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Testing 400Gbps-1Tbps architectures: How do we get there?

February 21, 2018

Testing 400Gbps-1Tbps architectures: How...

By Girish Baliga, Marketing Program Manager, Digital, Wireless and Aerospace Defense Test, Keysight Technologies

Analysts predict anything between 20 and 50 billion internet devices by the year 2020, ranging from machine-to-machine (M2M) devices that transmit a few bytes of data per day to applications that stream multiple high-definition video channels. Studies into future user demands give network operators the goal of creating an infrastructure that provides the impression of limitless capacity in any situation, even in sports stadia and concert arenas where there are dense user populations demanding high-rate mobile access. Regardless of the local connection to the device, traffic quickly flows into a fixed physical network: home asymmetric digital subscriber line (ADSL) router or cellular base station, for example. From that point, the network’s backbone is a high capacity system based to a large extent on interconnected optical fibre that is short of capacity for today’s needs and that must grow to support an estimated data volume of more than 40 zetabytes per year by 2020.

New Test Scenarios for Higher Data Rates

In the ultra high speed digital and optical communications world, capacity gains come essentially from three variables: more carriers through techniques such as polarization and multi-carrier orthogonal frequency-division multiplexing (OFDM) modulation, better spectrum efficiency through higher modulation density and higher symbol rate. (Figure 1 - find attached below)

Testing these higher order systems with data rates close to 1 Tbps requires test solutions capable of clean signal generation and analysis and a measurement bandwidth of at least 20 GHz, to be sure the measurements represent system performance and not the limitations of the test solutions.The test solutions must offer the flexibility to address many different modulation schemes on four synchronized channels for a dual-polarization I/Q signal.Traditionally, receiver tests such as phase noise, observed signal-to-noise ratio and polarization tests have been performed using a gold transmitter, giving a view of the device but lacking completely deterministic knowledge. (Figure 2 - find attached below).

Using an arbitrary waveform generator (AWG) allows the creation of test signals in the electrical domain, including clean signals and signals with specific, known, impairments. For transmitter test, these can be fed directly to a transmitter and the resultant error rate can be measured directly. For receiver test, they can be used directly to test digital signal processing (DSP) stages and be translated to the optical domain to create both clean and stressed deterministic optical signals for full receiver test. (Figure 3 - find attached below)

The key challenges in making measurements on coherent optical systems lie in providing known, repeatable, clean and distorted test signals at data rates in excess of 32 GBaud and with the flexibility to support diverse modulation formats. Test system calibration should be possible, not only at the front panel of the test signal generator and measurement solutions, but at any point in the signal chain through embedding and de-embedding techniques using the transmission system’s S-parameters.  Ability to measure cumulative effects is achieved with optical modulation analyzers.

Measuring Transmission Quality

The goal of every data transmission is to achieve highest possible transmission rates at a tolerable bit-error ratio (BER). Since BER measurements can be time-consuming there are other, faster signal quality metrics which closely correlate with the BER of a channel. A general quality metric which is well-defined for higher-level modulation formats is the error vector magnitude (EVM) which describes in short how close the actual symbol points are to their reference points on the complex plane. EVM is the magnitude of the vector connecting the measured and the expected vectors at any modulation point. (Figure 4 - find attached below)

Identifying Typical Transmission Problems

Closer analysis of the constellation and eye diagrams provide an excellent mechanism for finding and correcting poor EVM results. Gain Imbalance compares the amplitude of the I signal with the amplitude of the Q signal. Note the constellation width is different from its height (compare to the square yellow reference). IQ gain imbalance is best observed in the constellation diagram, though it will also show in eye diagrams as a difference between the amplitudes in I- and Q-Eye.  Other measurement challenges involve IQ skew, DC offsets, Quadrature errors, frequency errors, Symbol Rate Errors(SER)

EVM provides an overall figure of merit, and it is self-evident that higher-order modulation requires better fidelity since constellation points are closer together and therefore decoding errors are more likely.

The Q-factor describes the signal-to-noise ratio (SNR) ratio at decision points. It can be calculated from the EVM and also provides an estimate for the BER. It is used for on-off-keying signals and modulation formats up to quadrature phase shift keying (QPSK). The Q-factor is the BER a receiver would expect assuming that white Gaussian noise is the dominant impairment. The result is displayed in dB and is calculated as follows: Qfactor≈ 1/EVM

Addressing high speed digital & optical test challenges

Using waveform creation software like the optical modulation generator with the AWG to create, and vector signal analysis (VSA) software to analyze test signals ensures that both clean signals and ones with known impairments designed to stress-test the receiver can be generated. A known signal is generated with the arbitrary waveform generator and compared with the measured version of the signal. This way the frequency and phase response of the generator, amplifiers and cables can be derived.

Deterministic emulation and pre-distortion of two independent I/Q baseband 100G/400G/1Tbit communications test channels can be achieved with an high precision arbitrary waveform generator which has  the highest combination of speed, bandwidth and channel density working in tandem with optical modulation generator software.

A high-end turn-key optical modulation analyzer in single- and multi-carrier 400 Gbit/s and 1Tbit/s can be used for coherent receiver and optical modulation characterization. Vector signal analysis software would offer the most flexible analysis tools and smart setup for easiest measurement solution setup.

Furthermore you can add linear and non-linear impairments to your signal or compensate for distortions between the AWG and the system under test or even components inside your system.

In summary, a high precision arbitrary waveform generator coupled with a suite consisting of optical modulation analyzer, vector signal analysis tools among others can help mitigate the challenges offered by ultra-high speed digital and optical signals in the gigabit and terabit range.


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