This paper proposes a general framework of Space-Time-Frequency Codes (STFCs) for Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) Ultra-Wide Band (UWB) communications systems. A great similarity between the STFC MB-OFDM UWB systems and conventional wireless Complex Orthogonal Space-Time Block Code (CO STBC) Multiple-Input Multiple-Output (MIMO) systems is discovered. This allows us to quantify the pairwise error probability (PEP) of the proposed system and derive the general decoding method for the implemented STFCs. Based on the theoretical analysis results of PEP, we can further quantify the diversity order and coding gain of MB-OFDM UWB systems, and derive the design criteria for STFCs, namely diversity gain criterion and coding gain criterion. The maximum achievable diversity order is found to be the product of the number of transmit antennas, the number of receive antennas, and the FFT size. We also show that all STFCs constructed based on the conventional CO STBCs can satisfy the diversity gain criterion. Various baseband simulation results are shown for the Alamouti code and a code of order 8. Simulation results indicate the significant improvement achieved in the proposed STFC MB-OFDM UWB systems, compared to the conventional MB-OFDM UWB ones.
In this paper, we investigate antenna selection strategies for MIMO-OFDM wireless systems from an energy efficiency perspective. We first derive closed-form expressions of the energy efficiency and the energy efficiency-spectral efficiency (EE-SE) trade-off in conventional antenna selection MIMO-OFDM systems. The obtained results show that these systems suffer from a significant loss in energy efficiency. To achieve a better energy-efficiency performance, we propose an adaptive antenna selection method where both the number of active RF (radio frequency) chains and the antenna indices are selected depending on the channel condition. This selection scheme could be implemented by an exhaustive search technique for a small number of antennas. Moreover, we develop a greedy algorithm that achieves a near-optimal performance with much lower complexity compared to the (optimal) exhaustive search method when the number of antennas is large. In addition, the efficacy of power loading across subcarriers for improved energy efficiency in the conventional and proposed antenna selection MIMO-OFDM systems is considered. Monte-Carlo simulation results are provided to validate our analyses. AbstractIn this paper, we investigate antenna selection strategies for MIMO-OFDM wireless systems from an energy efficiency perspective. We first derive closed-form expressions of the energy efficiency and the energy efficiency-spectral efficiency (EE-SE) trade-off in conventional antenna selection MIMO-OFDM systems. The obtained results show that these systems suffer from a significant loss in energy-efficiency.To achieve a better energy-efficiency performance, we propose an adaptive antenna selection method where both the number of active RF (radio frequency) chains and the antenna indices are selected depending on the channel condition. This selection scheme could be implemented by an exhaustive search technique for a small number of antennas. Moreover, we develop a greedy algorithm that achieves a nearoptimal performance with much lower complexity compared to the (optimal) exhaustive search method when the number of antennas is large. In addition, the efficacy of power loading across subcarriers for improved energy efficiency in the conventional and proposed antenna selection MIMO-OFDM systems is considered. Monte-Carlo simulation results are provided to validate our analyses.
Analog least mean square (ALMS) loop is a promis-1 ing method to cancel self-interference (SI) in in-band full-duplex 2 (IBFD) systems. In this paper, the steady state analyses of the 3 residual SI powers in both analog and digital domains are 4 firstly derived. The eigenvalue decomposition is then utilized to 5 investigate the frequency domain characteristics of the ALMS 6 loop. Our frequency domain analyses prove that the ALMS loop 7 has an effect of amplifying the frequency components of the 8 residual SI at the edges of the signal spectrum in the analog 9 domain. However, the matched filter in the receiver chain will 10 reduce this effect, resulting in a significant improvement of 11 the interference suppression ratio (ISR). It means that the SI 12 will be significantly suppressed in the digital domain before 13 information data detection. This paper also derives the lower 14 bounds of ISRs given by the ALMS loop in both analog and 15 digital domains. These lower bounds are joint effects of the loop 16 gain, tap delay, number of taps, and transmitted signal properties. 17 The discovered relationship among these parameters allows the 18 flexibility in choosing appropriate parameters when designing the 19 IBFD systems under given constraints.
Analog least mean square (ALMS) loop is a promising structure for self-interference (SI) mitigation in full-duplex radios due to its simplicity and adaptive capability. However, being constructed from in-phase/quadrature (I/Q) demodulators and modulators to process complex signals, the ALMS loop may face I/Q imbalance problems. Thus, in this paper, the effects of frequencyindependent I/Q imbalance in the ALMS loop are investigated. It is revealed that I/Q imbalance affects the loop gain and the level of SI cancellation. The loop gain can be easily compensated by adjusting the gain at other stages of the ALMS loop. Meanwhile, the degradation on cancellation performance is proved to be insignificant even under severe conditions of I/Q imbalance. In addition, an upper bound of the degradation factor is derived to provide an essential reference for the system design. Simulations are conducted to confirm the theoretical analyses.
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