Modulation

/ˌɛf ˈɛm/

noun … “hiding information in the twists of a wave’s frequency.”

FM, short for Frequency Modulation, is a method of encoding information onto a carrier wave by varying its frequency in proportion to the instantaneous amplitude of a message signal, while keeping the carrier’s amplitude and phase constant. Unlike AM, which conveys information through changes in amplitude, FM shifts the carrier frequency above and below its nominal value based on the input signal, creating a waveform that is inherently more resistant to noise and amplitude-based interference.

Technically, an FM signal can be expressed as V(t) = Ac * cos(2πfct + 2πkf ∫ m(τ)dτ), where Ac is the constant carrier amplitude, fc is the carrier frequency, kf is the frequency sensitivity of the modulator, and m(t) is the message signal. The integral of the message signal determines how far and how fast the instantaneous frequency of the carrier deviates, producing sidebands at multiples of the message frequency. Unlike AM, where power is split between the carrier and sidebands in a predictable ratio, FM distributes energy across a theoretically infinite number of sidebands, but in practice most of the signal power is concentrated within a bandwidth defined by Carson’s Rule, calculated as BW = 2(Δf + fm), where Δf is the peak frequency deviation and fm is the highest frequency in the message signal.

In real-world applications, FM is widely used for radio broadcasting, two-way communications, telemetry, and audio signal transmission where clarity and noise resistance are critical. A transmitter processes the audio or data signal through a frequency modulator, adjusting the carrier’s frequency proportionally. The receiver uses a frequency discriminator or phase-locked loop (PLL) to track these frequency variations and reconstruct the original signal accurately. Because noise generally affects amplitude rather than frequency, the demodulated FM signal maintains fidelity even in environments where AM would be severely degraded.

Conceptually, FM can be visualized as a moving car on a road where speed changes encode the message: the car represents the carrier wave, and speeding up or slowing down in precise patterns carries information. Unlike amplitude-based signaling, this method preserves meaning even if the car’s size or paint is distorted, analogous to FM signals resisting static and interference. This dynamic makes FM ideal for high-fidelity audio transmission and robust communication in noisy environments.

Further refinements such as Wideband FM and Narrowband FM allow engineers to optimize for bandwidth efficiency or audio quality, while digital techniques like Digital Mobile Radio implement frequency modulation principles for digital data. At its core, however, FM remains a modulation technique that prioritizes signal clarity and resilience by encoding information in the twists and turns of a wave’s frequency rather than its amplitude, turning frequency deviations into a reliable carrier of meaning across the airwaves.

/ˌkjuː piː ɛs keɪ/

noun … “quadrature phase shift keying.”

QPSK is a digital modulation technique that encodes two bits per symbol by varying the phase of a carrier wave among four discrete states, typically 0°, 90°, 180°, and 270°. By using four phases instead of the two in BPSK, QPSK doubles the data rate for the same bandwidth while maintaining robustness to noise and interference.

At a conceptual level, QPSK divides the carrier into two orthogonal components, called the in-phase (I) and quadrature (Q) channels. Each component carries one bit of information. The combination of I and Q produces four distinct phase positions on a constellation diagram, allowing the receiver to infer both bits from a single symbol. This “quadrature” arrangement is the source of the name.

QPSK is widely used in modern communication systems where efficiency and reliability must coexist. Applications include satellite links, cellular networks, Wi-Fi, and FEC-protected data streams. Pairing QPSK with forward error correction enhances performance in noisy channels, allowing higher throughput without sacrificing signal integrity.

Mathematically, a QPSK signal can be expressed as:

s(t) = I(t)·cos(2πf_ct) + Q(t)·sin(2πf_ct)

where I(t) and Q(t) are the in-phase and quadrature components representing bit values, and f_c is the carrier frequency. The orthogonal sine and cosine components ensure that the two bits are independent yet transmitted simultaneously, maximizing spectral efficiency.

