Time Alignment Error (TAE)

Understanding Time Alignment Error

In the world of wireless communications, especially in the 5G-NR domain governed by 3GPP standards, precise timing is paramount. Time Alignment Error (TAE) refers to the discrepancies in frame timing between different antennas within a base station.

This is crucial in scenarios involving:

• Tx Diversity: Where multiple antennas transmit the same signal to improve reception.
• MIMO (Multiple Input Multiple Output): Where multiple antennas transmit different data streams simultaneously to boost data rates.
• Carrier Aggregation (CA): Where multiple frequency bands are combined to enhance bandwidth.

 Why Time Alignment Error (TAE) Matters

Inaccurate timing can lead to signal interference, degraded performance, and reduced data throughput. 3GPP has established strict specifications to ensure the proper functioning of wireless networks.

Time Alignment Error (TAE) Specifications as per 3GPP

The maximum allowable TAE depends on the specific transmission scenario:

Measuring Time Alignment Error (TAE): The 3GPP Methodology

3GPP outlines a specific method for measuring TAE, typically using a signal analyser (often referred to as a spectrum analyser):

1. Test Model: The base station is configured to transmit according to the NR-FR1-TM 1.1 test model.
2. Signal Analysis Software: Specific software designed for analysing 5G NR sub-6 GHz downlink signals are used.
3. Measurement Procedure:

• The signal is transmitted from Antenna Port 1, and its time offset is measured.
• The signal is transmitted from Antenna Port 2, and its time offset is measured.
• The time difference between the two measurements is calculated. This difference represents the TAE.

What is Spectrum Analyzer

A high-quality spectrum analyser is essential for accurate TAE measurement.

It captures the transmitted signals, analyses their timing characteristics, and facilitates the calculation of TAE.

This ensures that the base station adheres to 3GPP specifications, guaranteeing optimal network performance.

The Future of TAE Measurement and Mitigation

As 5G technology continues to evolve, the complexity of signal transmission and reception will increase. This necessitates ongoing research and development of innovative TAE measurement and mitigation techniques.  For example, advancements in machine learning could lead to more efficient and adaptive algorithms for TAE correction.

Calculation of TAE using Signal Analyzer Data

Modern signal analysers provide detailed information about the received signals, including the timing of each component. TAE can be calculated using this data in the following way:

• Identify Reference Point: Choose a specific component of the signal as the reference point. This could be the start of a frame, a specific symbol, or a particular pilot signal.
• Measure Time Offset: Measure the time difference between the arrival of each signal component and the reference point.
• Calculate TAE: The TAE is the maximum time difference observed among all signal components.

Results of a time alignment error (TAE) and phase error measurement performed using a Keysight signal analyser on a 5G NR (New Radio) signal.

Time Alignment Error:

• TAE = 24.41 ns
• This indicates a timing difference of 24.41 nanoseconds between two signal components (likely DMRS ports 0 and 1).

Phase Error:

• EVM = 1.2 deg
• This shows the Error Vector Magnitude (EVM) of the signal, which is a measure of modulation quality. In this case, the EVM is 1.2 degrees, indicating good signal quality.

Main Display:

• Spectrum: The top section displays the spectrum of the 5G NR signal cantered at 3.5 GHz with a bandwidth of 110.6 MHz. The two peaks likely represent the two DMRS ports of the MIMO transmission.
• Constellation Diagram: The circular diagram on the left likely represents the signal's constellation points. This is a visual representation of the signal's modulation scheme and its quality.
• IQ Measurement Time: The signal analyser is configured to measure the In-phase (I) and Quadrature (Q) components of the signal over a time interval, with markers indicating the start and stop points of the measurement.
• Measurement Setup: The signal analyser is set up to measure TAE, utilizing the NR-FR1-TM1.1 (QPSK) test model and focusing on the Downlink (DL) signal.
• Results Table:
• o   Input: Indicates the signal being analysed is coming from Channel 1.
• o   DMRS Ports: The analyser detects DMRS Ports 0 and 1.
• o   Power: Power level of each port (-49.61 dBm).
• o   TAE: Time alignment error between the two ports (24.41 ns).
• o   Frequency Offset: Frequency offset of each port relative to the centre frequency (220.1 MHz and 221.1 MHz).
• o   Phase Offset: Phase difference between the two ports (1.2 degrees).

Factors Affecting TAE Calculation Accuracy

Several factors can influence the accuracy of TAE calculation:

1. Signal Analyzer Resolution: The timing resolution of the signal analyser directly affects the accuracy of TAE measurement. Higher resolution analysers can provide more precise results.
2. Noise and Interference: Noise and interference in the received signal can introduce errors in the TAE calculation. Averaging techniques can be used to mitigate this effect.
3. Signal Processing Delays: Signal processing within the base station and the signal analyser can introduce delays that need to be accounted for in the TAE calculation.

The Role of Artificial Intelligence

Artificial intelligence (AI) and machine learning (ML) are increasingly being used in the telecommunications industry to optimize network performance.  In the context of TAE, AI/ML algorithms could be trained to analyse signal data, identify patterns, and predict potential timing errors before they occur. This could enable proactive mitigation measures, ensuring uninterrupted network operation.

Let’s take an example to explore and understand more

Assume a 5G NR base station is transmitting using two carriers:

• Carrier 1: 2 GHz (n258 band), 40 MHz bandwidth
• Carrier 2: 3.5 GHz (n78 band), 60 MHz bandwidth

We are using a signal analyser to measure TAE.

Measurement Data:

Let's say the signal analyser captures the following time offsets for specific signal components relative to the start of a frame:

TAE Calculation:

Carrier 1:

o   Maximum time offset: 20 ns (DMRS Port 1)

o   Therefore, TAE for Carrier 1 is 20 ns.

Carrier 2:

o   Maximum time offset: 80 ns (DMRS Port 1)

o   Therefore, TAE for Carrier 2 is 80 ns.

Combined TAE:

o   Since the TAE for Carrier 2 (80 ns) is greater than the TAE for Carrier 1 (20 ns), the combined TAE for the aggregated signal is 80 ns.

Points outlined:

1. The TAE for Carrier 1 (20 ns) is well within the 3GPP specification of 65 ns for MIMO transmissions at a single carrier frequency.
2. The TAE for Carrier 2 (80 ns) is also within the 3GPP specification of 260 ns for intra-band contiguous CA.
3. The combined TAE of 80 ns is acceptable because it falls within the limit for the worst-case scenario (Carrier 2 in this case).

This is a simplified example. In real-world scenarios, signal analysers capture data for numerous signal components, and the calculations involve statistical analysis to ensure accurate results.

Additional Considerations:

• Phase Alignment: In addition to TAE, phase alignment between different signal components is also important. Misaligned phases can cause signal cancellation and degradation of performance.
• Frequency Error: Frequency errors can also contribute to timing misalignment. Precise frequency synchronization is crucial to ensure accurate TAE measurements.
• Environmental Factors: Factors like temperature variations and cable lengths can also affect TAE. Careful calibration and compensation techniques are necessary to account for these effects.

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