• Journal of the Korea Society for Simulation
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• v.20 no.4
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• pp.81-89
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• 2011

Underwater wireless network can be useful in various fields such as underwater environment observation, catastrophe prevention, ocean resources exploration, ocean organism research, and vessel sinking exploration. We need to develop an efficient design for Medium Access Control (MAC) protocol to improve multiple data communication in underwater environment. Aloha protocol is one of the basic and simple protocols, but it has disadvantage such as collision occurs oftenly in communication. If there is collision occured in RF communication, problem can be solved by re-sending the data, but using low frequency in underwater, the re-transmission has difficulties due to slow bit-rate. So, Time Division Multiple Access (TDMA) based MAC protocol is going to be used to avoid collisions, but if there is no data to send in existing TDMA, time slot should not be used. Therefore, this paper proposes dynamic TDMA protocol mechanism with reducing the time slots by sending short "I Have No Data" (IHND) message, if there is no data to transmit. Also, this paper presents mathematic analysis model in relation to data throughput, channel efficiency and verifies performance superiority by comparing the existing TDMA protocols.

• The KIPS Transactions:PartC
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• v.9C no.1
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• pp.73-78
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• 2002

In WATM networks, in order to perform dynamic slot allocation required slots of mobile terminals are estimated based on DP (Dynamic Parameter) reflecting characteristics of traffic. In VBR (Variable Bit Rate) traffic, slot allocation is done at MT considering both time-dependent characteristics and QoS (Quality of Service) requirements. In this paper, DPs-buffer state information and buffer state change-are transmitted through in-band signaling. BS (Base Station) performs dynamic slot allocation considering traffic characteristics of each MT (Mobile Terminal), in other words, buffer state information informs the potentiality of 'buffer full state'to BS if MT buffer is over the specific threshold value and buffer state change notifies change in buffer state of incoming cells to MT. If buffer state information is equal to 'low (more than threshold)' and 'abrupt increase' it generates 'buffer full' state cell transmission delay or cell loss might occur. At this time BS should assign additional slots to MT, and then MT consumes cells in its buffer. In simulation, the proposed scheme shows better performance in cell delay and loss than EPSA (Estimation-Prorated Slot Assignment) in-band scheme.

• International Journal of Computer Science & Network Security
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• v.23 no.10
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• pp.1-10
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• 2023

This paper proposes a method to extend Inter-Carrier Interference (ICI) canceling Orthogonal Frequency Division Multiplexing (OFDM) receivers for 5G mobile systems to spatial multiplexing 2×2 MIMO (Multiple Input Multiple Output) systems to support high-speed ground transportation services by linear motor cars traveling at 500 km/h. In Japan, linear-motor high-speed ground transportation service is scheduled to begin in 2027. To expand the coverage area of base stations, 5G mobile systems in high-speed moving trains will have multiple base station antennas transmitting the same downlink (DL) signal, forming an expanded cell size along the train rails. 5G terminals in a fast-moving train can cause the forward and backward antenna signals to be Doppler-shifted in opposite directions, so the receiver in the train may have trouble estimating the exact channel transfer function (CTF) for demodulation. A receiver in such high-speed train sees the transmission channel which is composed of multiple Doppler-shifted propagation paths. Then, a loss of sub-carrier orthogonality due to Doppler-spread channels causes ICI. The ICI Canceller is realized by the following three steps. First, using the Demodulation Reference Symbol (DMRS) pilot signals, it analyzes three parameters such as attenuation, relative delay, and Doppler-shift of each multi-path component. Secondly, based on the sets of three parameters, Channel Transfer Function (CTF) of sender sub-carrier number n to receiver sub-carrier number l is generated. In case of n≠l, the CTF corresponds to ICI factor. Thirdly, since ICI factor is obtained, by applying ICI reverse operation by Multi-Tap Equalizer, ICI canceling can be realized. ICI canceling performance has been simulated assuming severe channel condition such as 500 km/h, 8 path reverse Doppler Shift for QPSK, 16QAM, 64QAM and 256QAM modulations. In particular, 2×2MIMO QPSK and 16QAM modulation schemes, BER (Bit Error Rate) improvement was observed when the number of taps in the multi-tap equalizer was set to 31 or more taps, at a moving speed of 500 km/h and in an 8-pass reverse doppler shift environment.