In satellite communications power consumption and reliability are key concerns. The unreliable nature of the wireless channel can cause erroneous transmissions, which must be compensated using retransmissions, at the expense of more power. This issue is aggravated by the low amplification efficiency of high data rate schemes such as Orthogonal Frequency Division Multiplexing (OFDM). Ideally, retransmissions could be avoided entirely by increasing transmission power, however, this would drastically lower the overall energy efficiency of the system. Sophisticated Hybrid Automatic Repeat Request (HARQ) techniques can simultaneously reduce the need for retransmissions and increase the success likelihood of each retransmission attempt (a critical aspect for delay-constraint services over satellite links), while minimizing average consumed power. In this work, we propose a novel HARQ technique where retransmitted signals are submitted to a soft clipping operation that lowers the Peak-to-Average Power Ratio (PAPR) of the OFDM signal, thereby increasing the amplification efficiency. A Generalized Approximate Message Passing (GAMP) based receiver makes use of the additional distortion in the retransmission to increase the likelihood of a successful decoding. It is shown that this approach can outperform Chase Combining (CC) schemes even for Super Quadrature Amplitude Modulation (Super-QAM), while decreasing the power expended during retransmissions.

Future wireless communication systems require increasing the data-rate, tremendously, given a limited bandwidth. One solution is to use super high order constellations in high signal to noise ratio (SNR) regimes. However, in practical systems, these constellations face many issues including hardware impairments and system non-linearities. In this paper we focus on the carrier frequency offset (CFO) and investigate its effects on the performance of an orthogonal frequency-division multiplexing (OFDM) system that applies a super quadrature amplitude modulation (super-QAM). Our analysis show that although super-QAM constellations are more sensitive to CFO than low-order QAM constellations, on the other hand, we can take advantage of the high SNR regime that super-QAM constellations are used in and obtain an accurate CFO estimate and consequently reduce error vector magnitude (EVM).

Non-linear distortion imposed to orthogonal frequency division multiplexing (OFDM) signals from non-linear power amplifiers (PAs), has always been a challenging problem in wireless communications. This is due to the high peak-to-average power ratio (PAPR) of OFDM signals. To overcome this issue, receiver side compensation techniques based on distortion cancellation and iterative detection have been proposed. However, these techniques require knowledge of the non-linearity (NL) parameters. Therefore, in this paper we focus on NL parameter estimation and propose a least squares (LS) based estimation technique by transmitting a training signal. We study the performance of the proposed technique for the solid state power amplifier (SSPA) model and the Bussgang model. The studies are done considering both perfect channel knowledge and imperfect channel knowledge at the receiver. The results show that the proposed technique has good accuracy and performance for distortion cancellation and iterative detection based receivers such as the Bussgang receiver. The results also indicate that the estimation technique is more sensitive to channel estimation error than the additive Gaussian noise.

Abstract Low cost video sensors used for streaming video signals to help firefighters, require high bit rate due to uncompressed images. To increase spectral efficiency given a limited bandwidth, very high order constellations in high signal to noise ratio regimes can be used. However, noise is not the only factor effecting the high order constellations. These constellations are also sensitive to hardware impairments and system non‐linearities. Therefore, in this paper, the effect of carrier frequency offset (CFO) on the performance of an orthogonal frequency division multiplexing (OFDM) system with high order quadrature amplitude modulation (QAM) is studied. A closed form expression is derived for the maximum normalized residual CFO that an OFDM system with M ‐QAM constellation can resist to have an error free symbol detection. Finally, the suitability of common previous CFO estimation techniques such as the cyclic prefix based technique and the Moose technique in these systems are investigate. The results show that the maximum residual CFO that an OFDM system with M ‐QAM constellation can resist is proportional to the inverse of . The results also show that very large order QAM constellations such as 4096‐QAM are very sensitive to even small residual CFO values and their performance degrades, significantly. However, the bit error rate analysis indicate that the Moose CFO estimation technique can be used in these systems to compensate the CFO effect, accurately.

Cell-free (CF) massive multiple input multiple output (MaMIMO) has been suggested to reach future wireless networks demands. Analyzing the effect of hardware impairments on performance of CF MaMIMO systems is of vital importance. In this paper, we investigate the effect of residual carrier frequency offset (CFO) on the uplink sum-rate of CF MaMIMO systems. Therefore, we derive closed form expression for the uplink achievable sum-rate of orthogonal frequency domain multiplexing (OFDM) based CF MaMIMO systems. Our results show that in a fully synchronized CF MaMIMO system, the sum-rate increases unlimitedly as the number of access points (AP) increases. However, when the system is hit by CFO the sum-rate is limited even when the number of APs tends to infinity. Our results also indicate that for finite number of APs, when the number of antennas at each AP tends to infinity, the sum-rate of the system in the presence of CFO is bounded. While, for the fully synchronized system, as the number of antennas at each AP grows large, the sum-rate increases, unlimitedly when the antennas of an AP are independent and is bounded when there is a correlation between the antennas of an AP.

Low complexity and spectral efficient carrier frequency offset (CFO) estimation in massive multiple input multiple output (MIMO) systems is a critical problem. Therefore, in this paper we propose a low complexity CFO estimation technique for single carrier frequency domain equalization (SC-FDE) and orthogonal frequency division multiplexing (OFDM) based Massive MIMO systems. In this technique, to increase the spectral efficiency, we use the pilots sent for channel estimation to estimate both the CFO and the channel, iteratively. Our analysis show that our proposed technique has better performance and lower complexity than previous techniques which utilize one pilot block for both CFO and channel estimation in massive MIMO systems.

In this paper, we investigate the suitability of single carrier frequency domain processing (SC-FDP) as the downlink transmission scheme in a massive multiple-input multiple-output (MIMO) system. By deriving the sum-rate of the SC-FDP massive MIMO system theoretically, we show that this method obtains a sum-rate similar to that of orthogonal frequency division multiplexing (OFDM) massive MIMO. We also derive the theoretical sum-rate of both SC-FDP and OFDM in a non-synchronized massive MIMO scenario and show that the rate of the former is significantly larger than that of the latter. Moreover, we theoretically analyze the sum-rate of both systems in the presence of power amplifier non-linearity. All the sum-rates are derived for both zero forcing and matched filter precoding schemes. The results show that the effect of power amplifier non-linearity on the sum-rate of both systems is similar when the number of users is large. We also compare SC-FDP with OFDM from the peak to average power ratio (PAPR) and complexity viewpoints. Although the PAPR of SC-FDP signals is lower than that of OFDM signals, for MIMO systems, the difference between their PAPR decreases as we increase the number of users. Thus, both techniques can have similar PAPR in massive MIMO systems. The overall complexity of SC-FDP and OFDM is similar. Due to the mentioned facts, SC-FDP can be considered as a promising transmission scheme for the downlink of the massive MIMO systems in the presence of carrier frequency offset and power amplifier non-linearities.

In this paper, we investigate the effect of carrier frequency offset (CFO) on the sum-rate of block-wise single carrier(SC) massive MIMO systems. Based on the user knowledge regarding the channel conditions, we consider three different scenarios. In the first scenario, the users know the expectation of the effective channel including the CFO effect in the time domain. In the second scenario, the users know the expectation of the effective channel including the effect of the CFO in the frequency domain. In the third scenario, the users know the expectation of the effective channel without the effect of the CFO. We show that in the first scenario, the effect of the CFO can be completely compensated. We also show that for finite number of base station (BS) antennas, the sum-rate in the second scenario is higher than that of the third scenario. However, as the number of BS antennas tends to infinity, the sum-rate in the former case is bounded and in the later case it tends to infinity.