3.2 Modulation techniques and signal propagation

Modulation techniques are crucial for wireless communication. They shape how data is encoded onto carrier signals for transmission. From simple AM radio to complex digital schemes like QAM, each method has unique strengths and applications in modern wireless systems.

Signal propagation affects how wireless signals travel through the environment. Factors like multipath, Doppler effect, and path loss impact signal quality and range. Understanding these effects is key to designing robust wireless networks and overcoming transmission challenges.

Analog Modulation Techniques

Amplitude Modulation (AM)

• AM varies the amplitude of the carrier signal in proportion to the message signal
• Commonly used in AM radio broadcasting (medium wave and shortwave)
• Susceptible to noise and interference due to the modulation technique
• Requires a larger bandwidth compared to FM for transmitting the same information
• Simple to implement and demodulate using envelope detection

Frequency and Phase Modulation (FM and PM)

• FM varies the frequency of the carrier signal in proportion to the message signal
• PM varies the phase of the carrier signal in proportion to the message signal
• FM is widely used in FM radio broadcasting (VHF band) and provides better noise immunity than AM
• PM is less common but can be used in certain applications like satellite communication
• Both FM and PM require a wider bandwidth than AM but offer improved signal-to-noise ratio (SNR)
• FM and PM can be demodulated using discriminators or phase-locked loops (PLLs)

Digital Modulation Techniques

Quadrature Amplitude Modulation (QAM)

• QAM combines both amplitude and phase modulation to encode digital data
• Widely used in high-speed data transmission systems like cable modems and wireless networks (Wi-Fi, LTE)
• Enables higher data rates by encoding multiple bits per symbol
• Common QAM schemes include 16-QAM, 64-QAM, and 256-QAM
• Requires a good SNR to maintain low bit error rates (BER)

Frequency-Shift Keying (FSK) and Phase-Shift Keying (PSK)

• FSK represents digital data by shifting the frequency of the carrier signal between two or more discrete values
• PSK represents digital data by shifting the phase of the carrier signal between two or more discrete values
• Binary FSK (BFSK) and binary PSK (BPSK) are the simplest forms, encoding one bit per symbol
• More advanced schemes like quadrature PSK (QPSK) and 8-PSK can encode multiple bits per symbol
• FSK and PSK are used in various applications such as Bluetooth, RFID, and satellite communication

Spread Spectrum Techniques

• Spread spectrum techniques spread the signal energy over a wider bandwidth to improve noise immunity and security
• Two main types: frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS)
• FHSS rapidly switches the carrier frequency among many channels according to a pseudorandom sequence
• DSSS multiplies the data signal with a higher-rate pseudorandom noise (PN) code before modulation
• Used in applications like Wi-Fi (802.11), Bluetooth, and GPS for improved performance and multiple access

Signal Propagation Effects

Multipath Propagation

• Multipath propagation occurs when a signal reaches the receiver through multiple paths due to reflections and scattering
• Causes constructive and destructive interference, leading to signal fading and distortion
• Can result in intersymbol interference (ISI) in digital communication systems
• Mitigated using techniques like equalization, diversity reception, and OFDM (Orthogonal Frequency-Division Multiplexing)
• Multipath effects are more prominent in urban environments with many obstacles (buildings, trees)

Doppler Effect and Free Space Path Loss

• The Doppler effect is the change in frequency of a signal observed when the transmitter and receiver are in relative motion
• Causes frequency shifts and can lead to synchronization issues in communication systems
• Compensated using techniques like frequency offset estimation and correction
• Free space path loss (FSPL) is the attenuation of a signal as it propagates through free space
• FSPL depends on the distance between the transmitter and receiver and the signal frequency
• Calculated using the Friis transmission equation: $FSPL = (\frac{4\pi d}{\lambda})^2$, where $d$ is the distance and $\lambda$ is the wavelength
• Higher frequencies experience more path loss, limiting the range of wireless systems