We present a predictive closed-loop power control scheme for delay-prone burst transmission systems. The scheme has a sample-by-sample predictor compensating burst delay and a built-in channel encoder reducing power control command bit error.

To realize dynamic spectrum access (DSA), spectrum sensing is performed to detect the presence or absence of primary users (PUs). This paper proposes a sensing architecture. This architecture enables use cases such as DSA with PU detection using a single spectrum sensor and DSA with distributed sensing, such as cooperative sensing, collaborative sensing, and selective sensing. In this paper we focus on distributed sensing. These sensing schemes employ distributed spectrum sensors (DSSs) where each sensor uses energy detection (ED) in Rayleigh fading environment. To theoretically analyze the performance of the three sensing schemes, a closed-form expression for the probability of detection by ED with selective combining (SC) in Rayleigh fading environment is derived. Applying this expression to the PU detection problem, we obtain analytical models of the three sensing schemes. Analysis shows that at 5-dB signal-to-noise ratio (SNR) and with a false alarm rate of 0.004, the probability of detection is increased from 0.02 to 0.3 and 0.4, respectively, by cooperative sensing and collaborative sensing schemes using using three DSSs. Results also show that the selected sensing scheme matches the performance of the collaborative sensing scheme. Moreover, it provides a low false alarm rate.

This letter provides a study on the end-to-end performance of multi-hop wireless communication systems equipped with re-generative (decode-and-forward) relays over Rayleigh fading channels. More specifically, the probability density function (pdf) of the tightly approximated end-to-end signal-to-noise ratio (SNR) of the systems is derived. Using this approximation allows us to avoid considering all possible combinations of correct and erroneous decisions at the relays for which the end-to-end transmission is error-free. The proposed analysis offers a simple and unifying approach as well as reduces computation burden in evaluating important multi-hop system's performance metrics. Simulations are performed to verify the accuracy and to show the tightness of the theoretical analysis.

In this letter, we propose an improved block-coded modulation scheme for the Rayleigh fading channel. The proposed coding structure is constructed by providing interblock coding between adjacent blocks. The error performance of an example is simulated. The simulation results show that the proposed method can achieve large coding gain with reasonable decoding complexity.

In this letter, we derive a very accurate closed-form approximate formula for the average achievable rate of stacked orthogonal space-time block code (OSTBC) in Rayleigh fading channels. Some simulations are performed to demonstrate that the derived formula shows better agreement with Monte-Carlo simulation results than the existing closed-form approximate expressions.

In incremental relaying, the destination uses a checking system and requires a retransmission from the relay in case an error happens. After receiving the signal from the relay, the destination combines the signals from the source and the relay and performs detection. However, the combined signal is actually worse because of the erroneous signal from the source. Our scheme eliminates the detrimental signal from the source and uses only the fresh signal from the relay, resulting in a large performance improvement and reduced complexity. The symbol error rate (SER) and its upper bound are established to analyze the power allocation strategy. Simulations verify the rightness of the theoretic studies, and many benefits of cooperative ARQ schemes are revealed.

Transmit diversity systems based on orthogonal space-time block coding (OSTBC) usually suffer from rate loss and power spreading. Proper antenna selection scheme can help to more effectively utilize the transmit antennas and transmission power in such systems. In this paper, we propose a new antenna selection scheme for such systems based on the idea of antenna switching. In particular, targeting at reducing the number of pilot channels and RF chains, the transmitter now replaces the antennas with the lowest received SNR with unused ones if the output SNR of space time decoder at the receiver is below a certain threshold. With this new scheme, not only the number of pilot channels and RF chains to be implemented is decreased, the average amount of feedback information is also reduced. To analyze the performance of this scheme, we derive the exact integral closed form for the probability density function (PDF) of the received SNR. We show through numerical examples that the proposed scheme offers better performance than traditional OSTBC systems using all available transmitting antennas, with a small amount of feedback information. We also examine the effect of different antenna configuration and feedback delay.