Like other phase-based modulation schemes, QPSK interacts with concepts such as Signal-to-Noise Ratio, Bit Error Rate, and Modulation theory. Engineers analyze these factors to balance throughput, reliability, and bandwidth usage.

Intuitively, QPSK is like a four-direction compass for signals: each symbol points in one of four directions, each representing a unique two-bit combination. By combining these directional choices, data flows efficiently over the channel, providing a balance between speed and resilience that is critical in real-world communication systems.

/ˌbiː piː ɛs keɪ/

noun … “binary phase shift keying.”

BPSK is a digital modulation technique that encodes binary data by shifting the phase of a carrier wave between two discrete states. Each state represents one bit … typically a phase of 0 degrees for binary 1 and 180 degrees for binary 0. Because only two phases are used, BPSK is conceptually simple, mathematically elegant, and exceptionally robust in noisy environments.

At its core, BPSK belongs to the broader family of digital Modulation schemes, where information is embedded into a carrier signal by altering a physical property such as amplitude, frequency, or phase. In BPSK, the property being manipulated is phase, which makes the signal resilient against amplitude distortion and certain types of interference.

Mathematically, BPSK can be described as multiplying the carrier wave by either +1 or −1 depending on the bit value. This simple inversion causes a 180-degree phase shift. When visualized on a constellation diagram, BPSK occupies just two points directly opposite each other, which maximizes the distance between symbols. That distance matters … it directly influences how well a receiver can distinguish bits in the presence of noise.

This robustness is why BPSK performs well at low Signal-to-Noise Ratio values. Compared to more complex schemes, it achieves a lower Bit Error Rate under the same conditions. The tradeoff is spectral efficiency. Because each symbol carries only one bit, BPSK transmits data more slowly than higher-order schemes.

In practical systems, BPSK is often used in environments where reliability is more important than raw throughput. Satellite communications, deep-space probes, GPS signals, and early wireless standards all rely on BPSK or closely related variants. When signals travel vast distances or through hostile noise environments, simplicity becomes a survival trait.

BPSK also serves as the conceptual foundation for more advanced phase-based techniques. For example, QPSK extends the same idea by using four phase states instead of two, allowing two bits per symbol. Understanding BPSK makes these higher-order systems feel less like magic and more like logical extensions.

From a signal-processing perspective, BPSK analysis frequently involves tools such as the Fourier Transform, which helps engineers understand bandwidth usage and spectral characteristics. These mathematical lenses reveal why BPSK’s clean phase transitions produce predictable and well-contained frequency behavior.

Conceptually, BPSK is a lesson in restraint. By using the smallest possible symbol set, it minimizes ambiguity and maximizes certainty. It does not chase speed; it prioritizes clarity. In communication theory, that balance between simplicity and performance is not a compromise … it is a design philosophy.

Even in an era dominated by dense constellations and adaptive modulation, BPSK remains relevant. It is the digital equivalent of speaking slowly and clearly in a noisy room. Sometimes, two choices are exactly enough.

/ˌpiː ɛs ˈkeɪ/

n. "Carrier phase modulation encoding bits via discrete phase states unlike GFSK frequency modulation."

PSK, short for Phase Shift Keying, encodes data by discretely shifting carrier phase to represent symbols—BPSK toggles 0°/180° (1 bit/symbol), QPSK uses 0°/90°/180°/270° (2 bits), 8PSK/16APSK pack 3/4 bits via 8/16 phase states. Requires coherent demodulation with phase-locked carrier recovery unlike noncoherent GFSK, enabling higher spectral efficiency (2-6 bits/Hz) for satellite/cellular but sensitive to phase noise and demanding precise constellation tracking.

Key characteristics of PSK include: Constellation Mapping BPSK=2/QPSK=4/8PSK=8 points on unit circle; Coherent Demodulation PLL/Carrier Recovery tracks phase reference; Spectral Efficiency 2b/Hz QPSK vs 1b/Hz GFSK; Phase Noise Sensitivity requires oscillator phase noise <-90dBc/Hz; Differential Encoding π/4-DQPSK avoids absolute phase ambiguity.

