Browsing by Author "Kazemi, Mohammad"

Now showing 1 - 15 of 15

Open Access
A slotted pilot-based unsourced random access scheme with a multiple-antenna receiver
(Institute of Electrical and Electronics Engineers, 2023-08-30) Özateş, Mert; Kazemi, Mohammad; Duman, Tolga Mete
We consider unsourced random access over fading channels with a massive number of antennas at the base station, and propose a simple, yet energy-efficient solution by dividing the transmission frame into slots. We utilize non-orthogonal pilot sequences followed by a polar codeword for transmission in each slot. At the receiver side, we first detect the transmitted pilot sequences by employing a generalized orthogonal matching pursuit algorithm and utilize a linear minimum mean square error solution to estimate the channel vectors. We then perform an iterative decoding based on maximal ratio combining, single-user polar decoding, and successive interference cancellation with re-estimation of the channel vectors to recover the data bits. We also analyze the performance of the proposed scheme using normal approximations and provide a detailed complexity analysis. Numerical examples demonstrate that the proposed scheme either outperforms the existing schemes in the literature or has a competitive performance with a lower complexity. Furthermore, it is suitable for fast-fading scenarios due to its excellent performance in the short blocklength regime.
Open Access
Analysis and optimization of spectral and energy efficiency in underlaid D2D multi-cell massive MISO over rician fading
(Institute of Electrical and Electronics Engineers Inc., 2023-02-15) Hashemi, R.; Kazemi, Mohammad; Mohammadi, A.
This paper investigates the uplink spectral efficiency (SE) characterization and energy efficiency (EE) optimization of device-to-device (D2D) communications underlying a multi-cell massive multiple-input single-output (m-MISO) network, assuming that the channels are modeled with Rician fading. First, an analytical expression for the lower-bound of the ergodic capacity of a typical cellular user (CU) is derived with imperfect channel state information in the presence of D2D links' interference. Then, a closed-form approximate achievable SE expression for a typical D2D pair is derived considering Rician fading between D2D pairs. The asymptotic SE behavior is analyzed in strong line-of-sight (LOS) conditions for CU and D2D pairs. Also, simplified expressions are obtained for the special case of Rayleigh fading. Next, to optimize the total EE, a transmit data power allocation based on the derived rate expressions is formulated. Since the optimization problem has a non-linear and non-convex objective function, it is intractable to be solved straightforwardly. Therefore, we resort to utilizing an iterative algorithm relying on fractional programming. The numerical results show that a stronger LOS component (i.e., larger Rician K-factor) between a typical CU and its serving BS leads to a significant improvement in the system performance, while it has less effect on the D2D network.
Open Access
Capacity bounds for the poisson-repeat channel
(Institute of Electrical and Electronics Engineers, 2023-08-22) Kazemi, Mohammad; Duman, Tolga M.
We develop bounds on the capacity of Poisson-repeat channels (PRCs) for which each input bit is independently repeated according to a Poisson distribution. The upper bounds are obtained by considering an auxiliary channel where the output lengths corresponding to input blocks of a given length are provided as side information at the receiver. Numerical results show that the resulting upper bounds are significantly tighter than the best known one for a large range of the PRC parameter ? (specifically, for ? =0.35). We also describe a way of obtaining capacity lower bounds using information rates of the auxiliary channel and the entropy rate of the provided side information.
Open Access
Channel reconstruction based multiuser precoding with limited feedback
(IEEE, 2021-09-06) Özateş, Mert; Kazemi, Mohammad; Göken, Çağrı; Duman, Tolga M.
We consider the downlink of a multiuser multiple-input multiple-output (MU-MIMO) system, where each user feeds back a partial channel state information (CSI), namely, the quantized version of the dominant eigenvector of its channel covariance matrix, to the base station (BS) for precoding. Specifically, we propose a downlink multiuser precoding scheme by first reconstructing the equivalent channel matrix of each user via a limited feedback, and then by employing a precoder to suppress the multiuser interference at the receivers. For the single stream case, a signal-to-leakage-and-noise ratio (SLNR) based precoding is employed, while for the full stream case with limited feedback, we employ a lattice reduction aided block diagonalization type precoding with suitable modifications at the receiver side. Extensive numerical examples which are provided using the 5G new radio (5G-NR) channel models demonstrate that the proposed schemes outperform the existing eigenvector based algorithms, and they are more robust against the downlink channel estimation errors.
