mmWave & 5G

CPRI

Q: How do you calculate the CPRI line rate for a given antenna configuration?

The aggregate CPRI payload rate scales with sampling rate, sample resolution, number of antenna carriers, and the IQ word width. A useful approximation is R = f_s times bits-per-sample times 2 (I and Q) times AxC count, then divided by 0.9846 to account for the 8B/10B (or 64B/66B at higher rates) line coding and control-word overhead, and rounded up to the nearest standard option. For example, a single 20 MHz LTE carrier sampled at 30.72 Msps with 15-bit IQ resolution needs about 30.72 times 15 times 2 = 921.6 Mbps of payload, which maps to CPRI option 3 at 2.4576 Gbps once overhead and additional antenna paths are included. A 4T4R 20 MHz radio typically lands at option 5 (4.9152 Gbps), and a 100 MHz 64T64R massive MIMO radio would require roughly 157 Gbps of raw CPRI, which is why such radios moved to eCPRI.

Q: What is the latency and synchronization budget on a CPRI fronthaul link?

CPRI is a constant-bit-rate, deterministic link, so its dominant latency contribution is fiber propagation at about 5 microseconds per kilometer one way. The 3GPP fronthaul round-trip processing budget for HARQ is roughly 250 microseconds, which limits practical CPRI reach to about 15 to 20 km of fiber. CPRI also distributes timing and frequency: the link must maintain a frequency accuracy of plus or minus 2 parts per billion and a round-trip delay accuracy on the order of plus or minus 8 nanoseconds so the radio unit can recover phase for TDD frame alignment and MIMO. Cable-length calibration (T14 delay measurement) compensates for the fixed fiber delay.

Q: How does CPRI differ from eCPRI, and why did 5G adopt eCPRI?

Classic CPRI sends raw time-domain IQ over a dedicated point-to-point fiber at a constant bit rate, so the bandwidth scales linearly with the number of antennas and the channel bandwidth. A 100 MHz, 64-antenna 5G radio would demand well over 100 Gbps of CPRI, which is uneconomical. eCPRI moves the functional split higher (typically split 7-2x) so that frequency-domain or partially processed data is sent instead of raw IQ, cutting the bitrate by 5 to 10 times. eCPRI is also packetized and rides over standard Ethernet or IP transport, allowing statistical multiplexing, switching, and shared transport, whereas CPRI requires dedicated wavelengths. CPRI remains common on legacy 4G radios; eCPRI dominates 5G massive MIMO.

/ˈsee-pry/ · Common Public Radio Interface

Short for Common Public Radio Interface, this is the constant-bit-rate serial standard that carries digitized IQ baseband samples, control and management words, and synchronization timing across the fronthaul link between a baseband unit and a remote radio unit. Because CPRI transports raw time-domain IQ rather than processed bits, its bandwidth scales directly with sampling rate, sample width, and antenna count, with standardized line rates running from 614.4 Mbps up to 24.33 Gbps over fiber. The link enforces a frequency accuracy of ±2 ppb and a round-trip delay accuracy near ±8 ns so the radio can recover phase for TDD frame alignment and MIMO. For dense 5G massive-MIMO radios CPRI is increasingly replaced by the packetized eCPRI interface.

Category: mmWave & 5G

Line Rates: 614.4 Mbps to 24.33 Gbps

Frequency Accuracy: ±2 ppb

How CPRI Moves Radio Samples Over Fiber

CPRI was defined in 2003 by a cooperation of base-station vendors (Ericsson, Huawei, NEC, Nokia, and others) to standardize the internal interface between the baseband processing of a base station and its radio frontend. Before CPRI, that connection was proprietary and forced operators to source the baseband and the radio from a single vendor. By digitizing the antenna signal into IQ samples and shipping them over a generic serial fiber link, CPRI enabled the radio head to be mounted at the antenna (cutting the lossy coaxial feeder run) and the baseband to sit in a sheltered cabinet, the architecture that underpins centralized RAN deployments.

