ATC algorithm-Automatic Time offset Control
This page describes Automatic Time offset Control(ATC) algorithm basics which covers time offset estimation and time offset correction in wireless receiver.
Wireless systems incorporate delay spread between transmit and receive end due to multipath distortion and also transit time of signal through electronic circuits, which will introduce time offset in the signal. ATC stands for Automatic Timing offset correction, used in wireless communication systems to find out start of the frame (mainly TDMA frame) or to estimate time offset introduced. It is used in wireless systems having frame structure either TDD or TDMA based. The algorithm is used in signal processing chain of wireless receiver.
ATC Algorithms or time offset estimation/correction algorithms are categorized into two main parts viz.
1. Threshold detection or coarse time offset estimation and correction and 2. Fine time offset estimation and correction The algorithms vary system to system due to different access technique, frame structure and channel model requirements.
Coarse Time offset estimation
For OFDM based systems, usually power/energy of time domain signal is continuously checked within a small window frame (small numbers of samples are acquired to do this). Coarse time offset estimation algorithm basically differentiates actual signal from the noise energy. Noise signal will be much down usually lesser than -100dBm or so. This level depends on rf link.
Power calculation is simple for a time domain sample, as mentioned below.
P = (real^2 + imag^2)^1/2
For OFDMA and Single carrier based systems, the same as above is done. OFDM physical layer is used in WiMAX-IEEE 16d or IEEE 802.16-2004, WLAN-IEEE 802.11a, 11g, 11n, 11ac, 11ad standards. OFDMA physical layer is used in Mobile WiMAX-IEEE 802.16e and LTE-3GPP standards.
Fine Time offset estimation
For Fine time offset estimation, received frame of data is correlated with known standard pattern at the receiver. This known standard pattern is known as 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 and so on.
Time offset correction
After the time offset estimation both coarse and fine, the starting of the frame is known which can be taken for further processing after skipping the samples.This is called as time offset correction.