Combining relaying and multi-input multi-output (MIMO) transmission is a generic way to overcome the channel-fading impairments. Best antenna selection is a simple but efficient MIMO method that achieves the full diversity and also serves as a lower bound reference of MIMO performance. For a dual-hop MIMO system with an ideal amplify-and-forward (AF) relaying gain and best antenna selection, we provide a probability density function (PDF) of received signal-to-noise ratio (SNR) and an analytic BER equation when using M-ary PSK in Rayleigh fading channels. The analytic result is shown to exactly match with simulated one. Furthermore, the effect of link unbalance between the first hop and the second hop, due to differences in the number of antennas deployed in both hops as well as in the average power of channel coefficients, on the BER performance is numerically investigated and the results show that the links with better balance give better performance.

In terms of outage behavior, it has been shown that incremental relaying achieves the best performance among cooperative diversity protocols such as: Amplify-And-Forward (AF), Decode-And-Forward (DF), and Selection Decode-And-Forward (SDF). Exploiting a limited feedback from the destination, incremental relaying lets the relay forward the signal received from the source whenever an error happens at the destination, then, the signals from both the source and the relay are combined to make a decision. Noticing that the signal from the source, indicated by the error, is detrimental, we propose a new scheme not using that signal but only making a decision based on the fresh signal from the relay. Large performance improvement and complexity reduction are attained as we show in the analysis and simulation results. Theoretical results are derived and shown to match with the simulation counterparts.

Multiple-input multiple-output (MIMO) maximum-SNR (max-SNR) system employs the maximum ratio combiner (MRC) at the receiver side and the maximum ratio transmitter (MRT) at the transmitter side. Its performances highly depend on MIMO channel characteristics, which vary according to both the number of antennas and their distribution between the transmitter and receiver sides. By using the decomposition of the ordered Wishart distribution in the uncorrelated Rayleigh case, we derived a closed-form expression of the largest eigenvalue probability density function (PDF). The final result yields to an expression form of the PDF where polynomials are multiplied by exponentials; it is worth underlining that, though this form had been previously observed for given couples of antennas, to date no formally-written closed-form was available in the literature for an arbitrary couple. Then, this new expression permits one to quickly and easily get the well known largest eigenvalue PDF and use it to determine the binary error probability (BEP) of the max-SNR.

The average bit error rate performances of M-ary orthogonal code shift keying (CSK) in Rician fading environments are analyzed in this letter. CSK is a digital modulation scheme that uses a code set as M-ary signals. In CSK, one code is selected from a code set containing M codes according to the information data. A signal is modulated by using this code and the effect of fading can be reduced by applying interleaving to the elements of the codes. In the analysis, the bit error probability is derived in closed form expression by using the Chernoff bound. The analysis results show that the error probability decreases when the code length is increased and that an arbitrarily small error probability is achieved as the code length approaches infinity, provided that Eb/N0 exceeds 1.42 dB.

For simulating i.i.d. time-varying MIMO channels using multiple Jakes' rings, it is desirable to generate channels having stable statistics with fewer scatterers. The statistical property of the conventional Jakes' model may depend on the initial phase set assigned to scattering points. In this letter, we present simple and effective conditions on arrangement of scattering points to achieve stable fading properties. The results show that the proposed arrangement provides higher statistical stability in generating time-varying channels.

Orthogonal frequency division multiplexing (OFDM) systems for mobile applications suffer from inter-carrier-interference (ICI) due to frequency offset and to time-variation of the channels and from high peak-to-average-power ratio (PAPR). In this paper, we revisit symmetric cancellation coding (SCC) proposed by Sathananthan et al. and compare the effectiveness of SCC with a fixed subtraction combining and the well-known polynomial cancellation coding (PCC) over Rayleigh fading channels with Doppler spread in terms of the signal-to-interference plus noise power ratio (SINR) and bit-error-rate (BER). We also compare SCC with subtraction combining and SCC of Sathananthan et al. with maximum ratio combining (MRC). Our results show that SCC-OFDM with subtraction combining gives higher SINR than PCC-OFDM over the flat Rayleigh fading channel and that this superiority is not maintained under multi-path induced frequency-selective fading unless diversity combining is used. A simulation result shows, however, that SCC-OFDM with subtraction combining may perform better than PCC-OFDM for a certain range of Doppler spread when differential modulation is employed. Finally, we also demonstrate that the SCC-OFDM signal has less PAPR compared to the normal OFDM and PCC-OFDM and hence may be more practical.

