This paper presents a new CLSP/CC (Channel Load Sensing Protocol/Channel Clearance) in order to obtain better performances in spread slotted ALOHA networks. This protocol can decrease a wasteful utilization of channel by prohibiting continuous transmission of the packets destroyed in previous slot, with little added network complexities. For channel clearance technique, hub station broadcasts NAK to all mobile stations when the number of packets transmitted in a slot exceeds the channel capacity, and mobile stations cease own packet transmissions at NAK broadcast. The performances of the proposed CLSP/CC are obtained and compared with those of conventional CLSP by simulations. The performances include network throughput, average processing time and the number of mobile stations in backlogged state. As simulation results, the proposed CLSP/CC outperforms conventional CLSP, especially in highly offered load environment. Also as the proposed CLSP/CC obtain maximum throughputs at higher offered loads and the larger number of mobile stations in backlogged state than the conventional CLSP, it is found that our protocol can support more mobile stations and higher occurrence probabilities.

In this letter, a blind frequency offset estimation algorithm is proposed for OFDM systems. The proposed method exploits the intrinsic phase shift between neighboring samples in a single OFDM symbol, incurred by a frequency offset. The proposed algorithm minimizes a novel cost function, which is the squared error of the candidate frequency offset compensated signals from two different observation windows. Also, the solution of the proposed algorithm is given by an explicit equation, which does not require any iterative calculation. It is shown that the performance of the proposed method is better than those of the conventional methods, especially in the presence of multipath channels. This is due to the fact that the proposed method is insensitive to inter-symbol interference (ISI).

In a multi-bandwidth CDMA system, the performance of a multiple order selection combining rake receiver is analyzed according to the spreading bandwidth of the system and the delay spread of a Rayleigh fading channel. The results for various channel environments indicate a tradeoff between total received signal energy and multipath fading immunity. Increasing the occupied bandwidth of the system (wide-bandwidth spreading) gives better performance for small delay spread environments, while gathering more energy (narrow-bandwidth spreading) gives better performance for large delay spread environments. It is also shown that the performance difference between low and high order selection combining grows larger as the spreading bandwidth is increased. It is noted that performance degrades by increasing the bandwidth above a certain point and the optimum spreading bandwidth for each channel environment decreases as the delay spread of the channel increases.

As noncoherent direct sequence code-division multiple-access (DS-CDMA) mobile satellite communications, two typical transmission schemes are compared; one is a quasi-synchronous differential BPSK (QS-DBPSK) where orthogonal signals are used for reducing the multiple access interference and the other is M-ary orthogonal signaling (MOS) scheme where orthogonal signals are used for exploiting more efficient modulation. The performances are evaluated in additive white Gaussian noise (AWGN) and shadowed Rician fading (SRF) channels and the effects of timing misalignments in the QS-DBPSK system and the amount of Doppler shifts of a SRF channel are investigated. The results show that MOS much outperforms QS-DBPSK in the region of low system loading up to about 50% and a precise chip synchronization is required for QS-DBPSK. In a SRF channel, it is also shown that QS-DBPSK much outperforms MOS in a slow fading channel but MOS has a performance gain against the large Doppler shift.

In this letter, the probability of error performance improvements by rain fade countermeasure techniques is analyzed in a Ka-band geostationary satellite communication system using synchronous CDMA scheme, when power control and macrodiversity are used as rain fade countermeasures. Numerical results show that the composite power control plus macroscopic selection diversity system is better than that of utilizing only one technique as a countermeasure of rain fading.

A new stopping criterion of turbo decoding based on the maximum a posteriori (MAP) decoder is proposed and applied to the turbo processing system. The new criterion suggested as the combined parity check (CPC) counts the sign difference between combined parity bits and the re-encode parity bits determined from decoded information bits. The CPC requires decoded parity bits and a turbo encoder but it performs better in terms of the bit error rate and the average number of iterations in the turbo processing system.

In this letter, an algorithm that estimates one of the most important channel parameters, maximum Doppler frequency, fD, is proposed. The algorithm uses phase variations of received pilot signals, which is strongly related with fD in a fading environment. In addition, a phase variation measurement method for binary phase shift keying (BPSK) modulated signals is also proposed and it makes possible to estimate fD from BPSK modulated information signals as well as unmodulated pilot signals. The results show that the proposed algorithm is very simple and shows good performance over wide Doppler frequency range.

