4G-LTE: Base station and repeater overlap and cover on the same frequency, and the maximum allowable delay difference between the two signals on the terminal side

Core: LTE relies on cyclic prefix CP to resist multipath delay, and cannot directly use CP limit, leaving engineering margin.

LTE CP basic parameters

1. Ordinary CP (95% current network, urban/suburban)

CP length: 4.7 μ s (first symbol 5.21 μ s)

Theoretical maximum multipath window: 4.7 μ s, beyond which ISI/ICI occurs, throughput drops sharply, and 64QAM demodulation fails

2. Expand CP (rural MBMS， Very rarely used)

CP length: 16.67 μ s

Note: This CP is the "protection window for multipath within the same cell"; The repeater belongs to two different physical paths within the same logical cell, and the time difference between the two signals reaching the mobile phone cannot exceed CP.

The inherent hardware delay of the repeater itself

RF wireless repeater: 2-5 μ s

Digital Fiber Optic Repeaters (GRRU): 7-12 μ s (brought by Near End ADC Fiber Optic Remote DAC)

Many digital repeater stations already have hardware latency close to/exceeding that of ordinary CP (4.7 μ s), so digital fiber repeater stations are strictly prohibited from overlapping and covering with donor base stations on the same frequency in large areas, which is a key point of network regulations.

Acceptable delay difference on the construction site (terminal side: direct signal from the base station vs. forwarding signal from the repeater)

① Ordinary CP (the vast majority of current network FDD LTE)

• ✅ Ideal goal: ≤ 3 μ s, with almost no degradation in performance in the overlapping area

• ⚠️ Barely usable upper limit: ≤ 4 μ s, approaching the CP boundary, high-order modulation is prone to dropout

• ❌＞ 4.7 μ s: Absolutely prohibited, causing inter symbol interference, subcarrier orthogonal destruction, rate diving, disconnection, and abnormal switching

The digital fiber optic repeater itself has 7-12 μ s, and as long as there is overlap with the original base station, the delay difference will directly exceed the standard; So in the planning of digital repeater stations, it is necessary to achieve coverage isolation and not overlap. The hardware latency of wireless repeater is small, which is the condition for small-scale overlap.

② Extended CP (rarely configured)

Ideal goal: ≤ 10 μ s

Maximum allowable: ≤ 14 μ s (leave a margin, do not exceed the theoretical limit of 16.67 μ s)

Compare with TETRA for easier memory retention

TETRA: Equalizer window ≈ 27.8 μ s, engineering maximum allowable 25 μ s

LTE ordinary CP: only 4.7 μ s, with much lower tolerance than TETRA, very sensitive to repeater overlap

Engineering treatment measures

1. Digital fiber optic repeater: prioritize coverage cutting and avoid overlapping with the donor sector; In the overlapping area, the power of the repeater should be lowered to allow the base station signal to dominate and the repeater signal to be weak.

If the repeater supports delay compensation function, align the overall delay of the repeater with the base station signal and reduce the time difference between the two signals on the terminal side.

3. Wireless repeater (analog) has low latency and can overlap in a small range, while also controlling the latency difference to be less than 3 μ s.

The report can directly extract short sentences

In LTE normal CP mode, when the donor base station overlaps with the repeater on the same frequency, it is recommended that the delay difference between the two signals received by the terminal should be ≤ 3 μ s, and the maximum should not exceed 4 μ s; Exceeding 4.7 μ s will result in inter symbol interference and subcarrier interference, leading to decreased throughput and abnormal business operations. The fixed time delay of digital fiber optic repeater is relatively large, and efforts should be made to avoid overlapping coverage with the donor base station on the same frequency.