ATC Algorithm: Automatic Time Offset Control Explained

This page explains the basics of the Automatic Time Offset Control (ATC) algorithm. It covers both time offset estimation and time offset correction within a wireless receiver. Wireless systems experience delay spread between the transmitting and receiving ends, caused by multipath distortion. Additionally, the transit time of the signal through electronic circuits introduces time offset.

ATC, short for Automatic Timing Offset Correction, is used in wireless communication systems to identify the start of a frame (particularly in TDMA frames) or to estimate the time offset that’s been introduced.

It’s a vital component in wireless systems employing a frame structure, whether TDD or TDMA based. The algorithm is applied within the signal processing chain of the wireless receiver. ATC algorithms, or time offset estimation/correction algorithms, are primarily divided into two categories:

1. Threshold detection or coarse time offset estimation and correction.
2. Fine time offset estimation and correction.

The specific algorithms vary depending on the system, as they are tailored to different access techniques, frame structures, and channel model requirements.

Coarse Time Offset Estimation

For OFDM-based systems, the power/energy of the time domain signal is continuously monitored within a small window frame (by acquiring a small number of samples). The coarse time offset estimation algorithm essentially distinguishes the actual signal from the noise energy. Noise signal levels are typically significantly lower, often below -100dBm. This level depends on the RF link.

Power calculation for a time domain sample is straightforward, as shown below:

P = √(real² + imag²)

The same principle applies to OFDMA and single carrier-based systems.

OFDM physical layers are utilized in standards such as WiMAX-IEEE 802.16d or IEEE 802.16-2004, WLAN-IEEE 802.11a, 11g, 11n, 11ac, 11ad. OFDMA physical layers are employed in Mobile WiMAX-IEEE 802.16e and LTE-3GPP standards.

Fine Time Offset Estimation

For fine time offset estimation, the received data frame is correlated with a known standard pattern at the receiver. This known standard pattern goes by different names:

• Preamble in WiMAX
• Short training sequence/long training sequence in WLAN
• Training Sequence Code (TSC) in GSM
• Primary and secondary synchronization signals (PSS and SSS) in LTE
• Channelization codes (SF) in CDMA

Time Offset Correction

After both coarse and fine time offset estimations, the start of the frame is identified. This information is then used to skip the offset samples and proceed with further processing. This process is known as time offset correction.