In this letter, a novel blind CFO estimation algorithm for the uplink of an OFDMA system is proposed. The proposed method exploits the inherent redundant information in OFDMA symbols and does not require additional pilot or preamble overhead. Since it is a post-FFT estimator, it does not use filter banks to separate the desired user's signal from the others in the time domain. Hence, the subcarriers of a certain user are not restricted to be clustered in the frequency domain. Therefore, the proposed estimator can be applied to OFDMA systems with an arbitrary subcarrier assignment over the entire bandwidth, including IEEE 802.16e, to obtain sufficient frequency diversity in a frequency selective fading channel. The proposed method can be efficiently used for continuous tracking of all active users' CFOs only with two FFT windows within a single OFDM symbol. From simulation results, the performance of the proposed scheme is shown better than that uses preamble symbols.

We propose an enhanced packet access scheme for IMT-2000/UMTS random access channel (RACH). In the proposed scheme, 2-level preamble detection threshold and 2-level message transmission power are used to mitigate the power imbalance in RACH. Simulation results demonstrate that the proposed scheme improves the interference characteristics of the conventional RACH and makes wider range of the detection threshold available.

This paper introduces an improved multistage parallel interference cancellation (PIC) technique that uses the reverse-link synchronous transmission technique (RLSTT) to improve the estimation of data at the initial stage. Because the subtraction of an interfering signal based on an incorrect bit decision quadruples the interference power for that signal, the relatively high decision bit error rate (BER) may lead to a poor cancellation or even a higher BER at the following stages. The RLSTT is a robust approach which takes into account the fact the tentative decision at the earlier stages is less reliable than the following stages and makes the earlier cancellation more reliable. The analysis demonstrates that a better transmission performance can be achieved by using the RLSTT at the initial stage of PIC.

To achieve high link availability at Ka-band geosynchronous satellite communication systems, an adaptive CDMA scheme as a rain fade countermeasure is proposed. The proposed adaptive CDMA scheme reserves and shares orthogonal sequences to countermeasure a rain fade. In the adaptive CDMA scheme, the base station allocates a set of orthogonal sequences to each user, and the number of sequences in a set is determined by the amount of rain attenuation. A symbol repetition technique is also used to compensate signal energy loss due to the rain fade. Our results show that when the base station has 64 free orthogonal codes, the adaptive CDMA scheme can save on its power by 4.2 dB compared to conventional CDMA scheme. The occupation rate of codes also shows that the adaptive CDMA can support multiple users simultaneously in even severe rain fade condition.

In this paper, an efficient partial incremental redundancy (P-IR) scheme is proposed for an H-ARQ using block type low density parity check (B-LDPC) codes. The performance of the proposed P-IR scheme is evaluated in an HSDPA system using IEEE 802.16e B-LDPC codes. Simulation results show that the proposed H-ARQ using IEEE 802.16e B-LDPC codes outperforms the H-ARQ using 3GPP turbo codes.

In this letter, a new rapid converging method based on orthogonalization is proposed. Our approach is to find the near-optimum coefficient values during training period, and then use them as the initial values of the LMS algorithm. The numerical results show that the rapid convergence speed of the proposed scheme does not depend on the eigenvalue spread.

A transceiver employing hierarchical constellation encodes two hierarchies with different levels of protection and selectively decodes one or both of them, resulting in constellation inconsistency of encoding and decoding. Therefore, a conventional ordered successive interference cancellation (OSIC) receiver, which restores the signals as they are transmitted, can not be compatible with the constellation inconsistency. To mitigate this problem, an OSIC detector with the individual received bit rate per data stream is first designed. To further improve the error performance, the proposed detector is modified, for which distinct criteria are used for demodulation and cancellation. It is shown that the proposed detector achieves spectrally efficient detection while guaranteeing reliable communication.

