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In this paper, power adaptation for direct-sequence code-division multiple-access communications that employs a successive interference cancellation (SIC) receiver is considered. The transmission power is adapted so that, with the channel variations, the received power levels of each user have appropriate disparities. Under the constraint of average transmission power, we consider two strategies in adjusting the disparity between received signal powers. With the first strategy, the average bit-error rate (BER) for a given user averaged over channel fading statistics is minimized, while with the other, the instantaneous BER is equal for all users. We find that the performance difference between the two strategies becomes negligible as the average transmission power or line-of-sight component increases. We also discuss the impact of appropriate disparity in received power levels on the BER performance of SIC receivers.

The performance of spatial multiplexing (SM) systems is significantly degraded in transmit correlated multiple-input multiple-output (MIMO) channels. In communication systems that are available for feedback, precoding can enhance performance by exploiting channel state information (CSI) at the transmitter. However, broadcasting systems have no reverse channel nor any CSI. Furthermore, channel mismatches in communication systems severely affect performance. For these systems, open-loop precoders that require no feedback CSI were proposed. However, an analytical precoder design method was not presented in the previous works. In this paper, we present an open-loop precoder design method for SM systems with a maximum-likelihood receiver in transmit correlated MIMO channels. We first describe how to design an open-loop precoder using a rotational matrix over the amplitude of the correlation. Our precoder design method is derived by observing the effect of correlation mismatch between the feedback CSI and the actual channel condition on a minimum Euclidean distance-based precoder. Next, we present a method to design an open-loop precoder that takes into account the impact of the phase of the correlation. For this precoder, a phase rotation matrix is added on the rotational matrix, and optimum parameters are obtained by maximizing the worst-case minimum distance for the phase of the correlation. The simulation results illustrate the validity of proposed open-loop precoder in transmit correlated MIMO channels.

In this paper, we consider a layered decoding algorithm for a performance improvement of LDPC. The proposed algorithm is based on a serial scheduling algorithm for the layered decoding of LDPC in which we consider check node degree, variable node degree, and the edge connection between the each node. Simulation results show that the proposed method improves about 0.2dB SNR gain compared with random schedule in a layered decoding. Especially when the maximum number of iteration and the code rate are low, the performance gain gets larger. This is desirable to the system which requires low per consumption and low latency such as WPAN system.

We consider power adaptation for constant rate transmission in Nakagami fading channels under the assumption that channel state information is provided at both the transmitter and the receiver. We derive average BER expression for channel inversion (CI) with peak and average transmission power constraints, and compare it to optimal power adaptation policy in minimizing the average BER. We find that the CI provides nearly optimal performance as the line-of-sight component and/or available average transmission power increase, and there exists an optimal peak-to-average power ratio for each channel parameters when the CI is employed.

Recently mobile communication systems such as 3GPP LTE, IEEE802.16e have been growing interest in the femto-cell systems for increase of data rates and enhanced call quality. In a femto-cell system, frequent handover among femto-cells may happen. Also, the probability of handover failure may increase during the frequent handover. In this paper, we propose a load-balanced handover with adaptive hysteresis in a femto-cell system considering channel allocation status of target base station. The simulation results show that the proposed schemes reduce the ping-pong rate and improve the MS’s handover-related performance in terms of handover failure probability compared with the conventional handover method in a femto-cell system.

The optimal space-time domain power adaptation that minimizes the average bit error rate in a transmit antenna diversity system is analytically derived subject to a fixed average power constraint. Our results show that the proposed space-time domain power adaptation scheme has a significant performance gain over the conventional power adaptation in the space domain only.

We examine efficient feedback signaling schemes in adaptive double space-time transmit diversity (STTD) systems over Rayleigh fading channels. In adaptive double STTD systems, transmit power weights are adaptively adjusted relative to channel fading based on the feedback information. In our paper, the optimum power weights are determined, which are functions of the number of feedback bits and the modulation size employed. We apply symmetric and asymmetric modulation methods in each space-time block encoder in cooperation with adaptive antenna weights. It is shown that the performance of adaptive double STTD with asymmetric modulations depends on the number of available feedback bits, while the same system with symmetric modulations has a relatively robust performance to variable feedback bits.

