Effrina Yanti Hamid, S.T., M.T., Ph.D.
Pelaksana | Sekolah Teknik Elektro dan Informatika
Abstract
The 6th Generation (6G) mobile communication technology is expected to support connectivity for mobile terminals/users such as autonomous vehicles, high-speed trains, and others. One of the main challenges for these services is providing reliable communication in high-mobility environments at higher frequencies that have significant challenges from high Doppler effects . One emerging candidate that has gained considerable attention is Orthogonal Time Frequency Space (OTFS) modulation. This scheme employs two-dimensional modulation, making it more robust against high Doppler effects . This research aims to develop an synchronization techniques for OTFS modulation systems. The proposed synchronization technique employs the preamble method, utilizing two types of preambles: the Zadoff-Chu (ZC) sequence and the IEEE 802.11a standard. Testing was conducted by transmitting data in the form of images and text. The performance of the synchronization technique was evaluated using Mean Square Error (MSE). The test results indicate that the ZC sequence preamble method provides better outcomes compared to other methods, and longer preamble lengths also positively impact the performance of CFO synchronization tech
Keyword: carrier frequency offset, mean square error, preamble, synchronization.
Introduction
In recent years, Orthogonal Time Frequency Space (OTFS) has attracted significant attention from researchers in the field of wireless communication due to its potential to serve as a more robust modulation scheme to address challenges in wireless environments. Unlike typical modulation schemes that operate in the frequency domain, OTFS works in the delay-Doppler domain. When users are moving at high speeds, such as on high-speed trains or airplanes traveling at speeds of up to 1000 km/h, frequency shifts occur, which can affect the performance of conventional modulation schemes. In OTFS, even in the presence of high mobility, this modulation ensures that the signal can still be transmitted effectively. OTFS helps maintain communication quality in dynamic conditions.
Although OTFS has advantages in overcoming high mobility challenges in wireless environments, when a signal is transmitted through the channel, there remains the possibility of Carrier Frequency Offset (CFO). CFO can occur due to several reasons, including equipment drift, environmental changes, or imperfections in synchronization between the transmitter and receiver. The presence of CFO can impact the performance of OTFS. Therefore, synchronization is still required to correct CFO, ensuring that the signal is received and processed correctly in dynamic wireless environments.
Research Mode
This research aims to implement effective synchronization techniques for accurately estimating Carrier Frequency Offset (CFO), thereby ensuring robust signal reception at the receiver. Synchronization will be performed prior to the demodulation process, as illustrated in Picture 1. The transmitted signal, upon reaching the channel, will enter the receiver, where the synchronization process will take place before proceeding to the Orthogonal Time Frequency Space (OTFS) demodulation stage.
The synchronization technique proposed in this research utilizes identical preambles, 𝒀𝟏 and 𝒀𝟐. Two types of preambles are used, IEEE 802.11a and the Zadoff-Chu sequence. The preamble is placed at the beginning of the OTFS block, preceding the Cyclic Prefix (CP) and the data. The CP itself is a copy of the end portion of the data placed at the beginning, with the purpose of reducing Intersymbol Interference (ISI). The structure of one OTFS block with identical preambles is illustrated in Picture 2.
The synchronization process is performed by comparing two identical preambles, 𝒀𝟏 and 𝒀𝟐. The first segment 𝒀𝟏 and the second segment 𝒀𝟐 , originate from the same preamble signal. However, upon reception, both segments experience phase shifts due to CFO. CFO estimation is obtained by calculating the angle of the product of 𝒀𝟏 and 𝒀𝟐, which represents the phase difference between them. Since CFO causes phase changes or shifts, this angle provides information about the extent of the
signal’s change or shift. The result is then adjusted with a scaling factor that considers the length of the preamble and the amount of transmitted data to achieve an accurate frequency shift estimation.
Discussion & Result
The testing was conducted under Additive White Gaussian Noise (AWGN) and Rayleigh fading channel conditions to evaluate the effectiveness of the ZC sequence and IEEE 802.11a preamble. This testing covers an SNR range from 0 dB to 30 dB and 10 different CFO values for each SNR. The data obtained from this testing is presented in quantitative graphs, illustrating the impact of preamble length variations on MSE. This testing also aims to identify the effect of preamble length on the estimation accuracy of each method used. The results of this testing will provide insights into the performance comparison between the ZC sequence and IEEE 802.11a methods in accurately estimating CFO values.
Picture 3. presents the MSE graph for the performance of the ZC sequence and IEEE 802.11a preamble with a length of 319 under two channel conditions. From the graph, it can be observed that both the ZC sequence and IEEE 802.11a preamble improve with each increase in SNR. However, the ZC sequence demonstrates better performance compared to the IEEE 802.11a preamble under both channel conditions.
Conclusion
The performance of the OTFS system’s synchronization technique on an AWGN channel with an SNR of 30 dB using the Zadoff-
Chu (ZC) sequence preamble with a length of 319 provides a better Mean Square Error (MSE) value of 10−7, compared to the IEEE 802.11a preamble, which produces an MSE value of 10−6 . For future work, synchronization and equalization techniques can be implemented on the OTFS system receiver using two Software-Defined Radios (SDRs), where the receiver device is placed on a moving receiver.
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