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An open-loop precoder, which does not require any feedback channel information, is proposed to enhance the performance of spatial multiplexing in transmit-correlated multi-input–multi-output (MIMO) channels. Different from previous open-loop precoders based only on the amplitude parameter of channel correlation, the proposed precoder, designed with both amplitude and phase parameters, is more suitable for practical environments. The proposed scheme performs better than previous schemes in MIMO channels with random phase correlation.

We consider a hybrid direct-sequence frequency-hopped (DS/FH) code division multiple access (CDMA) communication system, where the transmission power and data rate are adapted relative to the channel variation. Instead of random frequency hopping, hopping pattern is adaptively adjusted to obtain the maximum channel gain among available frequency slots. Transmission power and/or data rate are also adapted such that a target transmission quality is maintained. It is shown that the proposed scheme provides a higher average data rate than pure DS/CDMA with power and rate adaptations, subject to the identical average transmission power and bandwidth constraints.

An iterative noncoherent receiver that does not exploit channel statistics is proposed for the detection of differential space-time block codes (DSTBCs) in fast-fading channels. For robustness to channel variation, it employs an iterative architecture incorporating a priori information from a channel decoder in the Viterbi detection. Computer simulation shows that the performance of this iterative scheme is comparable with that of ideal coherent detection as the number of iterations increases.

We consider power and rate adaptations 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 propose, as a power allocation strategy in the frequency domain, to transmit 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 adopt channel inversion power adaptation in the time domain, where the target user's received power level maintains at a fixed value. We also investigate 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 power and rate adaptations with fixed average transmission power, and show that power adaptation in both the frequency and the time domains or combined power adaptation in the frequency domain and rate adaptation in the time domain make significant performance improvement over the power adaptation in the frequency domain only. We also compare the performance of the proposed power and rate adaptation schemes in MC-DS/CDMA systems to that of power and rate adapted single carrier DS/CDMA systems with RAKE receiver.

We consider transmission power adaptations in multicarrier code-division multiple-access communications, where the transmission power is adapted in frequency and/or time domain in response to channel variations. With frequency-domain power adaptation, we propose to allocate the transmission power over N' (1 les N' les N) strongest subcarriers (which have the highest channel gains) rather than over all possible N subcarriers. We examine the effect of choosing N' on the performance of the frequency-domain power adaptation scheme and find that there exists an optimal N' that minimizes the average bit error rate (BER). In the time domain, we consider adapting the transmission power so that the desired signal strength at the receiver output remains at a fixed level. We analyze the BER performances of frequency-, time-, and combined frequency-time-domain power adaptations with an average transmission power constraint. Our results show that the frequency-and time-domain power adaptation schemes outperform the nonadaptive (i.e., constant uniform power over all subcarriers) scheme, particularly for the noise-and interference-limited regions, respectively. The combined frequency-time-domain power adaptation is shown to have a significant performance gain over the power adaptation in only the frequency or time domain, as well as over the nonadaptive scheme. We also present numerical results to investigate the effect of channel estimation error on the performance of the proposed power adaptation schemes.

The frequency hopping (FH) based ultra-wideband (UWB) communication system divides its available frequency spectrum into several sub-bands, which leads to inherent disparities between carrier frequencies of each sub-band. Since the propagation loss is proportional to the square of the transmission frequency, the propagation loss on the sub-band having the highest carrier frequency is much larger than that on the sub-band having the lowest carrier frequency, resulting in disparities between received signal powers on each sub-band, which in turn leads to a bit error rate (BER) degradation in the FH UWB system. In this paper we propose an adaptive receiver for FH based UWB communications, where the integration time is adaptively adjusted relative to the hopping carrier frequency, which reduces the disparity between the received signal energies on each sub-band. Such compensation for lower received powers on sub-bands having higher carrier frequency leads to an improvement on the total average BER of the entire FH UWB communication system. We analyze the performance of the proposed reception scheme in Nakagami fading channels, and it is shown that the performance gain provided by the proposed receiver is more significant as the Nakagami fading index m increases (i.e., better channel conditions).

