK, QAM

9.2 Amplitude Modulation and Demodulation

9.3 Frequency Modulation

Modulation: Use a VCO

Demodulation: Use a discriminator

9.4 Digital Shift-Keying Modulation

ASK:

MSK, BPSK:

IQ modulator:

9.5 Bit Error Rate and Bandwidth Efficiency

9.6 Sampling and Pulse Code Modulation

1) Sampling

: Nyquist sampling theorem

Sampling rate = signaling rate: number of samples per second

PCM bandwidth: f_s/2

1Hz band can accommodate 2 samples per second

PCM bandwidth = channel bandwidth

Symbol rate

Shape of the pulse to represent the symbols (line code): the bits are represented by Sinc pulses of duration Tb.

2) Pulse code modulation

Quantizing = Rounding-off to the nearest discrete value

Encoder = Conversion of a quantized value into a digital form (analog-to-digital conversion)

N: number of pulses to represent a sampled value

Source coding

Linear pulse-code modulation (LPCM) is a specific type of PCM where the quantization levels are linearly uniform. This is in contrast to PCM encodings where quantization levels vary as a function of amplitude (as with the A-law algorithm or the μ-law algorithm). Though PCM is a more general term, it is often used to describe data encoded as LPCM.

A PCM stream has two basic properties that determine the stream's fidelity to the original analog signal: the sampling rate, which is the number of times per second that samples are taken; and the bit depth, which determines the number of possible digital values that can be used to represent each sample.

Some forms of PCM combine signal processing with coding. Older versions of these systems applied the processing in the analog domain as part of the analog-to-digital process; newer implementations do so in the digital domain. These simple techniques have been largely rendered obsolete by modern transform-based audio compression techniques.

Linear PCM (LPCM) is PCM with linear quantization.

DPCM encodes the PCM values as differences between the current and the predicted value. An algorithm predicts the next sample based on the previous samples, and the encoder stores only the difference between this prediction and the actual value. If the prediction is reasonable, fewer bits can be used to represent the same information. For audio, this type of encoding reduces the number of bits required per sample by about 25% compared to PCM.

Adaptive DPCM (ADPCM) is a variant of DPCM that varies the size of the quantization step, to allow further reduction of the required bandwidth for a given signal-to-noise ratio.

Delta modulation is a form of DPCM which uses one bit per sample.

Encoding for serial transmission (= Line coding); related = T-carrier, E-carrier

PCM can be either return-to-zero (RZ) or non-return-to-zero (NRZ). For a NRZ system to be synchronized using in-band information, there must not be long sequences of identical symbols, such as ones or zeroes. For binary PCM systems, the density of 1-symbols is called ones-density.^[30]

Ones-density is often controlled using precoding techniques such as Run Length Limited encoding, where the PCM code is expanded into a slightly longer code with a guaranteed bound on ones-density before modulation into the channel. In other cases, extra framing bits are added into the stream which guarantee at least occasional symbol transitions.

Another technique used to control ones-density is the use of a scrambler polynomial on the raw data which will tend to turn the raw data stream into a stream that looks pseudo-random, but where the raw stream can be recovered exactly by reversing the effect of the polynomial. In this case, long runs of zeroes or ones are still possible on the output, but are considered unlikely enough to be within normal engineering tolerance.

In other cases, the long term DC value of the modulated signal is important, as building up a DC offset will tend to bias detector circuits out of their operating range. In this case special measures are taken to keep a count of the cumulative DC offset, and to modify the codes if necessary to make the DC offset always tend back to zero.

Many of these codes are bipolar codes, where the pulses can be positive, negative or absent. In the typical alternate mark inversion code, non-zero pulses alternate between being positive and negative. These rules may be violated to generate special symbols used for framing or other special purposes.

9.7 Waveform Design for the State-of-Art Digital Mobile Communication

2) 5G Waveforms

- OFDM (orthogonal frequency division multiplexing)

- FBMC (filter bank multi-carrier)

- UFMC (universal filtered multi-carrier modulation)

- GFDM (generalized frequency division multiplexing)

- F-OFDM (filtered OFDM)

- CP-OFDM (cyclic-prefix OFDM)

- CP-DFT-s-OFDM (cyclic-prefix discrete Fourier transform spread OFDM)

- ZT-DFT-s-OFDM (zero-tail DFT-s-OFDM)

- UW-DFT-s-OFDM (unique word DFT-s-OFDM)