Open Access
Deep learning based interference exploitation in 1-bit massive MIMO precoding
(Institute of Electrical and Electronics Engineers, 2023-02-14) Hossienzadeh, M.; Aghaeinia, H.; Kazemi, Mohammad
In this paper, we focus on one-bit precoding approach for downlink massive multiple-input multiple-output (MIMO) systems, where we exploit the concept of constructive interference (CI) employing deep learning (DL) techniques. One of the main performance limiting factors in wireless communication systems is interference, which needs to be minimized or mitigated. By controlling the interference signals in order to add up constructively at the receiver side, there is a possibility to improve the system performance. This paper presents a DL-based one-bit precoding scheme that improves the massive MIMO performance via CI exploitation in the presence of one-bit digital to analog converters (DAC) as a hardware impairment. More precisely, for phase shift keying signaling, we first formulate the optimization problem in order to maximize the CI effects in the case of a base station equipped with one-bit DACs. Then, after solving the optimization problem and creating a large enough dataset, a DL network is trained to do the precoding. Numerical results show that the DL-based solution approaches the performance of the conventional interference exploitation one-bit precoding schemes in the massive MIMO systems while having an order of magnitude less complexity.
Open Access
Double-zipper: multiple access with zigzag decoding
(IEEE, 2020) Kazemi, Mohammad; Duman, Tolga M.; Medard, M.
As a building block toward a simple and scalable solution to massive random access, we consider two-user multiple access with ZigZag decoding, with no need for any coordination or codebook differentiation. We derive closed-form bounds on the achievable sum-rates of the original ZigZag and a modified version of it, called double-zipper ZigZag, for both cases of perfect and imperfect channel state information (CSI). We also show that performances of both versions of ZigZag approach that of optimal coordinated time-sharing in the high signal to noise ratio regime, even in the presence of CSI errors.
Open Access
Hierarchical over-the-air federated edge learning
(IEEE, 2022) Aygün, Ozan; Kazemi, Mohammad; Gündüz, D.; Duman, Tolga M.
Federated learning (FL) over wireless communication channels, specifically, over-the-air (OTA) model aggregation framework is considered. In OTA wireless setups, the adverse channel effects can be alleviated by increasing the number of receive antennas at the parameter server (PS), which performs model aggregation. However, the performance of OTA FL is severely limited by the presence of mobile users (MUs) located far away from the PS. In this paper, to mitigate this limitation, we propose hierarchical over-the-air federated learning (HOTAFL), which utilizes intermediary servers (IS) to form clusters near MUs. We provide a convergence analysis for the proposed setup, and demonstrate through experimental results that local aggregation in each cluster before global aggregation leads to a better performance and faster convergence than OTA FL.
Open Access
One-bit massive MIMO precoding using unsupervised deep learning
(Institute of Electrical and Electronics Engineers, 2024-02-01) Hosseinzadeh, Mohsen; Aghaeinia, Hassan; Kazemi, Mohammad
The recently emerged symbol-level precoding (SLP) technique is a promising solution in multi-user wireless communication systems due to its ability to transform harmful multi-user interference (MUI) into useful signals, thereby improving system performance. Conventional symbol-level precoding designs have a significant computational complexity that makes their practical implementation difficult and imposes excessive computational complexity on the system. To deal with this problem, we suggest a new deep learning (DL) based approach that utilizes low-complexity designs of symbol-level precoding. This paper focuses on DL-based one-bit precoding approaches for downlink massive multiple-input multiple-output (MIMO) systems, where one-bit digital-to-analog converters (DACs) are used to reduce cost and power. Unlike previous works, the optimized one-bit precoder for multiuser massive MIMO system (HDL-O1PmMIMO) for a wide range of signal-to-noise-ratio (SNR) has a low computational complexity, making it suitable for real precoding scenarios. In this paper, we first design an unsupervised DL-based precoder (UDL-O1PmMIMO) to address the low SNR scenarios, using which we then design a hybrid DL-based precoder (HDL-O1PmMIMO) to address both low and high SNR scenarios. The method suggested in this article utilizes a novel residual DL network structure, which helps overcome the problem of training very deep networks. Additionally, a novel customized cost function, specifically for one-bit precoding in massive MIMO systems, is introduced to optimize the performance of the system in handling interference. The results of an experiment conducted on a general test set using Python and MATLAB show that the proposed approach outperforms existing methods in three aspects: it has a lower bit error rate, it takes less time to generate the precoded vector, and it is more resistant to imperfect channel estimation.