We consider combined rate and power adaptation schemes in direct-sequence code division multiple access (DS/CDMA) communications, where the transmission power and the data rate are jointly adapted relative to channel variations. We propose to allocate the transmission power proportional to \(G_i^p\), where G i is the channel power gain and p is a real number, and to adapt the data rate simultaneously so that a desired transmission quality is maintained. We analyze the average data rate of the proposed adaptation scheme, and show that the proposed joint adaptation scheme provides up to 3.7–5.4 and 1.5–3.2 dB of power gains over power only adaptation and rate only adaptation, respectively, when peak-to-average power ratio is 2 to 10, in a Rayleigh fading channel with δ = 0.5, where δ is the decaying exponent of multipath intensity profile (MIP).

We propose a multicarrier code-division multiple-access (MC-CDMA) system with adaptive transmission power allocation. The transmission power is allocated over M′ strongest subcarriers among M available ones. Using only M′ subcarriers of which channel gains are relatively large indicates that the transmission power to maintain the desired received signal strength can be reduced, but decreasing M′ also lead to a reduction of effective spreading gain to mitigate the multiple access interference. We analyze the trade-off between M′ and effective spreading gain, and find the optimal M′ that minimizes the average bit error rate (BER). Our results show that the BER can be significantly reduced if M′ is properly chosen, and transmit signals over only the strongest subcarrier (M′=1) is the best especially for low SNR regions.

We investigate channel inversion power adaptation in Rayleigh fading channels under the assumption that channel state information is provided at both the transmitter and the receiver. We derive average bit error rate (BER) for the channel inversion subject to fixed peak and average transmission power constraints, and compare it to optimal power adaptation that minimizes the average BER with the same constraints. It is found that the performance difference between the channel inversion and the optimal adaptation becomes negligibly small as available average transmission power increases and/or peak-to-average power ratio decreases. At BER of 10− 3, the channel inversion is only 0.2 dB inferior to the optimal adaptation when peak-to-average power ratio is 10.

We present a joint power and rate adaptation scheme in multicarrier (MC) direct-sequence code-division multiple-access (DS/CDMA) communications under the assumption that channel state information is provided at both the transmitter and the receiver. We consider, as a power allocation strategy in the frequency domain, transmitting each user's DS waveforms over the user's sub-band with the largest channel gain, rather than transmitting identical DS waveforms over all sub-bands. We then consider rate adaptation in the time domain, where the data rate is adapted such that a desired transmission quality is maintained.We analyze the BER performance of the proposed joint power and rate adaptation scheme with fixed average transmission power, and compare the performance to that of power and rate adapted single carrier DS/CDMA systems with RAKE receiver.

Recently, cooperative diversity has been introduced to wireless adhoc networks for power saving and for diversity gain over fading channel. The space diversity gain is obtained from the multiple copies of signals from the several cooperating relays. Consequently this diversity gain saves the power of the networks. However, a receiver generally has fixed number of branches at a destination. Therefore the excess number of transmit relays does not contribute to the performance improvement, and only waste the network power and radio resources. Therefore, we propose a method which is adjusting the average number of transmit relays, and controlling the transmit power of the relays locally to satisfy the required performance in double opportunistic transmit (DOT) relaying system. Numerical analysis shows that average number of transmit relays decreases with the increase of threshold and vice versa. And we noticed that the transmit power of relays for the required system performance can be controlled locally.

In this paper, we consider a layered decoding algorithm for a performance improvement of LDPC. The proposed algorithm is based on a serial scheduling algorithm for the layered decoding of LDPC in which we consider check node degree, variable node degree, and the edge connection between the each node. Simulation results show that the proposed method improves about 0.2dB SNR gain compared with random schedule in a layered decoding. Especially when the number of iteration is low, the performance gain gets larger, which case is applicable to the WPAN system which requires low power consumption and low latency.

