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Example of the set of equivalent channels obtained with each solution in the frequency domain from simulated wireless channels. Notice that there are only two smooth sets out of 2 N FFT
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In this paper, we present an experimental study on the performance of spatial interference alignment (IA) in indoor wireless local area network scenarios that use orthogonal frequency division multiplexing (OFDM) according to the physical-layer specifications of the IEEE 802.11a standard. Experiments have been carried out using a wireless network t...
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... However, in real-world systems, detection and synchronization are performed right after the RF demodulation and analog-to-digital conversion, i.e., at sample level, thus they are affected by interference. The pre-FFT IA approach in [29,30] overcomes this issue by operating at sample level, that is, in the time domain. Nevertheless, since the synchronization mismatches have been already studied in the aforementioned works, we rely on an external clock and oscillator for time and frequency synchronization (see Section 3), and apply IA precoders and decoders in the frequency domain. ...
... As in[29,30], the channel estimates in steps 1 and 4 are obtained by dividing the received long training symbols by the original LT symbols on a per-subcarrier basis. ...
In this paper, we present an experimental evaluation of the performance benefits provided by flexible duplexing, an access technique that allows uplink and downlink cells to coexist within the same time-frequency resource blocks. In order to replicate a wireless multi-tier network composed of 1 macro-cell and 2 small cells, a measurement campaign has been conducted using an indoor wireless testbed comprised of a total of 6 multiple-input multiple-output (MIMO) software-defined radio (SDR) devices. Since each cell has a single active user, each uplink/downlink configuration can be identified with a different interference channel, over which interference alignment (IA) is used as an inter-cell interference management technique and compared to other existing methods. The obtained results show that flexible duplexing clearly outperforms the conventional time-division duplex (TDD) access approach, where all cells operate synchronized either in uplink or dowlink mode. Additionally, interference alignment consistently provides better results in most of the interference regimes when compared to minimum mean square error (MMSE)-based schemes. The impact of channel estimate quality on the different communication strategies is also studied. It is worth highlighting that the presented over-the-air (OTA) experiments represent the first implementation of IA with real-time precoding and decoding.
... Interference Alignment (IA) can effectively solve the interference problem in the interference channel and increase the achievable sum rate of the channel [21,22]. However, it requires global CSI to design the interference suppression matrix, usually using the feedback of the receiver to inform the CSI required by the transmitter [23,24]. ...
Aiming at the limitations of the existing Limited Feedback Interference Alignment algorithms, this paper proposes a direct codeword selection scheme that maximizes the lower-bound of the user rate and reduces the sum rate loss by integrating the Bit Allocation algorithm. The target signal is decoded using the maximum signal to interference plus noise ratio (MAX-SINR) algorithm. Moreover, low complexity and global searching mechanisms are deployed to select the optimized codewords from the generated sets of codewords that approach the ideal precoder. Simulation results show that the proposed algorithm effectively improves the rate lower-bound of the system user as compared with the existing state-of-the-art algorithms.
... The CSI acquisition problem is mitigated in time-division duplexing (TDD) systems by exploiting channel reciprocity, although calibrated RF equipment is required [2]. For frequency-division duplexing (FDD) systems, IA experiments with perfect [3,4] and realistic analog wireless [5] feedback channels have been reported in the literature. Furthermore, in most of the theoretical IA works, the block-fading channel model assumption plays a pivotal role due to its mathematical tractability [1]. ...
... In practical systems, however, frame detection and synchronization are applied immediately after the analog-to-digital conversion, hence being affected by interference and yielding a strong performance degradation of such tasks, thus impacting dramatically on the final system performance. For the specific case of spatial IA in multicarrier systems, IA decoding can be implemented in the time domain or following a more conventional per-subcarrier approach in the frequency domain [3,4]. Given that time-domain IA decoding suppresses most of the multiuser interference at the very beginning of the receiver signal processing chain, the effective SINR is improved, whereas synchronization tasks perform similar to the interference-free case because they operate after filtering out the interference [4]. ...