Open Access
Over the air federated edge learning with hierarchical clustering
(IEEE, 2024-12) Aygün, Ozan; Kazemi, Mohammad; Gündüz, Deniz; Duman, Tolga Mete
We examine federated learning (FL) with over-the-air (OTA) aggregation, where mobile users (MUs) aim to reach a consensus on a global model with the help of a parameter server (PS) that aggregates the local gradients. In OTA FL, MUs train their models using local data at every training round and transmit their gradients simultaneously using the same frequency band in an uncoded fashion. Based on the received signal of the superposed gradients, the PS performs a global model update. While the OTA FL has a significantly decreased communication cost, it is susceptible to adverse channel effects and noise. Employing multiple antennas at the receiver side can reduce these effects, yet the path-loss is still a limiting factor for users located far away from the PS. To ameliorate this issue, in this paper, we propose a wireless-based hierarchical FL scheme that uses intermediate servers (ISs) to form clusters in the areas where the MUs are more densely located. Our scheme utilizes OTA cluster aggregations for the communication of the MUs with their corresponding IS, and OTA global aggregations from the ISs to the PS. We present a convergence analysis for the proposed algorithm, and show through numerical evaluations of the derived analytical expressions and experimental results that utilizing ISs results in a faster convergence and a better performance than the OTA FL alone while using less transmit power. We also validate the results on the performance using different numbers of cluster iterations with different datasets and data distributions. We conclude that the best choice of cluster aggregations depends on the data distribution among the MUs and the clusters.
Open Access
RIS-aided secure millimeter wave communication under RF-chain impairments
(IEEE, 2024-01) Ragheb, Mohammad; Kuhestani, Ali; Kazemi, Mohammad; Ahmadi, Hossein; Hanzo, Lajos
The effects of hardware impairments (HWIs) on the secrecy performance of a reconfigurable intelligent surface (RIS)-assisted millimeter wave system are investigated, where a multi-antenna base station (BS) transmits confidential messages to a single-antenna mobile user (MU) in the presence of a single-antenna passive eavesdropper (Eve). Artificial noise (AN)-aided transmission by the BS is proposed for improving the achievable secrecy rate. As the first step, we construct a system model in the absence of HWIs. For this case, we present optimal solutions for the signal and AN powers, the beamforming design at the BS, and the phase shifts of the RIS elements. Then closed-form expressions are derived for the cumulative distribution functions of the signal-to-noise-ratios experienced at the legitimate and eavesdropping nodes. Finally, a compact solution is presented for the ergodic secrecy rate (ESR) performance. At the next step, we extend our designs and analysis by considering the HWIs of the radio-frequency (RF) modules of the BS, MU and Eve. Our results highlight the detrimental impact of HWIs on the achievable ESR. Finally, we find that the ESR can be further increased by beneficially distributing the tolerable HWIs between the transmission and reception RF chains
Open Access
Robust joint precoding/combining design for multiuser MIMO systems with calibration errors
(Institute of Electrical and Electronics Engineers, 2023-01-05) Kazemi, Mohammad; Göken, Çağrı; Duman, Tolga Mete
We consider the downlink of a multiuser system operating in the time-division duplexing mode, for which base station (BS) and users are equipped with multiple antennas, and provide a robust precoding/combining design against imperfect channel state information (CSI) and calibration errors due to hardware mismatch. Towards this end, we first formulate a robust joint precoder and combiner design as a stochastic minimum mean squared error optimization problem. Then, employing an alternating optimization approach, we propose an algorithm to obtain the precoding and combining matrices assuming imperfect CSI and calibration errors at both the BS and the user sides. We also provide asymptotic closed-form expressions for the mean squared error (MSE) and the achievable sum-rate in the massive MIMO regime. The results indicate that while the MSE linearly increases with the calibration errors at the user side, the sum-rate is asymptotically independent of them. Extensive simulation results show that the proposed robust joint precoder/combiner outperforms the existing solutions while having the same order of complexity. Moreover, when the BS sends a quantized version of the combining coefficients to the users, it is observed that the proposed solution is more robust to the quantization errors than the existing algorithms.
