Radar Waveform Design

Design and select radar waveforms using the Phased Array System Toolbox. Use the decision tree to select the correct waveform object based on requirements, follow correct function-to-object pairings, and avoid common mistakes.

When to Use

Primary keywords (any of these alone can trigger the skill): waveform, signal, chirp, LFM, NLFM, FMCW, PMCW, pulse compression, transmit signal, M-sequence, MLS, PN code, spread spectrum, phase code, joint radar-communication, dual-function waveform

Sensing-context words (confirm sensing domain when paired with primary keywords): target, jammer, jamming, pulse, pulses, spectrum, sidelobe, Doppler, range resolution, detection, clutter, matched filter, ambiguity, sweep, PRF, PRI

Trigger logic:

• Primary keyword + sensing-context word → use this skill directly
• Primary keyword alone → ask: sensing or communications? (communications → skill does not apply)
• Sensing-context words + vague language ("a signal that changes each time") → use this skill

After triggering, clarify purpose and scope:

1. Purpose — application-driven or exploration/learning?

   • Exploration/learning (student, paper reproduction, comparing properties): proceed with given parameters, suggest radarWaveformGenerator. Do not push for application context.
   • Application-driven → clarify dimensions below.

2. Application dimensions (ask what's unknown — applies to any sensing application): Range scale | Target motion and velocity | Resolution need | Environment (clutter, jamming, interference) | Hardware limits (ADC, duty cycle) | Primary metric (detection, resolution, accuracy, or ambiguity-free)

3. Requirements — Derive waveform parameters from the answers above (see table below)

When NOT to Use

• Full radar system simulation (transmitter → channel → receiver chain)
• Beamforming or array design (use phased array skills)
• Target detection / CFAR processing
• Simulink waveform generation blocks
• Communications waveforms (OFDM, QAM, etc.)

Escalation / Boundary Conditions

Do not answer as if waveform choice alone solves:

• Range/velocity ambiguity resolution strategy (staggered PRF scheduling, medium-PRF processing)
• Tracker-level Doppler/range association
• Detailed receiver chain design (noise figure, dynamic range budgets)
• Clutter suppression design (MTI, STAP)
• Antenna/array pattern issues
• CFAR or detector performance questions

Instead, explain that the issue is system-level and identify what waveform-related part can still be addressed.

Workflow

1. Clarify requirements — Gather what the user hasn't specified (see table below)
2. Select waveform object — Use the decision tree below
3. Configure the waveform — Set properties based on requirements
4. Analyze — Use appropriate analysis function (ambgfun, pambgfun, sidelobelevel)
5. Suggest interactive exploration — Recommend radarWaveformGenerator app

Construction Rule (non-negotiable)

Always generate signals using toolbox System objects and functions — never manually construct exp(1j*...) or write custom LFSR/sequence generators. If mlseq, legendreseq, phased.LinearFMWaveform, or another toolbox function can produce what you need, use it. Manual construction is only acceptable when no toolbox equivalent exists for the specific operation (e.g., applying element-wise phase modulation to an existing waveform vector). For hybrid waveforms (e.g., LFM + communication encoding), generate the base with the appropriate object, apply the modification, then wrap the result with PhaseCodedWaveform using Code='Custom'. If no built-in object fits (e.g., exact DFT-bin nulls, non-contiguous bands), escalate to custom IQ synthesis but keep toolbox functions for parameter derivation, wrapping, and analysis. Always state which parts are toolbox-based and which are custom.

Analysis Rule (non-negotiable)

Never use xcorr for radar waveform autocorrelation or matched filter response visualization. Instead:

• Matched filter response / autocorrelation: Use ambgfun with 'Cut','Doppler' (zero-Doppler cut = response vs delay).
• PSL measurement: Use sidelobelevel on the dB-converted cut.
• Doppler tolerance: Use ambgfun with 'Cut','Delay' (zero-delay cut = response vs Doppler).
• Downstream signal processing: Use phased.MatchedFilter when you need the actual filtered signal (range processing, bit recovery, etc.).

