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In this letter, a synchronization signal design technique for millimeter-wave cellular vehicle-to-everything (C-V2X) communications is proposed. The proposed vehicle synchronization signal generated using a generalized reciprocal sequence of m-sequence is shown to have desirable properties for orthogonal frequency division multiplexing (OFDM)-based C-V2X, particularly for vehicle-to-vehicle (V2V) synchronization. It is shown that the detection probabilities of synchronization signals in C-V2X are not degraded when base stations and vehicles coexist, owing to minimal interference between the synchronization signals. Moreover, the computational complexity required for signal detection and synchronization in V2V links can be significantly reduced with the proposed vehicle synchronization signal.

This paper proposes a downlink synchronization and cell identity (CID) estimation technique for cellular systems based on orthogonal time-frequency space (OTFS). In the proposed technique, each base station (BS) transmits one detection preamble (DP) and two OTFS symbols (pilot and secondary-CID (SCID) signal (SS)). The DP based on a linear frequency-modulated (LFM) waveform, called proposed-LFM (pLFM), carries the primary-CID (PCID) for symbol timing (ST) synchronization, whereas the SS based on the Zadoff-Chu (ZC) sequence mapped in the delay-Doppler domain carries the SCID. The pLFM is obtained by discretizing the LFM waveform, increasing the frequency sweeping parameter beyond the limit of the operational bandwidth, and incorporating the PCID into the LFM waveform. The autocorrelation and cross-correlation functions of the pLFM are analyzed to examine its correlation properties under the influence of a high Doppler shift. To reduce the undesirable correlation properties (side peak and time ambiguity), a receiver processing (RP) scheme is developed for the pLFM. It is found that the pLFM with RP is suitable for the DP design because it can provide an accurate ST in high-mobility scenarios. In addition, the influence of OTFS modulation on the ZC sequence is derived and a low-complexity detection algorithm based on message passing is applied to detect SCID at the user equipment. The simulation results demonstrate that the proposed downlink synchronization and CID estimation techniques are suitable for OTFS-based cellular systems in high-Doppler environments.

Due to short wavelength and weak diffraction ability, millimeter-wave (mmWave) signals are highly susceptible to blockage, which results in significant degradation in received signal power. As a possible solution for overcoming the blockage problem in millimeter-wave communication systems, the deployment of a relay station (RS) has been considered in recent years. In this paper, we discuss the problems to be considered in a relay-assisted mmWave cellular system based on orthogonal frequency division multiplexing. We describe a frame structure and a pilot-based training method to achieve efficient RS selection during blockage. In addition, a method designed to overcome the inter-symbol interference problem caused by different symbol time offsets of pilot signals received from adjacent RSs in the relay-assisted mmWave cellular system is discussed. Then, we propose two different types of pilot sequences that allow a mobile station to distinguish among the pilot sources in multi-cell multi-relay environments: pilot signals based on the Zadoff-Chu sequence (PS1) and pilot signals based on the m-sequence (PS2). The correlation property of PS2 is derived and compared with that of PS1 and another sequence (Gold sequence). Simulations are performed using a blockage model to verify the properties, constraints, and advantages and disadvantages of the proposed pilot sequences in RS-assisted mmWave cellular systems.

The deployment of relay stations (RS) has been regarded as a possible solution to the blocking problem in millimeter wave (mmWave) communication systems. In this paper, we discuss issues to be considered in relay-assisted mmWave cellular systems based on orthogonal frequency division multiplexing. We describe a pilot-based training method to achieve effective RS selection during blockage. We propose two different types of pilot sequences, which allow mobile stations to distinguish pilot sources in a multi-cell multi-relay environment: pilot signals based on Zadoff-Chu sequence (PS1) and m-sequence (PS2). Correlation properties of PS1 and PS2 are derived and compared with that of the conventional sequence (Gold sequence). A blocking model is used to verify the properties, constraints, advantages, and disadvantages of the proposed pilot sequences in the RS-assisted mmWave cellular system.

A vehicle synchronization signal design technique for millimeter-wave cellular vehicle-to-everything (C-V2X) communications is proposed. The proposed vehicle synchronization signal generated using a generalized reciprocal sequence of m-sequence is shown to have desirable properties for orthogonal frequency division multiplexing (OFDM)-based C-V2X, particularly for vehicle-to-vehicle (V2V) synchronization. It is shown that the detection probabilities of synchronization signals in C-V2X are not degraded when base stations and vehicles coexist, and the computational complexity can be significantly reduced with the proposed vehicle synchronization signal.

In this paper, a beam-tracking technique for orthogonal frequency division multiplexing (OFDM)-based millimeter-wave cellular systems is proposed using subarray structures and pilot sequences. In order to track the variations in the angle of departure (AoD) and angle of arrival (AoA), an antenna array is configured into two different subarrays, which allow us to estimate AoD/AoA based on the phase difference between the two subarrays in the received signals. A pilot sequence is designed such that it can distinguish between different subarrays and track a beam for a mobile station in multicell multiuser environments. Two different implementation methods of AoD/AoA estimation are proposed and compared with computer simulations.

A pilot-based beam-tracking technique is proposed for OFDM-based millimeter-wave cellular systems. In order to track the variations in the angle of departure (AoD) and angle of arrival (AoA), an antenna array is configured into two different subarrays which allow us to estimate AoD/AoA based on the phase difference between the two subarrays in the received signals. A pilot sequence is designed such that it can distinguish between different subarrays and track a beam for a mobile station in multi-cell multi-user environments.