In OFDM based mobile communication systems, channel variation during one symbol period introduces intercarrier interference (ICI). Conventional pilot-aided equalization mitigates the ICI at the price of band inefficiency. On the other hand, the blind or semi-blind equalization method, which utilizes the known statistic properties of the transmitted data, will raise system complexity. In this letter, without bandwidth-consuming pilots, a novel channel estimation and tracking method based on an iterative equalization technique (IET) is proposed. The proposed approach successfully achieves a good compromise between bandwidth efficiency and system complexity, and its validity is demonstrated by numerical simulations, especially for fast fading channel.

Decode-and-forward cooperative communications protocol (DFP) allows single-antenna users in wireless medium to obtain the powerful benefits of multi-antenna systems without physical antenna arrays. So far, only signal-to-noise ratio (SNR) or square amplitude of path gain has been used to evaluate the reliability of received signals for relays to decide whether to forward the decoded data so as to prevent unsuccessful detection at the relays. In this paper, we propose using log-likelihood ratio (LLR) as an alternative to SNR in the conventional DFP. Closed-form BER expressions for different versions of DFP are also derived and verified by Monte-Carlo simulations. A variety of numerical results reveal the significant superiority of LLR-based DFP to SNR-based DFP regardless of threshold level and relay position under flat Rayleigh fading channel plus additive white Gaussian noise (AWGN).

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.

Cooperative transmission among users in multiple access wireless environments is an efficient way to obtain the powerful benefits of multi-antenna systems without the need for physical arrays. Its performance in relay networks were extensively analyzed but most papers only focused on the case of high signal-to-noise ratio (SNR) or loose power constraints. Based on the approximation of total SNR distributions of the propagation paths through relays as the exponential functions, we derive the analytical BER expression in a simple form for general relay networks under strict power restriction. Numerical and simulation results reveal the high accuracy of the distribution estimation as well as the high reliability of the deduced formula, especially at the low SNR.

This paper studies the optimization of the effective channel capacity of wideband code division multiple access (WCDMA) systems under Rayleigh fading environments. Firstly, the results for Shannon capacity of fading channels with channel side information are reviewed, where the capacity is achieved by using an optimal power control scheme. Secondly, an optimal interference threshold is set for a given system outage probability Pout to minimize total interference. Finally, the effective channel capacity of WCDMA is defined and a target SIR level γ* is derived with the Lagrangian multiplier method to maximize the effective channel capacity. It is shown that is dependent on the power control interference ratio (PCIR) ρ, the number of diversity paths identified by the receiver M, and the outage probability of the system. Simulation results are provided to validate the theoretical deductions. We conclude that the total effective channel capacity will be maximized as long as M4, and ρ0.5 for a proper value of .

In this paper, we propose and analyze a scheme for wireless channels, which is the combination of an automatic repeat request (ARQ) scheme and power ramping. The power ramping is used for more reliable downlink data transmission. This technique gradually increases the transmission power at each retransmission attempt. Simulation results demonstrate that when the power step size is 0.5 dB, the average throughput gain may be as high as 2.3% to 5.4% with properly selected parameters.

In this letter, a novel channel estimation method is proposed for frequency-domain equalization of OFDM systems in fast fading multipath channels. It is shown by computer simulations that the proposed method can not only estimate the channel impulse response (CIR) accurately but also achieve lower BER than conventional method at low signal-to-noise ratio (SNR).