How does rain affect CCDP link performance?

Rain degrades CCDP in two ways. First, differential attenuation: raindrops are oblate (flattened by aerodynamic forces), causing horizontally polarized signals to experience 0.5 to 3 dB higher attenuation than vertically polarized signals at the same rain rate, depending on frequency and drop size distribution. Second, differential phase: the asymmetric drop shape introduces different phase shifts for H and V polarizations, rotating the polarization plane and coupling energy between polarizations. The combined effect reduces XPD according to ITU-R P.618 models. At 18 GHz in 50 mm/hr rain (0.01% annual probability), XPD degrades from a clear-air value of 35 to 45 dB to 18 to 22 dB, requiring XPIC to maintain acceptable C/I. At 38 GHz, degradation is worse: XPD can drop to 12 to 15 dB. Link designers use rain-margin calculations specific to CCDP, which must account for both co-polar rain attenuation and cross-polar rain depolarization simultaneously.

Link Engineering

Co-Channel Dual Polarization

Q: How does XPIC work to enable CCDP?

Cross-Polarization Interference Cancellation (XPIC) is a digital signal processing technique that removes cross-polar interference in CCDP links. Each receiver gets two inputs: the desired signal from its own polarization and a reference of the interfering signal from the orthogonal polarization (received via a cross-connected port or a second receiver chain). An adaptive equalizer (typically 32 to 128 taps, updated at symbol rate) generates a replica of the cross-polar interference and subtracts it from the desired signal. The cancellation depth depends on the channel estimation accuracy: in clear air with stable XPD above 30 dB, XPIC achieves 25 to 35 dB additional cancellation, bringing total isolation above 55 dB. During rain with XPD degraded to 15 dB, XPIC still provides 20 to 25 dB cancellation for total isolation of 35 to 40 dB, sufficient for 256-QAM operation. XPIC requires tight timing synchronization between the two polarization receivers (within 1 to 2 symbol periods) and operates in conjunction with the main equalizer that handles multipath distortion.

Q: What capacity does CCDP achieve?

CCDP exactly doubles the capacity of a single-polarization link within the same channel bandwidth. A 56 MHz channel at single polarization with 256-QAM and rate 0.95 coding achieves approximately 400 Mbps. The same channel in CCDP configuration achieves 800 Mbps. With 4096-QAM (achievable in short hops with high fade margin), single-pol reaches 560 Mbps and CCDP reaches 1.12 Gbps per channel. Modern microwave radios from Ericsson (MINI-LINK 6000), Nokia (Wavence), and Huawei (OptiX RTN) support CCDP with up to 2x2 MIMO (4 spatial streams using 2 antennas with dual polarization), achieving 4x capacity multiplication for a total of 4 to 10 Gbps aggregate throughput using multi-carrier configurations across 2 to 4 channels. The theoretical maximum for a single 112 MHz channel at 4096-QAM CCDP is approximately 2.24 Gbps.

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CCDP doubles microwave link capacity by transmitting independent data on orthogonal polarizations (H and V) at the same frequency. Requires antenna XPD of 25 to 30 dB plus digital XPIC cancellation (20 to 30 dB additional). Rain depolarization above 10 GHz degrades XPD to 15 to 20 dB during heavy events, making XPIC essential. Standard in modern 6 to 42 GHz backhaul with 256-QAM to 4096-QAM, achieving 800 Mbps to 2.24 Gbps per channel.

Category: Link Engineering

Capacity gain: 2x (same spectrum)

XPD required: 25 to 30 dB

Understanding CCDP

Spectrum for microwave backhaul is expensive and scarce, with typical channel bandwidths of 28 to 112 MHz allocated by national regulators. Doubling the capacity of a link without additional spectrum is enormously valuable to mobile operators. CCDP achieves this by exploiting the orthogonality of horizontal and vertical linear polarizations: two independent data streams modulate two transmitters on the same frequency, one driving a horizontally polarized antenna feed and the other a vertically polarized feed. At the receiver, two corresponding receivers demodulate the H and V signals independently, with each receiver seeing the other polarization's signal as interference.

The key challenge is maintaining sufficient isolation between polarizations. A well-designed dual-polarized antenna achieves 25 to 40 dB XPD (cross-polarization discrimination) in clear air, meaning the cross-polar signal is 25 to 40 dB weaker than the co-polar desired signal. For low-order modulation (QPSK, needing ~10 dB C/I), this antenna XPD alone is sufficient. For high-order modulation (256-QAM needing ~28 dB, 4096-QAM needing ~35 dB), the antenna XPD is insufficient and digital XPIC must be employed. XPIC uses an adaptive filter to estimate and subtract the cross-polar interference, adding 20 to 30 dB of isolation for a total system XPI (cross-polarization isolation) of 45 to 60 dB. This total isolation enables 4096-QAM CCDP operation on short hops (<5 km) with sufficient fade margin.

CCDP Link Equations

System Cross-Polar Isolation:
XPI_total = XPD_antenna + XPIC_cancellation (dB)

CCDP Capacity:
C_CCDP = 2 × BW × log₂(M) × CR (bps)

Rain XPD Degradation (ITU-R P.618):
XPD_rain = U - V(f) × log(A_co) (dB)

Where BW = channel bandwidth, M = QAM order, CR = code rate, U and V are frequency-dependent empirical constants, A_co = co-polar rain attenuation. Example: 28 MHz, 256-QAM, CR = 0.95: C_CCDP = 2 × 28 × 8 × 0.95 = 425 Mbps.

CCDP Link Performance

Configuration	Modulation	56 MHz Channel	XPI Required	Rain Margin Impact
Single-pol	256-QAM	400 Mbps	N/A	Co-polar only
CCDP	256-QAM	800 Mbps	≥45 dB	+3 to 5 dB extra
CCDP	1024-QAM	1,000 Mbps	≥50 dB	+5 to 8 dB extra
CCDP	4096-QAM	1,120 Mbps	≥55 dB	Short hops only
CCDP + 2x2 MIMO	256-QAM	1,600 Mbps	≥45 dB + MIMO	Two antennas needed

Common Questions

Frequently Asked Questions

How does XPIC work to enable CCDP?

A 32 to 128 tap adaptive equalizer generates a replica of the cross-polar interference from a reference input and subtracts it. Clear air: 25 to 35 dB cancellation on top of 30+ dB antenna XPD = 55+ dB total isolation. In rain (XPD drops to 15 dB): XPIC adds 20 to 25 dB for 35 to 40 dB total, sufficient for 256-QAM. Requires tight timing sync within 1 to 2 symbols.

How does rain affect CCDP performance?

Oblate raindrops cause 0.5 to 3 dB differential H/V attenuation and differential phase, rotating the polarization plane. At 18 GHz in 50 mm/hr rain, XPD drops from 35 to 45 dB to 18 to 22 dB. At 38 GHz, XPD can reach 12 to 15 dB. Link design must budget both co-polar rain fade and cross-polar depolarization margins simultaneously.

What capacity does CCDP achieve?

Exactly doubles single-pol: 56 MHz at 256-QAM goes from 400 to 800 Mbps. With 4096-QAM: 1,120 Mbps per channel. CCDP + 2x2 MIMO (4 streams, 2 antennas) achieves 4x multiplication. Modern radios (Ericsson, Nokia, Huawei) support 4 to 10 Gbps aggregate using multi-carrier CCDP configurations.

Related Terms

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