1.Achievable Sum Rates of MIMO SC-FDMA Systems with Different Receivers
This paper investigates the achievable sum rates of multiple-input multiple-output (MIMO) single carrier frequency division multiple access (SC-FDMA) systems with different receivers in spatially uncorrelated frequency selective Rayleigh fading channels. Zero forcing (ZF), linear minimum mean squared error (MMSE) and the proposed per subcarrier maximum likelihood (PSML) receivers are considered. Closed form expressions for the upper and lower bounds on the achievable sum rates of ZF, MMSE and PSML receivers are derived. Through these expressions, we characterize the behavior of the receivers in various scenarios of different channel lengths and subcarrier numbers. It is found that compared with the optimal receiver, the PSML receiver can obtain the optimal sum rate with significantly reduced computational complexity in flat fading channels. In addition, for ZF or MMSE receivers, either the upper or lower bound can be an accurate approximation of the achievable sum rate under certain conditions and can also provide a theoretical reference to practical systems.
2.BER analysis in Wavelet based SC-FDMA for LTE uplink transmission
Long Term Evolution (LTE) has adopted Single Carrier Frequency Division Multiple Access (SCFDMA) technique for uplink and Orthogonal Frequency Division Multiple Access (OFDMA) for downlink. SC-FDMA is often called as Discrete Fourier Transform (DFT) spread SC-FDMA. In this paper, Wavelet based SC-FDMA is proposed for analyzing Bit Error Rate (BER) performance. Analysis is carried out using different wavelets and different modulation schemes under AWGN channel. This analysis will show that the reduction in BER takes place by using wavelet transform in SCFDMA. Thus wavelet based SC-FDMA provides better BER performance than that of DFT based SC-FDMA.
3.Spectrum Sensing of Interleaved SC-FDMA Signals in Cognitive Radio Networks
This paper develops a spectrum sensing technique for interleaved single-carrier frequency-division multiple access (SC-FDMA) systems. By designing a metric that exploits cyclostationary features of interleaved SC-FDMA signals, we establish a framework for signal detection. Using Gaussian approximation for this metric, the parameters of the metric distributions under two hypotheses are derived, and both hypotheses are examined by the Neyman–Pearson test. We validate the accuracy of the Gaussian approximation by comparing theoretical and simulated metric histograms. The performance of the proposed method is presented for additive white Gaussian noise and multipath Rayleigh fading channels. We also investigate the effect of the block length, the number of users, the metric window length, and the presence of the pilot signals on the detection performance. Through comparative performance evaluation, we demonstrate the superiority of our proposed detection scheme over energy detection and the detection method based on autocorrelation of the cyclic prefix (CP). We obtain similar detection performance to that of the mentioned methods at about 8–13 dB lower signal-to-noise ratio (SNR). It is noteworthy that the complexity of our method is comparable to that of the energy detection and slightly higher than that of CP detection.
4.XOR Encryption Versus Phase Encryption, an In-Depth Analysis
Encryptions are used in almost all standards to ensure the confidentiality of the data. Encryptions can be and indeed are implemented in the different layers of a network protocol stack. Conventional encryption performs the bitwise XOR operation between one message bit and one key stream bit to generate one ciphertext bit. Huo et al. have recently proposed to provide confidentialities on the user data by performing the phase encryption on the time domain OFDM samples in LTE system. Phase encryption is performed on the modulated symbols, different from the bit level of XOR encryption, i.e., stream cipher encryption. In this paper, we extend their study.We first generalize the phase encryption to general communication systems independent of the underlying modulation scheme. Then, we formulate the mathematical models for XOR and phase encryptions. Based on ourmodel, we compare these two encryption methods in terms of their security and encryption efficiency.We also show phase encryption can resist traffic analysis attack when implemented in the physical layer. Finally, we conduct simulations to compare the performance of these two methods in terms of their decoding symbol error rate.
5.A low-complexity cyclostationary spectrum sensing for Interference Avoidance in femto-cells LTE-A based Networks.
This correspondence focuses on a new spectrum sensing algorithm based on the exploitation of the primary signal cyclostationarity property, used to avoid the inter-cell interference in a heterogeneous LTE-A network. The performance of the proposed spectrum sensing method will be evaluated by means of a suitable analytical approach whose accuracy will be validated by comparing analytical predictions with simulation results under different communication channel propagation conditions. Furthermore, it will be shown that the proposed approach requires the lowest computational complexity among the cyclostationarity based methods and allows better performance, even in the case of noise uncertainty, in terms of false alarm, detection probability and deflection coefficient.
