N4917CPCA Optical Receiver Stress Test Solution

Repeatable Optical Receiver Stress Test for 4G/5G Fronthaul Networks

The Cloud RAN (cloud radio access network, C-RAN) paradigm enables mobile networks operators and mobile infrastructure vendors to meet the accelerating demand for more bandwidth and the growing need for network functions virtualization in the mobile network.In the C-RAN architectures, signal processing is exclusively performed at the baseband unit (BBU) while the conversion of the baseband signal into RF signal is performed at the remote radio head (RRH).

The common public radio interface (CPRI) protocol was defined to transmit digitized RF data over optical fiber between the RRH and BBU.This digital radio over fiber approach addresses the need for high-speed and increased transmission range, while meeting the strict synchronization and timing requirements between BBU and RRH:

CPRI requirements

• ±0.002 ppm jitter-induced frequency error
• 1E-12 bit-error-rate (BER)
• 5 μs maximum round trip delay per link (excluding propagation time)
• 10 km range

CPRI options 7, 8 and 10 support line rates of 9.8304, 10.1376 and 24.33024 Gb/s respectively.Optical transmission can be performed using transceivers modules compliant with the following IEEE 802.3 standards

• 10GBASE-LR/ER standard, clause 52
• 40GBASE-LR4/ER4 standard, clause 87
• 100GBASE-LR4/ER4 standard, clause 88

However, CPRI links present more stringent requirements, especially in terms of supported temperature ranges and jitter, compared to classical 100G Ethernet links.Indeed, the electrical baseband signal received at the RRH side has a huge impact on the quality of the recovered reference clock, which is of primary importance in C-RAN.For this reason, compliance tests are necessary to develop and qualify optical transceivers to be deployed in CPRI links.

Requirements for 5G radio equipment frequency synchronization agreed by 3GPP (3GPP TS 38.133), are similar to LTE (± 0.05ppm), resulting in tight requirements for the clock and data-recovery (CDR) in the Ethernet interface to provide a low-jitter signal to the IEEE 1588 digital PLL.

The N4917CPCA optical receiver stress test solution provides an automated stressed receiver sensitivity test in accordance with the IEEE 10/40/100GBASE and CPRI Specification V7.In order to perform this kind of test, several test instruments such as a bit error ratio tester, digital sampling oscilloscope, optical reference transmitter and tunable laser source are required to operate together to achieve a compliant, repeatable optical stressed eye.This stressed eye, emulating an optical eye after a worst case optical link, is then fed to the receiver under test where the bit error ratio is measured under the stress conditions as defined in the standard.The test setup can be easily extended to support additional tests such as jitter analysis of the received electrical baseband signal or analysis of the recovered clock.

Optical Stressed Eye Test Challenges

The N4917CPCA Optical Receiver Stress Test software supports automated optical stressed receiver sensitivity test for optical transceivers in a CPRI Fronthaul environment that are compliant with IEEE802.3-2015, clauses 52,87,88.The following table contains the salient conditions required for stressed receiver sensitivity test.

Setting up a stressed eye according to the standard can be a very time consuming task, even for experienced users it can take a half day.The reason is that the stressed eye parameters are interdependent and therefore several iterations of the optimization cycle are required to converge on the solution.In addition, it is important that the setup is repeatable and remains stable from the time of the stressed eye calibration through to the end of the DUT measurement.Otherwise a drift in the test setup, especially in the electro-optical converter will impair the test results.

The N4917CPCA optical receiver stress test solution provides a repeatable and stable measurement in a fraction of time compared to manual setup of the stressed eye.This offers major time saving during daily measurements and speeds up developing a standard compliant test solution when compared to a self made solution.

N4917CPCA Detailed Test Setup

The core instrument of the optical stressed eye test setup is the J-BERT M8020A High-Performance BERT system that provides an electrical signal with all the required impairments (random, sinusoidal and bounded uncorrelated timing jitter, ISI and sinusoidal amplitude interference) at the data signal output, using a combination of its built-in stress sources plus an external generator.This electrical signal is fed into the 8149xA electrical-optical converter to modulate the laser and create the stressed optical signal.An 8160xA tunable laser is used to set the wavelength for each of the four lanes in turn.The optical output of the 8149xA is then attenuated to get to the required optical power level at the input of the receiver under test.

Prior to testing the receiver, the optical signal for each wavelength is calibrated using the 86100D Infiniium DCA-X Wide-Bandwidth Oscilloscope with 86105D optical module or using the N1092x DCA-M.All of the equipment is under control of the N4917CPCA software, which takes care of the various equipment settings and performs an iterative adjustment of the BERT stressess until the optical signal measured on the DCA meets the required stressed eye parameters (ER, VECP, J2, J9).