Junwoo Jung

Chaotic Standard Map Based Frequency Hopping OFDMA for Low Probability of Intercept

In frequency hopping (FH)-OFDMA, the transmitted symbols on all subcarriers are demodulated by FFT at once at the receiver. Thus, these symbols can be easily intercepted by an adversary radiometer when the number of simultaneous transmitting users is small. In this letter, we design a chaotic standard map (CSM) based FH pattern for low probability of intercept. Through the proposed scheme, more than two symbols of the same user can be shown in the same symbol period, or none. Moreover, the order of these symbols is mixed. It makes the radiometer difficult in intercepting these symbols, and thus, it achieves a low probability of intercept. We show that the proposed scheme satisfies requirements of a desired FH pattern.

Superposition-based adaptive modulated space time block coding for MIMO-OFDM systems

We propose a superposition-based adaptive modulated STBC (SPAM-STBC) for MIMO-OFDM systems to improve adaptive modulation optimization of space time block coding (STBC). When transmit antennas have the different channel conditions, the previous adaptive modulated STBC selects the same modulation based on averaging of the multiple channel gains. If the different modulation is selected to each transmit antenna, the STBC decoding problem occurs. In this letter, we select the optimal modulation corresponding to each channel condition by the superpositioned space time encoding and decoding. Simulation shows the proposed SPAM-STBC scheme outperforms both the fixed and adaptive modulated STBC schemes by the maximum 0.407 bits/sec/Hz in terms of spectral efficiency.

A Reliable Multicast MAC Protocol Based on Spread Spectrum Technique in Wireless Ad-Hoc Networks

We propose a reliable multicast MAC protocol based on the direct-sequence code-division multiple access (DS-CDMA) scheme to enhance channel efficiency in wireless ad-hoc area networks. In conventional multicast MAC protocols based on IEEE 802.11 standards using RTS-CTS handshake exchange CTS and ACK packets consecutively for reliable transmission. Although this protocol guarantees transmission reliability by avoiding the hidden node (HN) problem, multiple CTSs and ACKs degrade network efficiency. The multicast sender must receive multiple CTSs and ACKs from all multicast receivers in its own multicast group for reliable multicast. In the proposed protocol, CTS/ACK messages of multiple receivers can be transmitted concurrently through the DS-CDMA mechanism. The proposed protocol reduces the total transmission delay due to the reduction of the overhead caused by consecutive CTSs and ACKs. The proposed protocol is thoroughly examined by simulation and theoretical methods. Results show that the proposed protocol significantly reduces the aforementioned overhead, and thus improves the performance of wireless ad-hoc networks.

A Dynamic Resource Allocation Scheme for Fairness Guarantee using Cooperative Diversity in OFDMA Systems

Orthogonal Frequency Division Multiple Access (OFDMA) is a promising technique which can provide high capacity in the wireless environments. Also cooperative communications are effectively used to achieve cooperative diversity for high reliability. In this paper, we propose the pair-based OFDMA frame structure in which cooperative users transmit their blocks in turns for achieving cooperative diversity in OFDMA systems. In order to guarantee the fairness of users we also propose a dynamic resource allocation scheme based on cooperative communications. Through the proposed scheme, user fairness can be achieved by sharing the remained capacity of users near the base station (BS) with users who are farther from the base station. For this purpose, the center-located users select their partner whose channel suffers the lowest SNR to BS among their neighbor set. Based on the partner selection results, subchannels are assigned to cooperative and non-cooperative users. Through simulations in the IEEE 802.16e system environments, it is shown that the proposed scheme outperforms the non-cooperation based scheme in terms of user fairness.

An Efficient Radio Resource Allocation Scheme for Minimum Outage Probability Using Cooperation in OFDMA Systems

Cooperation can increase the system performance by obtaining the spatial diversity. While most of the present works concentrate on the analysis of the cooperation based on the inter-user channel response and developing a scheme for higher cooperative diversity, in this paper, we focus on practical resource allocation in OFDMA systems. Since the user who uses the same center frequency can not receive the signal when transmitting, this constraint should be considered to apply the cooperation to OFDMA systems. In this paper, we propose the pair-based OFDMA frame structure that overcomes this constraint. Also in this frame structure to achieve the minimum outage probability of system, we select the best partner among the candidate neighbors and allocate the suitable subchannels to bandwidth requested users through a cooperative subchannel allocation (CSA) algorithm. In order to evaluate the proposed resource allocation scheme, we carry out simulations based on IEEE 802.16e. The simulation results show that our proposed algorithm offers smaller outage probability than one based on non-cooperative communications and we get the minimum outage probability when a threshold for selection of candidate neighbors is 10dB. We analyze that these results can be achieved by helping users located around the edge of the cell.

A Novel Hybrid ARQ Scheme Based on Shift Column Permutation Bit Interleaving for OFDM Systems

In this paper, a novel hybrid ARQ scheme based on shift column permutation bit interleaving is proposed for OFDM systems. When a packet is requested for retransmission, we can use bit interleaving based on a shift column permutation to maximize channel diversity gain, which increases the system performance. The proposed scheme improves the performance of retransmission of OFDM systems by maximizing the distance between bits so that the decoding gain is maximized. Maximizing the channel diversity gain leads to the reduced number of retransmissions, resulting in lower retransmission delay and higher throughput. Through simulations in the IEEE 802.16e system environments, the proposed hybrid ARQ scheme is compared with the other conventional hybrid ARQ schemes.

A Reliable Multicast MAC Protocol with LPD Capability for Tactical Ad-Hoc Networks

We propose a reliable multicast MAC protocol for channel efficiency and LPD (low probability of detection), a capability that enhances survivability, assisted by multicarrier direct-sequence code-division multiple access (MC-DS/CDMA) scheme in tactical ad-hoc networks. For a reliable multicast based on IEEE 802.11 standards constructing RTS-CTS handshake, a multicast sender has to receive consecutive CTSs and ACKs from all multicast receivers. These cumulative CTSs and ACKs degrade network efficiency. Furthermore, the cumulative signals of CTSs and ACKs can be easily detected by an enemy detector. In the proposed protocol, CTS/ACK messages can be transmitted concurrently through the MC-DS/CDMA mechanism. This can reduce the transmission overhead generated by consecutive CTSs and ACKs. Also, the cumulative signal strength of CTSs/ACKs can be reduced, since the mechanism can spread CTS/ACK transmission power by spreading gain of the MC-DS/CDMA code. The proposed protocol is thoroughly examined by theoretical methods and simulation. Results show that it significantly improves the performance and the survivability of tactical ad-hoc networks.

Path selection for spatial multiplexing gain with multiple relay stations in MIMO-OFDMA systems

The LOS components of the base station (BS) can provide a high SNR to mobile stations (MSs), but they cannot achieve a high spatial multiplexing gain in the MIMO-OFDMA system. The spatial multiplexing gain is obtained by degrees of freedom between MIMO antenna arrays. MIMO channels should be uncorrelated to achieve the maximum degrees of freedom, and it depends on rich scattering environments which are needed in making the channel spatially selective. For this environments, we deploy multiple relay stations (RSs) that can provide uncorrelated transmit antenna paths, called “paths” to each MS. In this paper, we select proper paths from BS and RSs to achieve the maximum degrees of freedom with the desired SNR under a LOS MIMO channel. To do this, the proposed scheme selects paths by the order of decreasing SNR among uncorrelated paths until the maximum degrees of freedom. By the simulation, we compare the proposed scheme to the system with and without RSs. We observe that the proposed scheme achieves a higher throughput of 33% and 53% for 50 MSs than the system with and without RSs, respectively.