In this paper, we suggest the interference cancellation (IC) technique suitable for turbo coded code division multiple access (CDMA) systems, that merges IC processes into turbo decoding processes to improve system performance and reduce system complexity. To ensure the reliability of the temporary decision bits for cancellation, we use cyclic redundancy code (CRC) check as a measure. Prior to design turbo coded CDMA system, we first derive the optimized polynomials of low-rate turbo codes appropriate to CDMA systems. According to the simulation results with setting the processing gain (PG) to 120, the turbo coded CDMA system with the proposed IC technique can accommodate 60 users over additive white Gaussian noise (AWGN) channel when signal to noise ratio (SNR) is about 2. 5 dB and required frame error ratio (FER) is 10-2. Compared this result with the performance of single user's system, it requires only additional 1 dB SNR.

Future satellite communication systems will be developed at Ka-band (20/30 GHz) owing to the relatively wide frequency allocation and current freedom from terrestrial interference for multimedia services. A serious disadvantage of the Ka-band, however, is the very high atmospheric attenuation in rainy weather. Quasi-synchronous CDMA drastically reduces the effect of self-noise with several interesting features of CDMA for mobile communications such as flexible frequency reuse, the capability of performing soft-handover and a lower sensitivity to interference. This paper evaluates the performance of a quasi-synchronous CDMA return link for a Ka-band geostationary satellite communication system. For a fixed satellite channel whose characteristics depend on weather conditions, the signal envelope and phase for this channel is modeled as Gaussian. The bit error and outage probability, and the detection loss due to imperfect chip timing synchronization is analytically evaluated and the system capacity degradation due to the weather condition is estimated. Two cases of general and worst conditions are evaluated, in which i) rain attenuation ii) nonlinearity of transponder are considered. The two cases consist of the general case in which all users are affected by rainy weather, and the worst case in which only the user of interest, not multiple access interferers, is affected by rain attenuation. The results for the two cases of rainy weather clearly show that quasi-synchronous CDMA eases the power control requirements and has less sensitivity to imperfect power control. When dealing with the impact of the satellite transponder nonlinearity in addition to the rain attenuation, the shift of optimum amplifier operating point is shown so that [Eb/N0]sat, defined as the sum of the Eb/N0 value required to obtain a BER equal to Pb at a given output backoff (OBO) and the value of the OBO itself, tends to decrease, and higher BER impairment is given, since the rain attenuation results in the same effect as the additive input backoff (IBO) at the satellite transponder input. As the BER increases, the optimum [Eb/N0]sat and IBO decrease that result in the shift of optimum operating point.

In this paper, a slot-averaged SIR model is derived to analyze the performances of 1/N-slotted DS-CDMA packet radio networks. DS-CDMA packet radio networks can be analyzed with channel threshold model, but it is not appropriate for a 1/N-slotted access scheme since the interference level varies slot by slot during a packet transmission time. A packet in a DS-CDMA system will be channel-encoded and interleaved so that an excessive interference of some slots does not result in a corruption of a packet directly. In a 1/N-slotted DS-CDMA packet system, interleaving and channel coding can cope with the excessive interference in some slots. Proposed slot-averaged SIR model reflects this effect and is simple enough to be used in the performance analyses of various packet access control schemes. This paper verifies the slot-averaged SIR model and evaluates the throughputs of 1/N-slotted access schemes using it. This paper also compares the results with those of the conventional channel threshold model. Packet access schemes such as slotted ALOHA, slotted CLSP and slotted CLSP/CC are analyzed. The results show that many different aspects from the results of the previous works can be evaluated and this work will offer more accurate view on the behavior of 1/N-slotted DS-CDMA packet radio networks.

In this paper, we investigate a detection ordering scheme of OSIC (Ordered Successive Interference Cancellation) systems suitable for power controlled MIMO transmission. Most studies about power controlled systems have mainly focused on strategies for transmitter, while the ordering scheme optimized at open-loop system has not been modified. In a conventional ordering scheme, the ordering process is done according to the largeness and smallness relation of each sub-stream's SNR. Unlike the conventional scheme, we derive an optimized detection ordering scheme that uses proximity to the optimal SNR. Because of error propagation, our proximity based algorithm is not valid for open-loop MIMO system in many cases. An optimization problem analysis and simulation results show that the system using the proposed ordering scheme outperforms the system using the conventional ordering scheme. Furthermore, due to the nature of QR decomposition, the proposed scheme shows not only lower implementation complexity but also better BER performance compared with the conventional scheme based on pseudo-inverse.