6.A Cooperative Spectrum Sensing Scheme based on Task Assignment Algorithm for Cognitive Radio Networks
In cognitive radio networks, dynamic spectrum access allows an unlicensed (secondary) user to use the frequency bands that are statically allocated to licensed (primary) users under condition of causing no harm to the primary transmission. For the process of the dynamic spectrum access to succeed, spectrum sensing becomes of great importance for the secondary user (SU) to capture the under-utilized frequency bands. This process must be as fast as possible to enable elliclent spectrum utilization by the SU. In order to decrease the sensing time especially for wide band spectrum while keeping acceptable sensing accuracy, it is recommended to distribute this sensing task across the SUs in a way that leads to reduction of the probability of misdetection and the probability of false alarm. This work presents a channel assignment scheme for cooperative spectrum sensing in cognitive radio networks. Based on the individual - statistically calculated- probability of error of each SU over all channels, the algorithm assigns the best k adjacent channels for each SU to decrease the probability of misdetection and the probability of false alarm. It is shown by MATLAB simulations that the presented scheme is able to reach acceptable sensing probability of error in terms of reduced sensing delay when compared to full spectrum sensing where all nodes cooperate to sense all channels (no assignment takes place). Additionally, It Is shown that the overall energy consumption of the cognitive radio network is reduced resulting in better network lifetime.
7.Energy Efficiency Optimization in MU-MIMO System with Spectral Efficiency Constraint
Multiuser MIMO (MU-MIMO), in which the base station transmits multiple streams to multiple users, has received significant importance as a way of achieving spectral efficiency (SE) and energy efficiency (EE). However, these two important design criteria conflict with each other and a careful study of their trade-off is mandatory for designing future wireless communication systems. This paper investigates the trade-off between EE and SE in downlink MU-MIMO system which is adopted by 3GPP LTE-Advanced to meet IMT-Advanced targets. The EE is measured as, “throughput (bits) per Joule” while both RF transmit power and circuit power consumptions are considered. In this paper, given the SE requirement, a constrained optimization problem where constraints redefined with cubic inequality is formulated to maximize EE. Then, a novel resource allocation algorithm is proposed to achieve maximum EE. Simulation results demonstrate the effectiveness of the proposed scheme and illustrate the fundamental trade-offs between energy efficient and spectral efficient transmission. Our analytical results shed light on future ”green” network planning in downlink MU-MIMO systems.
8.Bandwidth Reservation as a Coexistence Strategy in Opportunistic Spectrum Access Environments
We consider a scenario in which a licensed wireless operator or primary network (PN) coexists with an ad-hoc, cognitive, secondary network. To minimize harmful interference on the primary users (PUs), secondary users (SUs) sense the PU
activity on the licensed spectrum band before transmitting in spectrum opportunities. We study a bandwidth reservation (BR) scheme by which the PN keeps a set of adjacent channels free of PU transmissions. These reserved channels only accommodate PU traffic when all the non-reserved channels are used, and the Sus only occupy available channels within the reserved spectrum. Intuitively, this strategy reduces collision probability and simplifies the design of opportunistic spectrum access (OSA) mechanisms. However, from the point of view of the PN, BR entails a tradeoff
between the benefits of an improved coexistence with SUs, and the capacity reduction associated to having fewer options for PU channel allocation. The main objective of this paper is to determine when BR improves the overall PN performance under SU activity. The SUs are characterized by a hardware limited radio, imperfect spectrum sensing, bayesian estimation of PU activity and multichannel access. Because PU capacity is the central issue, we assume a PN capable of exploiting all the available bandwidth at every moment. By means of a Marco reward model, we compute the expected PU capacity with and without BR, considering propagation effects, interference and random locations. The results show that, in a non-congested PN with SU activity, the interference reduction capability of BR increases the overall capacity of the PN compared to not using.
9.The Multicell Multiuser MIMO Uplink with Very Large Antenna Arrays and a Finite-Dimensional Channel
We consider multicell multiuser MIMO systems with a very large number of antennas at the base station (BS).We assume that the channel is estimated by using uplink training. We further consider a physical channel model where the angular domain is separated into a finite number of distinct directions. We analyze the so-called pilot contamination effect discovered in previous work, and show that this effect persists under the finite-dimensional channel model that we consider. In particular, we consider a uniform array at the BS. For this scenario, we show that when the number of BS antennas goes to infinity, the system performance under a finite-dimensional channel model with P angular bins is the same as the performance under an uncorrelated channel model with P antennas. We further derive a lower bound on the achievable rate of uplink data transmission with a linear detector at the BS. We then specialize this lower bound to the cases of maximum-ratio combining (MRC) and zero-forcing (ZF) receivers, for a finite and an infinite number of BS antennas. Numerical results corroborate our analysis and show a comparison between the performances of MRC and ZF in terms of sum-rate.
