Designing RF Communication Systems Using SDRs with GNU Radio

Modulation and demodulation drive wireless communication in SDRs, converting microphone voice into baseband, shifting to a carrier to create a passband, then restoring it at the receiver.

Explore analog and digital modulation by how a message modulates a carrier's amplitude, frequency, or phase, covering amplitude, frequency, and phase modulation as the main techniques.

Explain how modulation shifts the center frequency from zero to a carrier, enabling smaller antennas. Demonstrate frequency division multiplexing, enabling multiple signals to share bandwidth with distinct carrier frequencies.

Explore amplitude modulation by multiplying the message with a carrier to produce an am signal, revealing a spectrum centered at the carrier and a bandwidth doubled relative to the baseband.

Demodulate amplitude modulated signal by multiplying with transmitter's carrier, use cos^2(ωt) to obtain the baseband term, and recover the original message centered at zero hertz with a low pass filter.

Recover the original carrier frequency and phase in amplitude modulated demodulation using a phase-locked loop, exemplified by the Costas loop, implemented in hardware or software.

Learn how quadrature amplitude modulation carries in-phase and quadrature components in the same bandwidth using cosine and sine carriers, with demodulation recovering i and q by orthogonality and low-pass filters.

Learn binary amplitude shift keying: map bits to symbols, modulate with cos(ωc t) for on-off keying, and view the I-axis constellation; zero symbol has no energy.

Explore amplitude shift keying 4-ASK digital modulation, mapping four symbols to four carrier levels and multiplying by the carrier to form constellation points along the I axis.

Demodulate the 4-ASK signal by multiplying with the carrier, applying a low-pass filter to recover baseband symbols, then sampling and thresholding to decode the transmitted bits.

Encode information with phase shift keying, where data resides in the carrier phase. Binary phase shift keying uses 0 and 180 degree phase, producing modulated samples and a real-axis constellation.

Demonstrate BPSK demodulation by mixing with the carrier, applying a low-pass filter, sampling baseband symbols, and thresholding the two-point constellation to recover transmitted bits.

Explore M-PSK phase shift keying, where M symbols encode log2(M) bits per symbol using gray coding to minimize bit errors, with equal-energy constellation on a circle.

The lecture explains qpsk with four symbols by encoding two bits into i and q bpsk components, modulating i with cos(ωt) and q with minus sin(ωt), then summing.

Demodulate a QPSK signal with two BPSK demodulators for the I and Q components, using carrier multiplication, low-pass filtering, and thresholding to recover bits.

Explore modulation with differential encoding for RF communication, measuring symbol phase relative to the previous symbol, using a differential encoder/decoder to tolerate 180-degree shifts, with tradeoffs like error propagation.

demonstrates quadrature amplitude modulation by varying amplitude and phase to form mqam signals, and shows 16-qam generation from two square-root-M like signals with i and q level mapping.

Generate 16-qam signals by using two ESK modulators to map input bits to IESK and QESK levels, modulate with a cosine carrier, and sum the results to form 16-qam symbol.

Demonstrates 16-QAM demodulation using two demodulators for the I and Q signals, handling noise, mixing with a carrier, low-pass filtering, threshold decisions, and bit mapping to 1101.

Demonstrate generating psk via qam by adjusting the i and q components to keep amplitude constant while varying phase, with constellation points forming a circle as psk emerges.

Generate amplitude shift keying using quadrature amplitude modulation by setting the Q component to zero and varying the I component to create different constellation points.

Quadrature amplitude modulation (QAM) uses the I and Q to create constellation points with distinct phase and amplitude, enabling SDRs to generate digital and analog modulation schemes.

Explore analog frequency and phase modulation in SDRs via PSS and QAM. Vary I and Q to make theta_T follow message for phase modulation, or its integral for frequency modulation.

Represent constellation points as complex numbers with real (I) and imaginary (Q) parts, illustrating mappings such as 2 plus j, minus 2 plus j, and so on.

Explore the SCR transmitter block, the rf front end, where baseband I/Q symbols are converted to analog, filtered, modulated onto cosine and sine carriers, amplified, and transmitted through the antenna.

Explore the sdr receiver block diagram where a received symbol is amplified, demodulated in the I and Q branches, filtered, digitized, and decoded into bits by the radio.

Set up shows a pc running radio connected to the sdr with ethernet or usb, with baseband processing in radio and sdr acting as rf transceiver handling complex samples.

Explore the National Instruments USRP hardware block diagram. Trace I and Q flows, 100 mega samples per second analog to digital converter, digital down conversion, and 40 MHz low-pass filtering.

Explore the simplified rtl-sdr block diagram for reception only, detailing the antenna, LNA, filter, mixer to intermediate frequency, a five megahertz bandwidth filter, and i/q demodulation with a USB interface.

Explore how GNU radio provides real-time signal processing on a PC, handling I and Q data for transmit and receive modes, with Python wrappers for C++ blocks.

Represent radio applications as GNU Radio flow diagrams, using Python scripts or the radio companion, connecting sources (SDR, microphone, files) to sinks (SDR, speaker, files) through signal processing blocks.

Learn to install the new radio on Linux via the repository method—adding the repository, updating, and installing the radio and development tools—and consult the resources for Windows or source installs.