... Hardware imperfections are ignored in many IA algorithm designs. However, nonlinear distortions, phase noise, IQ imbalance, and frequency offset degrade IA performance [3,9]. For instance, the measurement results of IA in the 3-user 2 2 MIMO interference channel show that hardware imperfections can reduce the maximum achievable SINR up to 10 dB compared to the theoretical predictions [9]. ...
Interference alignment has triggered high impact research in wireless communications since it was proposed nearly 10 years ago. However, the vast majority of research is centered on the theory of interference alignment and is hardly feasible in view of the existing state-of-the-art wireless technologies. Although several research groups have assessed the feasibility of interference alignment via testbed measurements in realistic environments, the experimental evaluation of interference alignment is still in its infancy since most of the experiments were limited to simpler scenarios and configurations. This article summarizes the practical limitations of experimentally evaluating interference alignment, provides an overview of the available interference alignment testbed implementations, including the costs, and highlights the imperatives for succeeding interference alignment testbed implementations. Finally, the article explores future research directions on the applications of interference alignment in the next generation wireless systems.
... However, because of the time-varying wireless channel quality, T c should not exceed the fluctuation periodicity of the SINR in the time domain. According to a recent study [29], the minimum and mean SINR fluctuation periods (for fixed users operating in an outdoor environment) are equal 7 and 25 ms, respectively. Since the length of one LTE slot is equal 1 ms, we can choose any value of T c , such that 7 ≤ T c ≤ 25 slots. ...
We study the problem of joint user association and inter-cell interference (ICI) mitigation in heterogeneous Long Term Evolution Advanced (LTE-A) networks presuming the absence of the precise global channel state information (CSI). We confront this problem from a contract-theoretic perspective and model the considered network as a labor market where the LTE evolved NodeBs (eNBs) are regarded as the employers offering a contract to the employees (cellular users). We explore a scenario where the wireless channels are ordered into different types according to their link gains and the cost of power consumption. It is assumed that each user knows the type of its channel but is not aware of the types of the channels belonging to other users. Hence, the channel type is regarded as the private information which is known only by the corresponding user. Under the presence of such asymmetric information, the eNBs formulate the optimal contract given by a set of the contract items (the combinations of user association, allocated spectrum, and power) and pass these items to the users, who then select the best contract items according to their channel types. The performance of a proposed contract-based resource allocation mechanism has been evaluated using the OPNET-based simulations. It has been demonstrated that the efficiency of a contract-based mechanism with asymmetric information is close to the optimal method (where the network resources are allocated based on the precise CSI). It has also been shown that the contract-based method outperforms the other relevant resource allocation techniques in terms of the total network service rate and signal-to-noise ratio (SINR).
... interference alignment (IA) has been recently proposed as a promising interference management technique. An experimental study on the performance of spatial IA in indoor wireless local area network scenarios that use OFDM according to the IEEE 802.11a standard is presented in [12]. Experiments have been carried out using a wireless network testbed capable of implementing a 3-user MIMO interference channel and considering IA decoding schemes designed according to distinct criteria (e.g., zero-forcing or MaxSINR). ...
This editorial sums up relevant topics on the assessment of wireless communication systems covered by the especial issue entitled “Experimental Evaluation in Wireless Communications.” The topics include practical aspects on the implementation of distributed asynchronous non-linear kernel methods over wireless sensor networks; localization methods based on the exploitation of radio-frequency identification (RFID) wireless sensors and cellular networks or on sparsity approximations; channel sounding and assessment of broadband orthogonal frequency-division multiplexing (OFDM)-based wireless systems in high-speed vehicular communications; coexistence analysis of femtocell-based and outdoor-to-indoor systems; techniques for peak-to-average power ratio (PAPR) reduction; new solutions for baseband and radio frequency (RF) hardware impairments in full-duplex wireless systems; and, finally, suitability of interference alignment for broadband indoor wireless communications.
... Then, at any slot t, we keep the SINR in all wireless channels above this target level. That is, Based on (22), the interfering users are allowed to transmit at the level According to a recent study [43], the minimum and mean SINR fluctuation periods (for fixed users) are equal 7 and 25 ms, respectively. Consequently, for LTE slot size T s = 1 ms, we can choose any value of T, such that 7 ≤ T ≤ 25. ...