Recently mobile communication systems such as 3GPP LTE, IEEE802.16e have been growing interest in the femto-cell systems for increase of data rates and enhanced call quality. Cell management method for handover in a femto-cell system is very important issue because it includes heavy control management information of macro-cell and a large number of femto-cells and can be a burden to a system. In this paper, we propose an adaptive construction of control management message (MOB_NBR-ADV) and cell management method in a femto-cell system. The proposed scheme provides fast scanning and efficient handover by means of preselecting the candidate target femto-cells. The simulation results show that the proposed schemes improve the MS’s handover-related performance in terms of scanning power and scanning time compared with the conventional management scheme of femto-cell systems.

The Digital Video Broadcasting second generation Terrestrial (DVB-T2) standard purposes to provide high definition digital video and high rate services for the same spectrum allocation of the current Digital Video Broadcasting Terrestrial (DVB-T) standard. The implementation guideline is provided by the DVB-T2 standardization community, the Digital Video Broadcasting (DVB) Project. The DVB Project is requesting the many submission technologies which could be considered as candidates for a second generation terrestrial transmission system. The most challenging requirements introduced by the DVB-T2 specification are an Fast Fourier Transform (FFT) size of up to 32k samples, 256-Quadrature amplitude modulation (QAM) and Low-Density Parity-Check (LDPC) coding with a block size of 64k bits. The increasing concern in Multiple-Input Multiple-Output (MIMO) systems has given rise to numerous research activities in recent years. In this paper, we propose the MIMO architecture and coded cooperative MIMO transmission scheme for DVB-T2 system. The simulation result of the proposal MIMO architecture in DVB-T2 system shows a definite performance improvement for coded cooperative MIMO system.

Conventional opportunistic transmit (COT) relaying system does not consider the received SNR between R-D path. For that reason, it can't guarantee all the received SNRs exceed the threshold at destination. In this paper, we propose double opportunistic transmission relay (DOT) system which considers the received SNR both from S-R and R-D paths. It is noticed that the proposed system can be reduced to 6.9, 20.9, 32.4, and 41.4 % of power saving to that of COT relay system for K = 3, 5, 7, and 9, respectively at a given condition of P out =1×10−3 and γ SR /Γ = 30 dB. And the performance of DOT system is superior than that of COT system for the identical number of active transmit relays under the same condition of γ RD /Γ RD .

In this paper, power minimization for Poisson distributed random relay networks in Rayleigh fading channels is considered. We investigate two power allocation methods; one is a minimum power allocation (MPA) strategy and the other is an equal outage power allocation (EOPA) strategy. We analyze the total transmit power of two allocation methods under the given end-to-end outage probability constraint. Our results show that the MPA achieves more power saving than EOPA, and the power saving is more significant as the number of relay nodes increases.

We propose new routing scheme, Cooperative Diversity-based Routing (CDR) which utilize the cooperative space diversity for power saving and for performance enhancement of wireless ad-hoc networks. The end-to-end performance of the proposed routing, CDR, is analyzed based on the Haenggi's link model. The improved performance is compared with Multi-hop Relay Routing (MRR) by analytical methods. When the required outage probability is 1times10-3 at the destination node in ad-hoc networks with 7 nodes, we noticed that each node can save power consumption by 21.5 dB in average, by using our proposed CDR compared to MRR.

In a closed-loop transmit antenna diversity system, a closed- form expression for the optimal transmitter weights that minimize the average bit error rate (BER) is derived with fixed average and peak transmit power constraints. It is shown that the peak power limitation degrades the average BER performance more significantly as the available average power and/or the number of transmit antennas increase.

We propose a combined power and rate adaptation scheme in direct-sequence code division multiple access (DS/CDMA) communications over Nakagami fading channels. The transmission power proportional to Gi p is allocated to the data transmission of user i, where Gi is the channel gain for user i, and p is a real number, and the data rate (i.e. spreading gain) is jointly adapted so that a desired transmission quality is maintained. We analyze the average data rate of the proposed adaptation scheme subject to fixed average and peak transmission power constraints. Our results show that the proposed joint adaptation scheme has considerable performance improvements over power only adaptation and rate only adaptation for a range of system parameters.