In this paper, we propose an adaptive data transmission scheme for DS/CDMA packet radio communication systems in bandlimited indoor multipath fading channels. We first analyze the relationship between the code rate and the processing gain (defined as the number of chips per coded bit) in maximizing the normalized throughput in connection with the channel state of the indoor multipath fading channels. One observation made is that the maximum throughput with BPSK modulation is attained when the code rate is chosen as low as possible irrespective of the channel state, and the processing gain is increased (decreased) as the channel becomes worse (better). The other observation made is that when DPSK modulation is employed, there exists an optimal combination of the code rate and the processing gain in maximizing the normalized throughput for each channel state. Based on these observations, we propose to adapt the processing gain and/or code rate according to the fading conditions in order to maximize the normalized throughput. We analyze the performance of the proposed scheme and compare it with the non-adaptive data transmission scheme. Our results show that the adaptive transmission scheme yields a significant performance improvement over the nonadaptive scheme, and increasing the adaptation level is more effective as the channel gets worse, but the 3-state adaptation seems to be practically optimum.

We consider a hybrid direct-sequence frequency-hopped (DS/FH) code division multiple access (CDMA) communication system, where the transmission power, data rate (i.e. spreading gain), and hopping frequency are adapted relative to the channel variations. Instead of random frequency hopping, hopping pattern is adaptively adjusted to obtain the maximum channel gain among available frequency slots. Transmission power and/or data rate are also adapted such that a target transmission quality is maintained. It is shown that the proposed scheme provides a higher average data rate than pure DS/CDMA with power and rate adaptations, subject to the identical bandwidth and average transmission power constraints.

We investigate the optimal chip rate of power or rate adapted direct-sequence code division multiple access (DS/CDMA) communication systems in Nakagami fading channels. We find that the optimal chip rate that maximizes the spectral efficiency depends upon both the channel parameters, such as multipath intensity profile (MIP) and line-of-sight (LOS) component, and the adaptation scheme itself. With the rate adaptation, the optimal chip rate is less than 1/T m irrespective of the channel parameters, where T m is multipath delay spread. This indicates that with the rate adaptation, correlation receiver achieves higher spectral efficiency than RAKE receiver. With the power adaptation, however, the optimal chip rate and the corresponding number of tabs in RAKE receiver are sensitive to MIP and LOS component.

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.

We consider combined rate and power adaptations in direct-sequence code-division multiple-access communications, where the transmission power and the data rate are adapted relative to channel variations. We discuss the power gain that the combined adaptations provide over power adaptation. Then, we consider an integrated voice and data transmission system that offers a constant bit rate voice service, using power adaptation and a variable bit rate data service with rate adaptation. We present an expression for the required average transmission power of each traffic type having different quality-of-service specifications and discuss the capacity gain over power adaptation for voice and data.

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.

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 study the optimal chip rate of adaptive power and rate direct-sequence code division multiple access (DS/CDMA) communication systems in Nakagami fading channels. We find that the optimal chip rate that maximizes the spectral efficiency depends upon both the channel parameters, such as multipath intensity profile (MIP) and line-of-sight (LOS) component, and the adaptation scheme itself. With the rate adaptation, the optimal chip rate is less than 1/T m irrespective of the channel parameters, where T m is multipath delay spread. With the power adaptation, however, the optimal chip rate and the corresponding number of tabs in RAKE receiver are function of the MIP and the LOS component.

We investigate the impact of feedback signaling on the performance of adaptive double space-time transmit diversity (STTD) systems in wireless fading channels. We consider four transmit antennas with two space-time block encoders, where at least two receive antennas are required for detection. In adaptive double STTD systems, transmit power weights are adaptively adjusted relative to channel fading based on the feedback information. In our work, the optimum power weights are determined by a numerical search, 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. Our results also show that in adaptive double STTD with asymmetric modulations, the number of feedback bits can be reduced with little performance degradations.

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 .

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

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.