Conceptual example of PSK usage:

/* QPSK modulator I/Q mapping */
typedef enum {
    PSK_00 = 0,  // 45°
    PSK_01 = 1,  // 135°  
    PSK_10 = 2,  // 225°
    PSK_11 = 3   // 315°
} psk_symbol_t;

const complex float qpsk_constellation = {
    0.707f + 0.707fj,  // 45°  (00)
   -0.707f + 0.707fj,  // 135° (01)
   -0.707f - 0.707fj,  // 225° (10)
    0.707f - 0.707fj   // 315° (11)
};

void qpsk_modulate(complex float *samples, uint8_t *bits, int num_symbols) {
    for (int i = 0; i < num_symbols; i++) {
        uint8_t dibit = (bits[i*2] << 1) | bits[i*2+1];
        samples[i] = qpsk_constellation[dibit];
    }
    
    // Raised cosine pulse shaping
    pulse_shape(samples, rc_filter, num_symbols);
}

Conceptually, PSK imprints data onto carrier phase via I/Q vector rotation—QPSK packs 2 bits per symbol doubling BPSK throughput while π/4-DQPSK rotates constellation each symbol preventing origin trajectory. Demodulators project onto I/Q axes making hard decisions amid AWGN; contrasts GFSK FM discrimination by demanding clean LO unlike constant-envelope Class-E PAs thriving on nonlinear distortion.

/ˌɛf eɪtʃ ɛs ɛs/

n. "Pseudo-random carrier frequency switching spreading narrowband signal across wide spectrum for Bluetooth anti-jamming."

FHSS, short for Frequency-Hopping Spread Spectrum, transmits data bursts across rapidly changing carrier frequencies (1600 hops/sec in Bluetooth) following a PN sequence known to TX/RX, minimizing narrowband interference impact since each hop duration (~625μs) sees only fraction of total energy. Contrasts WiFi DSSS by occupying full channel briefly vs spreading chip sequence continuously, enabling multiple piconets sharing 2.4GHz ISM band with low mutual interference.

Key characteristics of FHSS include: Pseudo-Random Hop Sequence generated via LFSR across 79 x 1MHz channels (Bluetooth Classic); Fast Hop Rate 1600 hops/sec (400ms period/79 channels); Adaptive FHSS blacklists interfered channels; Short Dwell Time 625μs/classic slot limits jammer effectiveness; CDMA Capability multiple transmitters share band via unique hop patterns.

Conceptual example of FHSS usage:

/* Bluetooth Classic FHSS hop sequence generator */
#define NUM_CHANNELS 79
#define HOP_PERIOD   625  // microseconds

uint8_t hop_sequence[NUM_CHANNELS];
uint16_t clock;  // 12.5ms native + 625us slots
uint8_t current_channel;

void generate_bluetooth_hop_seq(uint16_t clk_28) {
    // Native clock → 5-bit channel select via permutation
    uint8_t page = (clk_28 >> 7) & 0x1F;  // Page hop bits
    uint8_t inquiry = (clk_28 >> 3) & 0x1F; // Inquiry bits
    
    current_channel = permute(page ^ inquiry ^ clk_28);
    set_rf_channel(current_channel);  // Synth tune 2402 + ch MHz
}

void transmit_slot() {
    // 625us dwell: transmit GFSK packet
    transmit_gfsk_packet(data_buffer, 366);  // 366us max
    delay_us(625);  // Slot time
    generate_bluetooth_hop_seq(++clock);  // Next hop
}

Conceptually, FHSS evades interference by transmitting brief packets across wide spectrum—Bluetooth piconets hop pseudorandomly while AFH detects/avoid busy channels, ensuring reliable PAN links amid WiFi 2.4GHz congestion. Military origins (Hedy Lamarr torpedo guidance) evolved to civilian WPANs where hop synchrony via master clock maintains TDD timing; contrasts SerDes fixed carriers requiring CTLE equalization, with spectrum analyzers revealing FHSS as impulsive wideband noise versus OFDM comb lines.