Agent Reasoning Policy

• Follow the clarification flow above — do not skip to code without sufficient context.
• User names a waveform family → treat as a constraint; configure and analyze it. Only override if requirements are impossible with that family.
• User gives only performance constraints → choose the simplest family that satisfies them and explain why.
• Performance requirements given → derive parameters before selecting objects.
• Waveform family given (e.g., "NLFM") → ask about the goal (sidelobes, spectral shaping, Doppler tolerance, hardware).
• Exploration/learning → help directly with given parameters. Still use toolbox objects — generate base waveforms via System objects, use mlseq/legendreseq/apaseq for sequences, wrap custom results in PhaseCodedWaveform with Custom code. Do not write manual signal construction. Suggest radarWaveformGenerator for interactive exploration.
• Application context only (no numeric requirements) → recommend the waveform family/object and explain why. If the domain has well-known defaults (e.g., automotive radar at 77 GHz), state assumptions and proceed. Otherwise ask the application dimensions. Do NOT silently invent parameters without stating them.
• Conflicting requirements → surface the conflict before proposing a waveform.
• "Best waveform" → explain it depends on resolution, ambiguity, sidelobes, Doppler, hardware, and processing.
• User states MATLAB release → check function availability; note radarWaveformGenerator requires R2026a.
• No debugging loops. If code errors or results don't match expectations, retry at most once with a targeted fix. If the second attempt fails, stop and report what went wrong, what you tried, and ask the user whether to adjust requirements, relax constraints, or provide additional information. Do not iterate beyond 2 attempts.
• Write complete scripts, not incremental snippets. Do not develop code through many small evaluate_matlab_code calls. Instead, design the full script, save it to a .m file, and run it once with run_matlab_file. To save the file: use the Write tool if available, otherwise use evaluate_matlab_code with MATLAB's writelines or fopen/fprintf/fclose. Never use Bash heredocs for MATLAB code — single quotes and format strings (%d, \n) break shell quoting. If you are uncertain about an API or parameter and find yourself wanting to "try things" in MATLAB, that is a signal to stop and ask the user for clarification rather than exploring interactively. Reserve evaluate_matlab_code for at most: (1) one setup/cleanup call, (2) saving and running the script, and (3) one retry if needed.
• PSL trade-off checkpoint. When PSL target is between -30 and -45 dB: calculate TBP. If TBP < 500, present the trade-off BEFORE generating code: (a) NLFM — no SNR loss but PSL limited by stationary-phase approximation at this TBP; (b) LFM + time-domain windowed matched filter — guarantees target PSL at any TBP but costs ~3-4 dB SNR; (c) increase TBP to enable NLFM. Let the user choose before proceeding.

Requirements to Clarify

Once the application dimensions are known, check for these specific gaps:

Requirement-to-Recommendation Heuristics

• Doppler tolerance priority → LFM-style solutions
• Low sidelobes without SNR loss → NLFM (sufficient TBP required)
• Spectral notching → amplitude-only bandstop for structured waveforms; shapespectrum only for PRO-FM. See references/spectral-notching.md
• Custom frequency profile for sidelobes → nlfmspec2freq + CustomFMWaveform
• Hardware BW limited → stretch processing or stepped FM
• External IQ → Custom IQ pattern (PhaseCodedWaveform with Code='Custom')
• CW/periodic analysis → pambgfun (not ambgfun)

See references/waveform-objects.md for the family summary table (strengths/tradeoffs).

Waveform Selection Decision Tree

Is the waveform continuous (CW)?
├── Yes: Does the user need linear FM sweep?
│   ├── Yes: Multiple targets where ghost targets are a concern?
│   │   ├── Yes → phased.MFSKWaveform (resolves range+speed without ghosts)
│   │   └── No → phased.FMCWWaveform (triangle sweep for range+speed)
│   └── No: Does the user need multiple frequency steps?
│       ├── Yes → phased.MFSKWaveform
│       └── No: No dedicated CW object for desired modulation?
│           └── Use pulsed object with PRF = 1/PulseWidth (see CW Pattern below)
│               ├── Nonlinear FM → phased.NonlinearFMWaveform or phased.CustomFMWaveform
│               └── Phase-coded → phased.PhaseCodedWaveform
│
└── No (pulsed): What modulation?
    ├── None (simple pulse) → phased.RectangularWaveform
    ├── Linear FM → phased.LinearFMWaveform
    ├── Nonlinear FM (built-in type) → phased.NonlinearFMWaveform
    │   (4 types: Polynomial, Hyperbolic, Hybrid Linear-Tangent, Stepped Price)
    ├── Nonlinear FM (custom shape) → phased.CustomFMWaveform
    │   (use with nlfmspec2freq for stationary-phase design)
    ├── Phase-coded → phased.PhaseCodedWaveform
    ├── Stepped frequency → phased.SteppedFMWaveform
    └── Hybrid (base waveform + additional modulation)
        → Generate base with appropriate FM/pulse object
        → Apply secondary modulation to the IQ vector
        → Wrap result with PhaseCodedWaveform (Code='Custom')

