spaxel/docs/research/01-csi-fundamentals.md

WiFi CSI Fundamentals

What CSI Is

Channel State Information (CSI) describes the propagation process of a wireless signal at the physical layer, with per-subcarrier granularity. It characterises how a signal is attenuated, phase-shifted, and dispersed by the environment as it travels from transmitter to receiver.

CSI is extracted from OFDM (Orthogonal Frequency Division Multiplexing) frames during the Long Training Field (LTF) preamble, which the receiver uses for channel equalisation. This same channel estimate — normally discarded after equalisation — is exposed via vendor APIs.

Mathematically, CSI is the Channel Frequency Response (CFR), expressed as a vector of complex numbers across subcarriers:

H = { H(f_j) | j ∈ [1, J] }
H(f_j) = |H(f_j)| · e^(j∠H(f_j))

where |H(f_j)| is amplitude and ∠H(f_j) is phase for subcarrier j.

---

CSI vs. RSSI

Property	RSSI	CSI
Granularity	Single scalar per packet	Complex vector (one value per subcarrier)
Frequency content	Aggregate bandwidth-averaged power	Per-subcarrier amplitude and phase
Multipath sensitivity	Cannot distinguish multipath components	Each subcarrier encodes a different multipath mix
Phase information	None	Full phase per subcarrier
Noise resilience	Single measurement; unreliable if faded	Other subcarriers remain informative when some fade
Temporal stability	Low (random fading)	Higher (multipath structure is more stable)

RSSI is the sum of power across all subcarriers, discarding all frequency-selective fading information. CSI exposes how amplitude and phase vary across frequency — this frequency-domain signature encodes geometry.

---

802.11n / HT20 Subcarrier Structure

Under 802.11n (Wi-Fi 4) with HT20 (20 MHz channel):

Parameter	Value
Total OFDM subcarriers	64 (indices −32 to +31)
Data subcarriers	52
Pilot subcarriers	4 (at indices −21, −7, +7, +21)
Null/guard subcarriers	8 (DC + edge guards)
Subcarrier spacing	312.5 kHz
Symbol duration	3.2 µs + 0.8 µs guard interval

For HT40 (40 MHz): 114 usable subcarriers (108 data + 6 pilots).

---

ESP32-S3 CSI API

Key Calls

// 1. Enable in menuconfig: Component Config → Wi-Fi → Wi-Fi CSI
// 2. Register callback
esp_wifi_set_csi_rx_cb(my_csi_callback, NULL);
// 3. Configure which LTF fields to capture
wifi_csi_config_t csi_cfg = {
    .lltf_en          = true,   // Legacy LTF (all modes)
    .htltf_en         = true,   // HT LTF (802.11n)
    .stbc_htltf2_en   = true,   // STBC HT LTF
    .channel_filter_en = false, // disable for independent subcarriers
    .manu_scale       = false,
    .shift            = 0,
};
esp_wifi_set_csi_config(&csi_cfg);
// 4. Enable
esp_wifi_set_csi(true);

wifi_csi_info_t Fields

Field	Type	Description
rx_ctrl	struct	RSSI, noise floor, channel BW, signal mode, STBC flag, timestamp, antenna index
mac	uint8[6]	Sender MAC address
len	uint16	Byte count of CSI buffer
buf	int8*	Raw CSI: (Im, Re) int8 pairs per subcarrier
first_word_invalid	bool	First 4 bytes may be invalid (hardware bug on original ESP32)

CSI Data Format

Each subcarrier is encoded as two consecutive signed 8-bit integers:

• buf[2k] = imaginary part (Im) for subcarrier k
• buf[2k+1] = real part (Re) for subcarrier k

Order: LLTF subcarriers, then HT-LTF, then STBC-HT-LTF (whichever are enabled).

Subcarrier Counts and Byte Totals

Packet type	Channel	LTF fields enabled	Subcarriers	Bytes
Legacy (non-HT)	HT20	LLTF only	64	128
HT20	20 MHz	LLTF + HT-LTF	128	256
HT40	40 MHz	LLTF + HT-LTF	192	384
HT40 w/ STBC	40 MHz	All 3 LTFs	320+	640+

Packet Rate

CSI is event-driven — one sample per received WiFi packet:

Mode	Practical rate
Passive beacon sniffing	~10 Hz (beacon interval = 100 ms)
Active ICMP ping loop	50–100 Hz
Typical sensing deployments	20–22 Hz

---

ESP32-S3 vs. Other Hardware

Tool	Chip	Subcarriers (HT20)	Phase access	Cost
Intel 5300 CSI Tool	IWL5300	30 groups	Yes	~$50 NIC + Linux host
Atheros CSI Tool	AR9390/AR9580	56	Yes	Moderate
Nexmon CSI	BCM4339/BCM4366	256	Yes	RPi / Android
ESP32-S3	ESP32-S3	64 (LLTF+HT-LTF)	Yes	~$5–10 per node
ESPARGOS	8× ESP32 array	Phase-coherent 8-antenna	Coherent	Research device

ESP32 advantages: more subcarriers than Intel 5300 per sample, standalone embedded operation, no host PC required, ~$5 per node.

ESP32 disadvantages: single antenna (no spatial MIMO diversity), lower phase stability, int8 limits dynamic range, no Linux userspace driver ecosystem.

ESPARGOS (University of Stuttgart) is an 8-antenna phase-coherent 2×4 array of ESP32 chips sharing a single 40 MHz reference clock. It supports MUSIC-based AoA estimation and is designed for research requiring true array processing.

Quality Ranking (Espressif's own)

ESP32-C5 > ESP32-C6 > ESP32-C3 ≈ ESP32-S3 > ESP32 (original)