Map bits to eye levels and form complex symbols in GNU Radio for BPSK, using I and Q components with thresholds to demodulate zero or one.

Generate bpsk samples with a gnu radio flow diagram by defining variables, creating random bits, throttling, converting bytes to floats, forming i and q, and using constellation and threshold blocks.

Generate QPSK complex symbols in GNU Radio by mapping two bits per sample to I and Q levels with gray coding, then transmit the symbols and add channel noise.

Generate qpsk constellation samples in GNU Radio by streaming i bits (0110) and q bits (0011), mapping to i and q levels, and adding gaussian noise to visualize the constellation.

Learn how to prepare the SDR hardware by upsampling I and Q samples, mitigating multipath-induced frequency distortion with pulse shaping and an equalizer, and recovering clock, frequency, and phase.

Explore upsampling of IQ symbols for a BPSC signal with only the I component, using an upsampling factor of four, generating impulses, and smoothing via a DAC before SDR modulation.

Illustrates how multipath in wireless channels causes intersymbol interference with direct and reflected paths, creating delayed echoes and frequency-selective distortion that lead to bit errors.

Explore pulse shaping to combat intersymbol interference from multipath by comparing sync pulses and raised cosine pulses, and learn how adjusting the roll-off factor r trades bandwidth for ISI reduction.

Explains how transmitter and receiver use root raised cosine pulse shaping with matched filters to produce raised cosine bandwidth for BPSK, followed by sampling and threshold demodulation.

Explore PSK modulation in GNU Radio by selecting constellation modulator and constellation object, configuring QPSK and upsampling four samples per symbol with 0.35 excess bandwidth for pulse shaping.

Demonstrate PSK modulation in GNU Radio with fourfold upsampling and pulse shaping using root raised cosine filters, yielding a four-point constellation and reduced intersymbol interference.

Simulate wireless channels in GNU Radio with the channel model block, adjusting noise voltage, frequency offset, and epsilon to model multipath taps and real and imaginary parts for each tap.

Simulate a wireless channel in GNU Radio using the channel model and qtgui range blocks to adjust timing and frequency offsets, noise, and six-tap multipath affecting the constellation.

Explains polyphase clock synchronization block in the receiver, delivering clock recovery to align transmitter and receiver clocks, root raised cosine filtering, and down sampling to align timing and sampling rate.

Explore the polyphase clock synchronization block in GNU Radio, its root raised cosine filter that removes intersymbol interference, and how channel noise and offsets affect clock recovery and synchronization.

The receiver uses an equalizer to invert the channel's frequency response and counter intersymbol interference; training-based equalizers use a known training sequence, while blind equalizers rely on received samples.

Explore linear equalizers with CMA and LMS, and configure the adaptive algorithm to select the equalizer type, creating the adaptive algorithm object with an ID for the linear equalizer.

Explore the constant modulus algorithm blind equalizer, a radio block for constant-amplitude signals, that uses tunable taps to mitigate intersymbol interference while balancing computational overhead.

Choose the constant modulus algorithm and transmitter constellation for CMA, then tune the step size, modulus, and upsampling to enable the blind CMA equalizer against six multipath interference.

Describe the least mean square decision directed equalizer, a blind receiver that removes intersymbol interference without training sequences by using known constellation information, with limitations at low signal-to-noise ratio.

The LMS equaliser in GNU Radio acts as a blind, decision-directed compensator that removes intersymbol interference in a six multipath channel, using a 0.005 step size and PSK modulation.

Master carrier recovery by synchronizing phase and frequency between transmitter and receiver with the Costas loop, adjusting loop bandwidth and loop order for bpsc, upsc, or ppsc signals.

Demonstrates a GNU Radio Costas loop for carrier recovery, analyzing a four-symbol modulation with a six-path channel, tuning loop bandwidth and sliders for frequency offset and timing error.

Explore packet transmission over radio using the tag stream block method in GNU Radio, detailing preamble, header, payload, and the CRC check block for error detection.

Learn to convert a file-based byte stream into tagged packets using the stream to tag stream block, configuring item size, vector length, and the packet length tag key.

Feed packets with length tags into the protocol formatter, which generates the header and preamble, then the tag stream max block multiplexes them with the packet.

Explore the constellation modulator block in GNU Radio, specify upsampling factor four, excess bandwidth, differential encoding, and define constellation types and points for baseband modulation.

Describe how a receiver uses a constellation decoder to map symbols to bits, removes preamble and header, unpacks two bits per byte, and repacks bits to reconstruct the payload.

Simulate end to end packet transmission and reception in a GNU Radio flow graph, from test.txt to decoded text through preamble, header, modulation (bpsk and gpsc), channel effects, and synchronization.

Demonstrates real-time wireless transmission using GNU Radio and SDR hardware, converting data to baseband I and Q symbols, transmitting with Lime SDR and receiving with RTL SDR.

Demonstrates end-to-end packet transmission and reception with GNU Radio, LimeSDR, and RTL-SDR. Uses 0.5 gain scaling before LimeSDR to prevent saturation, and implements constellation visualization, clock/phase synchronization, equalization, and decoding.

A 180-degree phase ambiguity at the receiver rotates the constellation, causing bit inversion and no data output until the costas loop is corrected; the next lecture addresses the solution.