Key Functions

Patterns

Parameter Derivation from Requirements

Derive all waveform parameters from system requirements — never hardcode.

fc = 10e9;                              % carrier frequency
rangeRes = 20;                          % required range resolution (m)
maxRange = 80e3;                        % max unambiguous range (m)
maxVel = 300;                           % max target velocity (m/s)

lambda = freq2wavelen(fc);
bw = rangeres2bw(rangeRes);             % bandwidth from range resolution
c = physconst('LightSpeed');
prfMax = c / (2 * maxRange);            % max PRF from range ambiguity
fdMax = 2 * speed2dop(maxVel, lambda);  % TWO-WAY Doppler (speed2dop is one-way)
tbp = 50;                               % time-bandwidth product
pw = tbp / bw;                          % pulse width from TBP

PRF conflict: If fdMax > prfMax, the velocity requirement demands a higher PRF than the range requirement allows — no single PRF satisfies both. Do NOT proceed with a single-PRF design. Present the conflict, explain the trade-off (range vs velocity), and recommend staggered PRF or medium-PRF with ambiguity resolution. Ask which strategy the user prefers before generating any waveform code. Waveform objects accept PRF as a vector for staggered operation, but the ambiguity resolution strategy is a system-level concern — see Escalation / Boundary Conditions.

Before generating code, verify:

• PulseWidth <= 1/PRF (pulse fits within PRI)
• SampleRate / PRF is integer (integer samples per PRI)
• For phase-coded: SampleRate * ChipWidth is integer
• Sample rate: fs = ceil(8*bw / prf) * prf guarantees integer samples/PRI + oversampling

Mainlobe broadening compensation: When using tapering (windowed MF or NLFM): bwComp = rangeres2bw(rangeRes, 'RangeBroadening', broadeningFactor) (Taylor ~1.3×).

NLFM Design via Stationary Phase

Use nlfmspec2freq to compute the frequency profile from a desired spectrum shape, then feed it to phased.CustomFMWaveform. Do NOT use with phased.NonlinearFMWaveform (which has fixed built-in types only).

% Design NLFM waveform with low sidelobes
bw = 5e6;
nSamples = 500;
desiredSpectrum = taylorwin(nSamples, 4, -40);
freq = nlfmspec2freq(bw, desiredSpectrum);

wav = phased.CustomFMWaveform( ...
    'PulseWidth', 10e-6, ...
    'SampleRate', 10e6, ...
    'FrequencyModulation', freq);

TBP-PSL trade-off: The stationary-phase approximation introduces Fresnel ripples that limit achievable PSL. The design spectrum (e.g., Taylor window) sets an upper bound, but actual PSL is always worse than the design SLL — the gap narrows as TBP increases. At low TBP (< 100), expect PSL significantly above the design target; at high TBP (200+), PSL approaches the design SLL.

If the achieved PSL doesn't meet the target, do not iterate on window parameters or design tweaks — the gap is a fundamental limitation of the stationary-phase method at that TBP. Instead, present the trade-off options:

• Increase pulse width or bandwidth to raise TBP
• Switch to a windowed matched filter (achieves target PSL at any TBP, at the cost of SNR loss that grows with the target SLL)
• Accept the achieved PSL if it meets system needs

CW from Pulsed Objects (100% Duty Cycle)

When no dedicated CW object exists for your modulation type, use a pulsed waveform object with PRF = 1/PulseWidth so the pulse fills the entire PRI.

