SRS — Comb, Cyclic Shift, Hopping, Usages

1 Why sound

Measure where data isn't

DM-RS only reveals the channel on the RBs you transmit. To schedule you on the best RBs — or to beamform the downlink in TDD by reciprocity — the gNB needs a wideband look. SRS is that wideband pilot: a low-PAPR Zadoff-Chu sequence on a comb, in the last symbols of a slot.

Freq-selective scheduling

Wideband sounding shows which RBs are strong → the scheduler places PUSCH on the good ones.

TDD reciprocity

UL channel ≈ DL channel in TDD → the gNB computes DL precoders without DL feedback.

Link adaptation

SRS SINR feeds UL MCS/rank, and antenna-switching probes each UE antenna.

2 The signal itself

Zadoff-Chu base + cyclic shift

SRS is a Zadoff-Chu sequence — constant amplitude (CAZAC), so it has near-zero PAPR and a perfect autocorrelation. Each SRS gets a cyclic shift α: a phase ramp e^jαn across the sequence. In the time domain that's a circular shift, which is what makes different shifts perfectly separable. Move the shift and watch the phase ramp.

Cyclic shift n_cs 0

Comb K_TC (sets n_cs,max)

α = 2π·n_cs/n_cs,max

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Time-domain shift

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PAPR

~0 dB (CAZAC)

SRS sequence (38.211 §6.4.1.4.2)r^(α)_u,v(n) = e^j·α·n · r̄_u,v(n) , α = 2π · n_cs / n_cs,max (group u, sequence v from group/sequence hopping)

3 Frequency structure

The comb

SRS occupies every K_TC-th subcarrier (comb 2 or 4) from a configured offset. Different offsets interleave several soundings in the same RBs; cyclic shifts stack more on the same comb.

Comb K_TC

Comb offset 0

SRS subcarriers / RB

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Multiplexable combs

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Max cyclic shifts

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4 Multi-port

Antenna ports = equally-spaced cyclic shifts

A multi-port SRS (1–4 ports) puts each port on the same comb but at a cyclic shift spaced n_cs,max/N_ap apart, so all ports stay orthogonal. Pick the port count and watch the shifts spread evenly around the circle.

SRS ports N_ap

Base shift n_cs 0

Per-port cyclic shiftn_cs^(p) = ( n_cs + n_cs,max·(p−1000)/N_ap ) mod n_cs,max

5 How wide

SRS bandwidth — the C_SRS / B_SRS tree

SRS bandwidth is a hierarchy. C_SRS (0–63, cell-level) selects a row of TS 38.211 Table 6.4.1.4.3-1, fixing the widest bandwidth. B_SRS (0–3, UE-level) then chooses how finely to subdivide it — larger B_SRS = a narrower sounding (m_SRS,b RBs) that hops to cover the whole band. Pick both.

C_SRS (row) 13

B_SRS (level)

m_SRS,b (RBs sounded)

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Widest (B=0)

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Hops to cover band

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6 Coverage

Frequency hopping (b_hop)

A power-limited UE can't sound a wide band at once. When b_hop < B_SRS, the narrow sounding hops across the band over successive SRS occasions, building a wideband picture piece by piece. Animate it.

Each narrow sounding hops to a new position; after a full hopping cycle the gNB has a complete wideband channel estimate — within the UE power budget.

7 Purpose & timing

Four usages, three time-behaviours

codebook

gNB picks a UL precoder from a codebook (like DL PMI) from the sounded channel.

nonCodebook

UE forms candidate beams (from DL CSI-RS); SRS sounds them; gNB selects via SRI.

beamManagement

Sweep UL beams so the gNB picks the UE's best transmit beam at FR2.

antennaSwitching

Sound each UE antenna in turn (1T2R/1T4R) for full DL-channel reciprocity.

Resource type & symbol position

Periodic

RRC period + offset; always on.

Semi-persistent

MAC-CE activated/deactivated.

Aperiodic

DCI-triggered, one-shot.

SRS sits in the last 1, 2, or 4 OFDM symbols of a slot (configurable startPosition), so it never collides with the front-loaded DM-RS.

8 Put it together

A full SRS resource

9 Knowledge check

Test yourself

UL Power Control PUSCH Mapping