10.Ergodic Sum-Rate Maximization for Fading Cognitive Multiple Access Channels without Successive Interference Cancellation
In this paper, the ergodic sum-rate of a fading cognitive multiple access channel (C-MAC) is studied, where a secondary network (SN) with multiple secondary users (SUs) transmitting to a secondary base station (SBS) shares the spectrum band with a primary user (PU). An interference power constraint (IPC) is imposed on the SN to protect the PU. Under such a constraint and the individual transmit power constraint (TPC) imposed on each SU, we investigate the power allocation strategies to maximize the ergodic sum-rate of a fading C-MAC without successive interference cancellation (SIC). In particular, this paper considers two types of constraints: (1) average TPC and average IPC, (2) peak TPC and peak IPC. For the first case, it is proved that the optimal power allocation is dynamic time-division multiple-access (D-TDMA), which is exactly the same as the optimal power allocation to maximize the ergodic sum-rate of the fading C-MAC with SIC under the same constraints. For the second case, it is proved that the optimal solution must be at the extreme points of the feasible region. It is shown that D-TDMA is optimal with high probability when the number of SUs is large. Besides, we show that, when the SUs can be sorted in a certain order, an algorithm with linear complexity can be used to find the optimal power allocation.
11.Chaos Communication System using MIMO Technique
The security of chaos communication system is superior to other digital communication system, because it has characteristics such as non-periodic, wide-band, unpredictability, easy implementation and sensitive initial condition. However, chaos communication system increases the number of transmitted symbols by spreading and transmitting information bits according to characteristic of chaos maps. So the research that improves data transmission speed is necessary for chaos communication system. If many antennas are applied to chaos communication system, the capacity of data is proportional to the number of antenna.so it is good way applying multiple-input and multiple-output (MIMO) to the chaos communication system. In this paper, we propose the correlation delay shift keying (CDSK) using 2 ´ 2 MIMO technique and evaluate BER performance over Rayleigh MIMO fading channel. Lastly we evaluate BER performance applying the boss map with 2x2 MIMO using MIMO detection algorithm such as zero forcing (ZF) and minimum mean square error (MMSE).
12.A Study on Inter-Cell Subcarrier Collisions due to Random Access in OFDM-Based Cognitive Radio Networks
In cognitive radio (CR) systems, one of the main implementation issues is spectrum sensing because of the uncertainties in propagation channel, hidden primary user (PU) problem, sensing duration and security issues. This paper considers an orthogonal frequency-division multiplexing (OFDM)- based CR spectrum sharing system that assumes random access of primary network subcarriers by secondary users (SUs) and absence of the PU’s spectrum utilization information, i.e., no spectrum sensing is employed to acquire information about the PU’s activity or availability of free subcarriers. In the absence of information about the PU’s activity, the SUs randomly access (utilize) the subcarriers of the primary network and collide with the PU’s subcarriers with a certain probability. In addition, inter-cell collisions among the subcarriers of SUs (belonging to different cells) can occur due to the inherent nature of random access scheme. This paper conducts a stochastic analysis of the number of subcarrier collisions between the SUs’ and PU’s subcarriers assuming fixed and random number of subcarriers requirements for each ser. The performance of the random scheme in terms of capacity and capacity (rate) loss caused by the subcarrier collisions is investigated by assuming an interference power constraint at PUs to protect their operation.
13.SHARP: Spectrum Harvesting with ARQ Retransmission and Probing in Cognitive Radio
In underlay cognitive radio, a secondary user transmits in the transmission band of a primary user without serious degradation in the performance of the primary user. This paper proposes a method of underlay cognitive radio where the secondary pair listens to the primary ARQ feedback to glean information about the primary channel. The secondary transmitter may also probe the channel by transmitting a packet and listening to the primary ARQ, thus getting additional information about the relative strength of the cross channel and primary channel. The method is entitled Spectrum Harvesting with ARQ Retransmission and Probing (SHARP). The probing is done only infrequently to minimize its impact on the primary throughput. Two varieties of spectrum sharing, named conservative and aggressive SHARP, are introduced. Both methods avoid introducing any outage in the primary; their difference is that conservative SHARP leaves the primary operations altogether unaffected, while aggressive SHARP may occasionally force the primary to use two instead of one transmission cycle for a packet, in order to harvest a better throughput for the secondary. The performance of the proposed system is analyzed and it is shown that the secondary throughput can be significantly improved via the proposed approach, possibly with a small loss of the primary throughput during the transmission as well as probing period.
14.Per-Antenna Constant Envelope Precoding for Large Multi-User MIMO Systems
We consider the multi-user MIMO broadcast channel with M single-antenna users and N transmit antennas under the constraint that each antenna emits signals having constant envelope (CE). The motivation for this is that CE signals facilitate the use of power-efficient RF power amplifiers. Analytical and numerical results show that, under certain mild conditions on the channel gains, for a fixed M, an array gain is achievable even under the stringent per-antenna CE constraint. Essentially, for a fixed M, at sufficiently large N the total transmitted power can be reduced with increasing N while maintaining a fixed information rate to each user. Simulations for the i.i.d. Rayleigh fading channel show that the total transmit power can be reduced linearly with increasing N (i.e., an O(N) array gain). We also propose a precoding scheme which finds near-optimal CE signals to be transmitted, and has O(MN) complexity. Also,in terms of the total transmit power required to achieve a fixed desired information sum-rate, despite the stringent per-antenna CE constraint, the proposed CE precoding scheme performs close to the sum-capacity achieving scheme for an average-only total transmit power constrained channel.