Do NOT use DutyCycle = 1 — this errors on all pulsed waveform objects. Instead, set PRF equal to the reciprocal of the pulse width.

% Phase-coded CW using Legendre sequence
seq = legendreseq(127);
chipWidth = 1e-6;
prf = 1/(numel(seq) * chipWidth);  % 100% duty cycle
wav = phased.PhaseCodedWaveform( ...
    'Code', 'Custom', ...
    'CustomCode', seq, ...   % Do NOT set NumChips — inferred from vector length
    'ChipWidth', chipWidth, ...
    'PRF', prf, ...
    'SampleRate', 10e6);

IMPORTANT: Never set NumChips when Code='Custom'. The chip count is inferred from the CustomCode vector length. Setting NumChips explicitly produces a warning and may cause unexpected behavior.

PMCW (Phase-Modulated Continuous Wave): PMCW = phase-coded CW with 100% duty cycle. Configure as PRF = 1/(numel(code) * ChipWidth). If the user says "PMCW", treat as CW — PRI in PMCW context means code repetition interval, not pulsed transmission with dead time.

Custom IQ as Waveform Object (external data OR constructed hybrids)

Use phased.PhaseCodedWaveform with Code='Custom' to wrap ANY complex IQ vector into the toolbox ecosystem — whether captured from hardware, loaded from a file, or constructed by combining/modifying other waveforms (e.g., LFM with embedded communication phase modulation). Despite the name, CustomCode accepts any complex-valued vector — not just phase values. Do not set NumChips when using Custom code — it is inferred from the vector length.

% Example: LFM + communication phase encoding as a toolbox object
wfLFM = phased.LinearFMWaveform('SampleRate', fs, 'PulseWidth', pw, ...
    'SweepBandwidth', bw, 'PRF', prf);
xLfm = wfLFM();
xHybrid = xLfm(1:nPulse) .* exp(1j * commPhaseVector);

wfHybrid = phased.PhaseCodedWaveform( ...
    'Code', 'Custom', ...
    'CustomCode', xHybrid, ...
    'ChipWidth', 1/fs, ...
    'SampleRate', fs, ...
    'PRF', prf);

% Now usable with toolbox processing
mfCoeffs = getMatchedFilter(wfHybrid);

When NOT to wrap: If you only need ambiguity analysis on IQ, pass it directly to ambgfun(iq, fs, prf) — no object needed. Use the wrapper only when you need toolbox integration (matched filter, range processing, etc.).

Sidelobe Reduction

Three approaches, depending on context:

1. Time-domain windowed matched filter (reliable, works with any waveform):

Apply a window directly to the matched filter coefficients. This reliably achieves the target PSL at any TBP, at the cost of mismatch loss (~3 dB for -35 dB target, ~4 dB for -40 dB). Use SampleRate >= 8 * bandwidth for accurate PSL measurement.

wav = phased.LinearFMWaveform('PulseWidth', 10e-6, 'SweepBandwidth', 5e6, ...
    'SampleRate', 40e6, 'PRF', 1e4);
mfCoeffs = getMatchedFilter(wav);
N = numel(mfCoeffs);
tWin = taylorwin(N, 4, -40);  % target -40 dB PSL
mf = phased.MatchedFilter('Coefficients', mfCoeffs .* tWin);

Note on SpectrumWindow property: The SpectrumWindow property of phased.MatchedFilter applies spectral weighting in the frequency domain. Testing indicates it achieves only ~50% of the specified attenuation in dB (e.g., Taylor-40 spec → ~-20 dB actual PSL). Use time-domain coefficient windowing when precise PSL control is required.

2. NLFM (inherent low sidelobes, no SNR loss):

Use the NLFM Design pattern above with nlfmspec2freq + CustomFMWaveform. No SNR loss unlike windowed matched filter. Mainlobe broadens (amount depends on the spectral taper shape). Requires sufficient TBP to approach target PSL.

3. Phase codes with good autocorrelation:

Use pnkcode for deep PSL (< -30 dB) at moderate lengths. legendreseq only achieves ~-17 dB PSL. See references/phase-code-reference.md for selection.