The processing time for beam training in millimeter-wave (mmWave) cellular systems can be significantly reduced by a code division multiplexing (CDM)-based technique, where multiple beams are transmitted simultaneously with their corresponding Tx beam IDs (BIDs) in the preamble. However, mmWave cellular systems with CDM-based preambles require a large number of cell IDs (CIDs) and BIDs, and a high computational complexity for CID and BID (CBID) searches. In this paper, a new preamble design technique that can increase the number of CBIDs significantly is proposed, using a preamble sequence constructed by a combination of two Zadoff-Chu (ZC) sequences. An efficient technique for the CBID detection is also described for the proposed preamble. It is shown by simulations using a simple model of an mmWave cellular system that the proposed technique can obtain a significant reduction in the complexity of the CBID detection without a noticeable performance degradation, compared to the previous technique.

In this paper, a new preamble design technique that can increase the number of cell IDs (CIDs) and beam IDs (BIDs) significantly is proposed, using a preamble sequence constructed by a combination of two Zadoff-Chu (ZC) sequences. An efficient technique for the CID and BID (CBID) detection is also described for the proposed preamble. It is shown by simulations using a simple model of an mmWave cellular system that the proposed technique can obtain a significant reduction in the complexity of the CBID detection without a noticeable performance degradation, compared to the previous technique.

In this paper, a new handover procedure for OFDM-based multi-hop relay systems is proposed to reduce handover overhead by distinguishing an inter-cell handover event from an intra-cell handover event at the level of the physical layer using a preamble with a hierarchical design. A Subcell ID concept used to identify relay station in a cell is proposed in the hierarchical design that works in conjunction with the existing Cell ID used to identify base station. The proposed handover procedure can simplify the scanning procedure and skip/simplify the network re-entry procedure, resulting in a significant reduction in handover overhead.

In this letter, a recursive ICI cancellation technique for cooperative space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) systems is proposed to mitigate the intercarrier interference (ICI) caused by multiple carrier frequency offsets (CFOs) in cooperative transmission. The proposed technique is shown to mitigate the noise enhancement effect on the STBC-OFDM signals caused by multiple CFOs, and to be effective in reducing ICI especially when the system has a large FFT size and multiple CFOs.

In a cellular system, efficient power saving techniques for a mobile station (MS) are necessary because of its inherently limited battery capacity. The paging indicator (PI) transmission scheme in CDMA cellular systems is known to be an effective power saving strategy. However, in OFDM-based cellular systems, the MS has to operate FFT for PI symbol detection, resulting in a significant power consumption. In this letter, a PI transmission technique with reduced power consumption using the preamble in OFDM-based cellular systems, especially for mobile WiMAX systems, is proposed for the MS under power saving mode. Simulations indicate a 30-50% power saving from our proposed PI transmission technique, at the expense of a slight increase in paging response delay.

In this paper, we propose a new digital duplexing scheme, called synchronous digital duplexing (SDD), which can increase data efficiency and flexibility of resource by transmitting uplink signal and downlink signal simultaneously in wireless communication. In order to transmit uplink and downlink signals simultaneously, the proposed SDD obtains mutual information among subscriber stations (SSs) with a mutual ranging symbol. This information is used for selection of transmission time, decision on cyclic suffix (CS) insertion, determination of CS length, and re-establishment of FFT starting point.

In this letter, the effect of a propagation delay resulting from the use of an OFDM system with a transparent mobile multi-hop relay (MMR) is initially analyzed. Then, a least square (LS) channel estimation technique for the OFDM system with throughput enhancement (TE) MMR or cooperative MMR is proposed. It is demonstrated by computer simulation that the proposed LS channel estimation technique for OFDM systems with transparent MMR is superior to the conventional technique in terms of mean square error (MSE) and bit error rate (BER).

In this paper, a new handover technique for OFDM-based mobile mesh networks (MMNs) is proposed to reduce the handover overhead by distinguishing an inter-cluster handover event from an intra-cluster handover event using a hierarchical design concept of a preamble. The proposed handover procedure can simplify scanning procedure and skip/simplify network entry procedure, resulting in a significant reduction of handover overhead.

In a cellular system, efficient power saving techniques for a mobile station (MS) are necessary because of its inherently limited battery of MS. The paging indicator (PI) transmission scheme in CDMA cellular systems is known to be an effective power saving strategy. However, in OFDM-based cellular systems, the MS has to operate FFT for PI symbol detection, resulting in a significant power consumption. In this paper, a reduced power consumptive PI transmission technique using the preamble in OFDM-based cellular systems, especially for mobile WiMAX systems, is proposed for the MS under power saving mode. Simulations indicate a 30∼50% power saving result from our proposed PI transmission technique, at the expense of a slight increase of paging response delay.

In this paper, the effect of a propagation delay resulting from the use of an OFDM system with a transparent mobile multi-hop relay (MMR) is initially analyzed. Additionally, a channel estimation technique for the OFDM system with throughput enhancement (TE) MMR or cooperative MMR is proposed. Secondly, the effect of a Doppler shift caused by a mobile MS is analyzed and a compensation technique for the carrier frequency offset (CFO) in the OFDM system with TE MMR is proposed.

In this paper, an iterative decoding technique for cooperative space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) systems is proposed to mitigate the intercarrier interference (ICI) caused by multiple carrier frequency offsets (CFOs) in cooperative transmission. The proposed iterative decoding technique is shown to mitigate the noise enhancement effect on the STBC-OFDM signals caused by multiple CFOs, and be effective in reducing ICI especially when the system has a large FFT size and a large multiple CFOs.