The frame structure is hierarchical and clocked to the LTE/NR 10 ms radio frame. A basic frame spans one UMTS chip period (260.42 ns) and contains 16 words; one control word per basic frame carries management, Layer 1 in-band signaling, and the vendor-specific channel, while the remaining words carry user-plane IQ. Sixty-four basic frames form a hyperframe, and 150 hyperframes form a 10 ms radio frame, giving the receiving end a deterministic reference for frame and phase alignment. Because the link is constant-bit-rate, there is no statistical multiplexing gain: a radio that needs 9.8 Gbps of payload occupies a full CPRI option 7 wavelength even when idle.

Line coding adds the only meaningful overhead. CPRI options 1 through 7 use 8B/10B coding (80 percent efficiency), while the higher options 8, 9, and 10 use 64B/66B coding (about 97 percent efficiency) to keep the optics practical at multi-gigabit rates. The protocol distributes a recovered clock to the radio, so the radio unit needs no local high-stability oscillator; the cable delay is measured and calibrated (the T14 round-trip measurement) so the radio can compensate for the fixed fiber propagation when timestamping.

Line-Rate and Latency Equations

Required IQ payload rate:
R_payload ≈ f_s × b_IQ × 2 × A_xC (bps)

CPRI line rate (with coding overhead):
R_line = R_payload / η_code, η_8B/10B = 0.8, η_64B/66B ≈ 0.97

One-way fronthaul fiber latency:
t_fiber ≈ 5 μs/km × L_km

Where f_s = sampling rate, b_IQ = bits per I or Q sample (typically 15), the factor 2 accounts for I and Q, and A_xC = number of antenna-carrier streams. Example: 30.72 Msps × 15 × 2 × 1 ≈ 0.92 Gbps payload → option 3 (2.4576 Gbps) after overhead and additional paths. The ±250 μs HARQ budget limits reach to roughly 15 to 20 km.

CPRI Line-Rate Options

Option	Line Rate	Line Coding	Coding Efficiency	Typical Use
Option 1	614.4 Mbps	8B/10B	80%	Single narrowband carrier
Option 3	2.4576 Gbps	8B/10B	80%	20 MHz LTE, 1 to 2 antennas
Option 5	4.9152 Gbps	8B/10B	80%	4T4R LTE radio
Option 7	9.8304 Gbps	8B/10B	80%	Multi-band macro radio
Option 8	10.1376 Gbps	64B/66B	~97%	High-capacity radio
Option 9	12.16512 Gbps	64B/66B	~97%	Wideband multi-carrier
Option 10	24.33024 Gbps	64B/66B	~97%	Peak CPRI; beyond this use eCPRI

Common Questions

Frequently Asked Questions

How do you calculate the CPRI line rate for a given antenna configuration?

Compute the payload as R ≈ f_s × b_IQ × 2 × A_xC, then divide by the coding efficiency (0.8 for 8B/10B) and round up to the nearest standard option. A single 20 MHz LTE carrier at 30.72 Msps with 15-bit IQ needs ~0.92 Gbps and maps to option 3 (2.4576 Gbps). A 4T4R 20 MHz radio typically lands at option 5 (4.9152 Gbps), while a 100 MHz 64T64R massive-MIMO radio would need ~157 Gbps of raw CPRI, which is why such radios moved to eCPRI.

What is the latency and synchronization budget on a CPRI fronthaul link?

CPRI is constant-bit-rate, so its dominant delay is fiber propagation at ~5 μs/km one way. The 3GPP HARQ round-trip budget of ~250 μs limits practical reach to about 15 to 20 km. The link also carries timing: it must hold ±2 ppb frequency accuracy and ~±8 ns round-trip delay accuracy so the radio can recover phase for TDD frame alignment and MIMO. The T14 cable-delay measurement calibrates out the fixed fiber propagation.

How does CPRI differ from eCPRI, and why did 5G adopt eCPRI?

Classic CPRI ships raw time-domain IQ over dedicated fiber at a constant rate, so bandwidth grows linearly with antennas and channel bandwidth; a 100 MHz, 64-antenna radio would need well over 100 Gbps. eCPRI raises the functional split (typically split 7-2x) and sends partially processed data, cutting the bitrate 5 to 10 times, and it is packetized over standard Ethernet or IP for statistical multiplexing and shared transport. CPRI persists on legacy 4G radios; eCPRI dominates 5G massive MIMO.

Related Terms

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