Ambiguity Function Analysis

Ambiguity shape drives waveform choice:

Do not assume waveform class guarantees thumbtack — phase-coded can still be Doppler sensitive; NLFM improves range sidelobes but may not decouple delay-Doppler. Always validate with ambgfun 2D cut.

Choose the right function: ambgfun for single pulses and pulse trains; pambgfun for CW/periodic waveforms (exploits periodicity for finer Doppler).

Critical: Pass the full PRI output to ambgfun (includes trailing zeros), not just the pulse portion — pulse-only will error:

sig = wf();          % full PRI — pass this to ambgfun

Cut direction (counterintuitive naming): The 'Cut' parameter specifies which variable is HELD CONSTANT, not which axis is returned:

Cuts return 2 outputs only — do not request 3 outputs with cuts.

See references/analysis-functions.md for detailed usage examples and plotting guidelines.

Stepped FM Processing

Processing stepped FM data requires coherent integration across frequency steps, not a single matched filter. Apply matched filter per step, then combine returns across steps (IFFT across the frequency dimension) to synthesize the full bandwidth and achieve the fine range resolution. The effective bandwidth is NumSteps × FrequencyStep. See references/waveform-objects.md for full code.

Stretch Processing (Wideband LFM)

When approximate target range is known, use stretch processing instead of matched filtering to avoid wideband ADC. Only available for LinearFMWaveform.

wav = phased.LinearFMWaveform('PulseWidth', 10e-6, 'SweepBandwidth', 100e6);
strproc = getStretchProcessor(wav, 5000, 200);  % refRange, rngSpan
slope = bw / pw;  % sweep slope for range conversion
rng = stretchfreq2rng(beatFreq, slope, refRange);

Polynomial NLFM Coefficient Convention

phased.NonlinearFMWaveform with Type='Polynomial': the polynomial defines instantaneous frequency, not phase. LFM = linear frequency → coefficients [0, 1, 0]. Do NOT use [1, 0, 0] — that gives quadratic frequency (not LFM). See references/waveform-objects.md for coefficient examples.

Receiver Architecture

Waveform-only PSL claims are incomplete — always state the receiver assumption:

Verification Outputs

Every waveform design script should report:

• Derived bandwidth and range resolution
• PRF, max unambiguous range, max two-way Doppler
• PSL (via sidelobelevel) and whether it meets the requirement
• Any SNR/mismatch loss from receive weighting
• Transmit spectrum plot (with mask if applicable)
• Zero-Doppler cut with target PSL reference line

Troubleshooting Patterns

• Custom code warnings → don't set NumChips (inferred from vector)
• Poor PSL → check SampleRate >= 8*bandwidth and dB input to sidelobelevel
• PRF/sample rate errors → ensure SampleRate/PRF is integer
• ambgfun size error → pass full PRI from wf(), not just pulse portion
• Wrong Doppler → use 2*speed2dop for two-way radar Doppler
• Phase-coded construction error → ensure SampleRate*ChipWidth is integer

See references/common-mistakes.md for the full catalog.

Multi-Step Reasoning Examples

• "Resolution > ADC bandwidth" → stretch processing (range known) or stepped FM (not)
• "CW NLFM" → CustomFMWaveform + PRF=1/PulseWidth + nlfmspec2freq → pambgfun
• "Deep PSL + Doppler tolerance" → LFM + time-domain windowed MF (~3-4 dB SNR cost)

Conventions

• Always use toolbox System objects and functions — never manual exp(1j*...) or custom sequence generators (see Construction Rule)
• Always recommend radarWaveformGenerator for interactive waveform exploration (requires R2026a)
• For phase-coded waveforms: clarify binary vs polyphase constraints before recommending a code (see references/phase-code-reference.md)
• When a sidelobe target is specified, add a yline(targetPSL, 'r--') on the zero-Doppler cut for visual pass/fail verification

References

• references/waveform-objects.md — All 8 waveform objects with properties and constraints
• references/phase-code-reference.md — Code types, binary vs polyphase, NumChips constraints
• references/analysis-functions.md — ambgfun, pambgfun, sidelobelevel usage details
• references/common-mistakes.md — Full catalog of common mistakes with corrections
• references/spectral-notching.md — Spectral notching for interference avoidance, shapespectrum API

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