In this paper we propose a new approach for the design of adaptive modulation and coding (AMC) for MIMOOFDM wireless communications. We justify a possible conflict on the joint extraction of diversity and adaptation benefits of a MIMO-OFDM channel known at the transmitter. The proposed scheme avoids this conflict by adapting as accurately as possible given the side information while using diversity to combat remaining uncertainty. This is accomplished by channel parallelization, sorting and grouping into modulation modes. A new power allocation strategy, named WF/CI, is also introduced and showed to perform better than conventional water-filling in a practical setting.

The effect of channel estimation errors and fading channel correlation on closed-loop transmit diversity (CTD) systems is investigated in frequency flat Rayleigh fading channels. A bit error rate expression for the CTD system is analytically derived as a function of the channel correlation coefficient, feedback delay, and fading index. Numerical results show that the performance degradation caused by the estimation errors is more severe than that caused by the fading correlation. It is interpreted that the performance of the CTD system is more sensitive to the beamforming error rather than to the fading correlation.

We investigate the impact of feedback signaling on an adaptive space-time transmit diversity (STTD) scheme for wireless mobile communication systems. 4 transmit antennas are considered with 2 space-time block encoders, where at least 2 receive antennas are required for detection. In adaptive STTD systems, transmit power weights are adaptively allocated to all antennas, based on the feedback information. Using the same feedback information, asymmetric modulation methods can then be applied to each encoder in accordance with antenna weights. We show that depending on the number of feedback bits available, the performance of adaptive STTD with asymmetric modulations varies, while the same system with symmetric modulations has little gain by variable feedback bits. The optimum weights are determined for adaptation by numerical search, and are different according to the degree of feedback loading. Simulation results show that in adaptive STTD with asymmetric modulations the number of feedback bits can be reduced without performance degradation.

In orthogonal frequency division multiple access (OFDMA) systems, the cyclic prefix (CP) length needs to be no less than the longest channel delay spread of many users, reducing bandwidth efficiency more significantly than in single user OFDM systems. We propose a novel receiver for OFDMA systems with an insufficient CP, exploiting the unused subchannels for a user. With the aid of computer simulations, we demonstrate that the proposed equalization method is superior to the conventional equalization method using a time domain channel shortening filter in terms of the achievable SNR and the error performance

In this paper, power adaptation for a transmit antenna diversity system is considered in Rayleigh fading channels. The optimal space-time domain power adaptation that minimizes the average bit error rate is analytically derived subject to a fixed average power constraint. Our results show that the proposed space-time domain power adaptation scheme has a significant performance gain over the conventional power adaptation in the space domain only.

We analyze the performance of closed-loop transmit diversity systems in the presence of feedback delay and channel estimation error in Rayleigh time-selective flat fading. We formulate both the feedback delay and the channel estimation error as a function of Doppler spread, and derive the average bit error probability under such imperfect channel information. It is shown that the channel estimation error is more sensitive to system performance than feedback delay in slow fading environments.

Two basic problems which the wireless mobile communication system designers are faced with are multipath fading of the radio link and interference from other users. The spread spectrum technique is known to be effective in mitigating multipath fading and interference. We consider a convolutionally coded DS/SSMA system in bandlimited slowly Rayleigh fading channel. We examine the relationship between the code rate and the processing gain (defined as the number of chips per coded bit) in maximizing the throughput in connection with the average signal-to-noise ratio (SNR). It is found that the maximum throughput is attained when the code rate is chosen as low as possible irrespective of the SNR, and the processing gain is increased (decreased) as the SNR decreases (increases). Based on this observation, we propose an adaptive transmission scheme that changes the effective processing gain according to an estimate of the SNR. It is found that a significant improvement can be obtained by using the proposed scheme over the nonadaptive scheme. The effects of the channel state estimation error and the user speed are examined.