Open Access
Robust joint transceiver design for multiuser MIMO systems with calibration errors
(Institute of Electrical and Electronics Engineers, 2022-08-11) Kazemi, Mohammad; Göken, Ç.; Duman, Tolga Mete
We consider the downlink of a multiuser multiple-input multiple-output (MIMO) system operating in the time-division duplexing (TDD) mode. In this mode, assuming reciprocity, the channel coefficients estimated during the uplink channel training are utilized by the base station (BS) in the downlink data transmission. However, due to hardware mismatches, the uplink and downlink channels are not exactly the same, and therefore, there are calibration errors, which degrade the system performance. In this paper, our goal is to provide a transceiver design which has a robust performance under imperfect channel reciprocity. To this end, we first formulate a robust joint precoder and combiner design as a stochastic minimum mean square error (MMSE) optimization problem. Then, employing an alternating optimization approach, we propose an algorithm to obtain the precoding and combining matrices assuming imperfect CSI and calibration errors at both the BS and user sides. Extensive simulation results show that the proposed robust joint precoder/combiner outperforms the existing solutions in the literature.
Open Access
Unsourced random access using multiple stages of orthogonal pilots: MIMO and single-antenna structures
(Institute of Electrical and Electronics Engineers , 2024-06-27) Ahmadi, Mohammad Javad; Kazemi, Mohammad; Duman, Tolga Mete
We study the problem of unsourced random access (URA) over Rayleigh block-fading channels with a receiver equipped with multiple antennas. We propose a slotted structure with multiple stages of orthogonal pilots, each of which is randomly picked from a codebook. In the proposed signaling structure, each user encodes its message using a polar code and appends it to the selected pilot sequences to construct its transmitted signal. Accordingly, the transmitted signal is composed of multiple orthogonal pilot parts and a polar-coded part, which is sent through a randomly selected slot. The performance of the proposed scheme is further improved by randomly dividing users into different groups each having a unique interleaver-power pair. We also apply the idea of multiple stages of orthogonal pilots to the case of a single receive antenna. In all the set-ups, we use an iterative approach for decoding the transmitted messages along with a suitable successive interference cancellation technique. The use of orthogonal pilots and the slotted structure lead to improved accuracy and reduced computational complexity in the proposed set-ups, and make the implementation with short blocklengths more viable. Performance of the proposed set-ups is illustrated via extensive simulation results which show that the proposed set-ups with multiple antennas perform better than the existing MIMO URA solutions for both short and large blocklengths, and that the proposed single-antenna set-ups are superior to the existing single-antenna URA schemes.
Open Access
Unsourced random access using multiple stages of Orthogonal Pilots: MIMO and single-sntenna structures
(IEEE, 2024-02) Ahmadi, Mohammad Javad; Kazemi, Mohammad; Duman, Tolga M.
We study the problem of unsourced random access (URA) over Rayleigh block-fading channels with a receiver equipped with multiple antennas. We propose a slotted structure with multiple stages of orthogonal pilots, each of which is randomly picked from a codebook. In the proposed signaling structure, each user encodes its message using a polar code and appends it to the selected pilot sequences to construct its transmitted signal. Accordingly, the transmitted signal is composed of multiple orthogonal pilot parts and a polar-coded part, which is sent through a randomly selected slot. The performance of the proposed scheme is further improved by randomly dividing users into different groups each having a unique interleaver-power pair. We also apply the idea of multiple stages of orthogonal pilots to the case of a single receive antenna. In all the set-ups, we use an iterative approach for decoding the transmitted messages along with a suitable successive interference cancellation technique. The use of orthogonal pilots and the slotted structure lead to improved accuracy and reduced computational complexity in the proposed set-ups, and make the implementation with short blocklengths more viable. Performance of the proposed set-ups is illustrated via extensive simulation results which show that the proposed set-ups with multiple antennas perform better than the existing MIMO URA solutions for both short and large blocklengths, and that the proposed single-antenna set-ups are superior to the existing single-antenna URA schemes.
Open Access
Unsourced random access using ODMA and polar codes
(Institute of Electrical and Electronics Engineers, 2024-04) Özateş, Mert; Kazemi, Mohammad; Duman, Tolga M.
We utilize the on-off division multiple access technique in unsourced random access and develop a high-performing solution operating on both Gaussian and fading multiple access channels based on polar coding. We propose a transmission structure where a fixed part of the transmission frame is utilized for pilot transmission while the users exploit a randomly-selected small fraction of the rest of the frame for data transmission. Our proposed scheme employs a simple, yet efficient pattern detection and single-user decoding algorithm at the receiver, which leads to a superior performance with similar complexity, or a competitive performance with a significantly lower complexity compared to the existing approaches.