This paper investigates the problem of resource allocation for Device-to-Device (D2D) Communication in a Third Generation Partnership Project (3GPP) Long Term Evolution Advanced (LTE-A) network. The users in the network can operate either in a traditional cellular mode communicating with each other via the eNB, or in D2D mode communicating with each other without traversing the eNB. In the considered model, the D2D users and cellular users share the same radio resources. Particularly, each resource block (RB) within the available bandwidth can be occupied by one cellular and several D2D users. Hence, the problem of interference management is crucial for effective performance of such a network. The twofold aim of the proposed algorithm is to i) mitigate the interference between cellular and D2D users and ii) improve the overall user-perceived quality of service (QoS). To control the interference, for each user we define a certain target interference level, and constrain the interference from the other users to stay below this level. The corresponding optimization problem maximizes the QoS of the users by minimizing the size of the buffers of user equipments (UEs). Performance of the algorithm has been evaluated using the OPNET-based simulations. The algorithm shows improved performance in terms of mean packet end-to-end delay and loss for UEs when compared to other relevant schemes.
... However, because of time-varying wireless channel quality, the observation period should not exceed the fluctuation periodicity of the SINR in time domain. According to a recent study [54], the minimum and mean SINR fluctuation periods (for fixed users operating in outdoor environment) are equal 7 and 25 ms, respectively. Consequently, we can choose any value of T, such that 7 ≤ T ≥ 25. ...
This paper explores the problem of joint mode selection, spectrum management, power control, and interference mitigation for device-to-device (D2D) communication underlaying a Long Term Evolution-Advanced (LTE-A) network. We consider a dynamic mode selection scenario, in which the modes (D2D or cellular) of the devices depend on optimal allocations. To improve the quality of service (QoS) for the users, the optimization objective in a corresponding problem is formulated in terms of buffer size of user equipments (UEs), which is estimated based on buffer status information collected by the UEs. The realizations of a resource allocation approach presented in the paper include its real-time and non-real-time implementations, as well as two modifications applicable to a standard LTE-Direct (LTE-D) network. Performance of the proposed algorithms has been evaluated using the OPNET-based simulations. All algorithms show improved performance in terms of mean packet end-to-end delay when compared to most relevant schemes proposed earlier.
... For each channel realization, the foregoing procedure is repeated for all individual data rates specified by the IEEE 802.11a standard. More details about this setup can be found in [15]. ...
In this paper we present an experimental study on the performance of spatial Interference Alignment (IA) in broadband indoor wireless local area network scenarios that use Orthogonal Frequency Division Multiplexing (OFDM) according to the IEEE 802.11a physical-layer specifications. Experiments have been carried out using a wireless network testbed made up of six nodes equipped with Multiple-Input Multiple-Output (MIMO) radio interfaces. This setup allows the implementation of a 3-user MIMO interference channel. We have implemented different IA decoding schemes that operate either before or after the Fast Fourier Transform block. IA has been experimentally evaluated comparing both approaches to analyze its performance in synchronous and asynchronous transmissions. Our results indicate that spatial IA performs satisfactorily in practical broadband indoor scenarios in which wireless channels often exhibit relatively large coherence times.
Several interference alignment (IA) testbeds have been reported by research groups that validate the theoretical findings thus demonstrating the feasibility of IA. The design complexity of the IA testbed is significant since many simultaneous wireless links have to be established to observe the benefits of IA. Therefore, the already enormous efforts needed for the development of multiple-input multiple-output (MIMO) testbeds are folded multiple times in IA testbeds. In general, testbeds can be categorized into three groups depending on the hardware platform, generic, specific and the hybrid of two. Clearly, developing on generic, hybrid and application specific platforms in turn are the safe steps towards real life prototyping. There are many research groups that have been developing IA on generic platforms [1]–[17]. To the best of our knowledge, there is only one research group that has developed on a hybrid platform [18], and none on the application specific platform. In this paper, we review the experimental IA evaluations in the literature, include important aspects from well-known prototyping principles in the picture, and give comparative discussion with the state-of-art wireless communications testbeds.
