How is C/I ratio related to frequency reuse distance?

In cellular systems, C/I is directly determined by the frequency reuse factor N and the path loss exponent n. For a hexagonal cell layout, C/I = (1/6) * (sqrt(3N))^n. With N=7 (GSM) and n=4 (urban propagation), C/I = (1/6) * (sqrt(21))^4 = 18.7 dB, which exceeds GSM's 9 dB minimum requirement with comfortable margin. Reducing the reuse factor to N=3 drops C/I to approximately 11 dB, marginal for GSM but acceptable for CDMA and LTE which use interference cancellation and scheduling to operate at lower C/I values. LTE with N=1 (full frequency reuse) achieves adequate C/I through ICIC, beamforming, and adaptive modulation.

What minimum C/I do different wireless systems need?

GSM requires minimum 9 dB C/I for acceptable voice quality using GMSK modulation. UMTS (WCDMA) operates at C/I as low as -15 to -20 dB because its spreading gain (typically 25 dB for voice) recovers the signal from below the interference floor. LTE needs approximately 0-5 dB C/I for QPSK at cell edge but 20+ dB for 256-QAM at cell center. Wi-Fi 6 (802.11ax) requires 25 dB C/I for MCS11 (1024-QAM) but operates down to -2 dB for MCS0 (BPSK 1/2). Satellite VSAT links typically require 10-15 dB C/I depending on the modulation and coding scheme.

What is the difference between C/I, C/N, and SINR?

C/I measures carrier power relative to interference only, ignoring thermal noise. C/N measures carrier relative to thermal noise only, ignoring interference. SINR (signal-to-interference-plus-noise ratio) combines both: SINR = C/(I+N). In noise-limited scenarios (rural cells, satellite links), SINR approaches C/N. In interference-limited scenarios (dense urban cells, co-channel satellite beams), SINR approaches C/I. Modern cellular networks are almost always interference-limited, making C/I the dominant factor. The combined metric is: 1/SINR = 1/(C/I) + 1/(C/N).

What is the C/I Ratio? | Carrier-to-Interference

Definition & Context

The carrier-to-interference ratio (C/I) quantifies the power of a desired received signal relative to the aggregate power of unwanted co-channel and adjacent-channel interfering signals, expressed in decibels. C/I is the primary design parameter for cellular frequency planning, satellite beam coordination, and any wireless system where multiple transmitters share or reuse the same frequency band.

In cellular networks, C/I determines the minimum frequency reuse distance: the spatial separation required between cells using the same channel to maintain acceptable signal quality. A higher C/I requirement forces a larger reuse factor (more unique frequencies per cluster), reducing spectral efficiency. Modern systems like 5G NR and LTE use full frequency reuse (N=1) by managing interference through beamforming, inter-cell interference coordination (ICIC), and adaptive modulation that adjusts the data rate to the instantaneous C/I at each user location.

Key Formulas

C/I (hexagonal cell reuse):

C/I = (1/6) × (√(3N))ⁿ

N = 7, n = 4: C/I = (1/6) × (4.58)⁴ = 73.5 → 18.7 dB

C/I from reuse distance:

C/I(dB) = n × 10 log₁₀(D/R) − 10 log₁₀(N_i)

D/R = 4.58, n = 4, N_i = 6 interferers: C/I = 18.7 dB

SINR from C/I and C/N:

1/SINR = 1/(C/I) + 1/(C/N)

Minimum C/I Requirements

System	Min C/I	Modulation	Reuse Factor	Interference Mgmt
GSM	9 dB	GMSK	N = 4-7	Frequency hopping
UMTS (WCDMA)	-15 to -20 dB	QPSK + spreading	N = 1	Spreading gain, power control
LTE (cell edge)	0-5 dB	QPSK	N = 1	ICIC, eICIC
LTE (cell center)	20+ dB	256-QAM	N = 1	Beamforming, scheduling
Wi-Fi 6 (MCS11)	25 dB	1024-QAM	Channel plan	OFDMA, BSS coloring
Satellite VSAT	10-15 dB	QPSK-16APSK	Beam separation	Polarization, orbital spacing

Practical Application

A cellular operator deploying LTE on a 10 MHz carrier at 1800 MHz in a dense urban environment measures C/I = 3 dB at cell edge users 500 m from the serving site and C/I = 25 dB at cell center users 100 m away. The eNodeB scheduler assigns QPSK with 1/3 coding rate (requiring C/I > 0 dB) to edge users, achieving 2 Mbps throughput, and 64-QAM with 4/5 coding (requiring C/I > 18 dB) to center users, achieving 30 Mbps. Enabling ICIC reserves the outer 25% of subcarriers for edge users while neighboring cells avoid those frequencies, improving edge C/I from 3 dB to 8 dB and enabling 16-QAM (7 Mbps), effectively doubling edge user throughput with minimal impact on cell center performance.

Frequently Asked Questions

How is C/I related to frequency reuse?

C/I = (1/6) × (√(3N))^n for hexagonal cells. N=7, n=4: C/I = 18.7 dB. Reducing N to 1 (LTE) lowers C/I but compensates with ICIC, beamforming, and adaptive modulation.

What minimum C/I do systems need?

GSM: 9 dB. WCDMA: -15 to -20 dB (spreading gain recovers signal). LTE: 0-5 dB edge, 20+ dB center. Wi-Fi 6 MCS11: 25 dB. Satellite VSAT: 10-15 dB.

C/I vs. C/N vs. SINR?

C/I = carrier vs. interference only. C/N = carrier vs. noise only. SINR = carrier vs. (interference + noise). Dense urban cells are interference-limited (SINR ≈ C/I). Rural/satellite are noise-limited (SINR ≈ C/N).

C/I Ratio