This application claims priority to Indian Patent Application No. 202231028390 filed on May 17, 2022, which is hereby incorporated herein by reference in its entirety.
The present invention relates to wireless communications in high mobility applications. More specifically, the present invention is directed to develop a new pilot pattern and channel estimation method for millimeter wave (mmWave) systems in high mobility scenarios with non-ideal power amplifier, which is applicable in the fifth generation (5G) and beyond networks.
To support the emerging intelligent digital society with high mobility communications, millimeter waves (mmWaves) are envisioned to be a strong contender. However, the channel estimation which is a prerequisite of any communication systems, at such high speeds is difficult and requires large pilot overhead. To deal with, orthogonal time-frequency space (OTFS) modulation has emerged as a strong paradigm, which is resilient to the delay-Doppler shift.
The true potential of mmWave OTFS systems can be utilized with efficient pilot design and channel estimation. Existing literature have developed several channel estimator and pilot design considering the ideal behavior of power amplifier, e.g.
However, it is well-known that power amplifier exhibits nonlinear response at mmWave frequency attributed to the high-frequency operation and large bandwidth. These nonlinear distortions distort the pilot and induce inter-symbol interference (ISI) that destroys the guard region, resulting in erroneous estimation of channel parameters. Hence, it is mandatory to formulate a pilot pattern and channel estimation methodology assuming nonlinear behavior of power amplifier for mmWave OTFS systems to realize its practical applicability in high mobility applications.
It is thus the basic object of the present invention is to develop a pilot pattern for mmWave OTFS systems which would operate involving nonlinear behavior of power amplifier to realize its practical applicability in high mobility applications.
Another object of the present invention is to develop a channel estimation method for mmWave OTFS systems which would operate involving nonlinear behavior of power amplifier to realize its practical applicability in high mobility applications.
Yet another object of the present invention is to develop a pilot pattern and channel estimation method for mmWave systems in high mobility scenarios with non-ideal power amplifier, which is applicable in the fifth generation (5G) and beyond networks.
Thus, according to the basic aspect of the present invention there is provided a pilot pattern for communication signal in wireless communications involving mmWave systems with non-ideal power amplifier comprising
The present pilot pattern includes multiplicative distortion and inter symbol interference (ISI) at receiver side due to the nonlinearity, wherein selective placement of the guards facilitates removal of the ISI, while the multiplicative distortion depends on the pilot magnitude and power amplifier parameters which is known at the receiver, enabling the receiver to evaluate the amplitude and phase of the distorted pilot to carry out the channel estimation.
The present pilot pattern includes a pattern A in presence of nonlinearity formulated as
Where d0 is the pilot, d(l, k) is the information-bearing symbol, and zero is used as the guard; wherein the pilot position is confined within the grid l0∈[lmax, M−1−lmax], k0∈[2kmax, N−1−2kmax], here, lmax and kmax denote the channel's maximum delay and Doppler index and also l=0,1, . . . , M−1 and k=0,1, . . . , N−1; or alternative ideal pattern B in presence of nonlinearity which includes less false and miss detection of paths is
In the present pilot patterns, the selective placement of the guards with respect to the pilots provides overhead, whereby the pilot and the guard overhead for the pattern A is (lmax+1)(2kmax+1)+N−2kmax−1 whereas the pilot and guard overhead for the ideal pattern B is N(lmax+1), thus pattern A is spectrally more efficient than pattern B.
According to a further aspect of the present invention, there is provided a transmitter for transmitting communication signal in wireless communications involving the above pilot pattern comprising
In the present transmitter, the Heisenberg transform modulator is implemented by passing the signal through IFFT and pulse shaping waveform; and for the rectangular pulse shaping waveform, the Heisenberg transform modulator is implemented in hardware in FPGA board involving IFFT, memory blocks, FIR filter, and parallel to serial convertors.
According to a further aspect of the present invention, there is provided a method for estimating channel for communication signal in wireless communications involving mmWave systems with non-ideal power amplifier involving the present pilot pattern comprising
In the above method, for the rectangular pulse shaping waveform, the input-output relation in the delay-Doppler domain with nonlinearity for an identified path corresponding to beamforming angle θi can be established as
wherein, (1, k) is the nonlinearly distorted OTFS sample in the delay-Doppler domain which can expressed as
Wherein X(l, k) is the OTFS sample in lth delay index and kth Doppler index, and
represents the multiplicative distortion, where
denotes the ISI from the nonlinear power amplifier; In the above method, the nonlinearly distorted pilot at the receiver can be written as
The above method includes determination of the Doppler shift corresponding to an identified path and can be given by
{circumflex over (k)}
θ
=NTv
s cos(θi)/λ
determination of the delay by scanning following region
θ
={(l, k0+{circumflex over (k)}θ
obtaining the delay index by
and
estimating the channel coefficient that incorporates the nonlinearly distorted pilot and the estimated delay and Doppler index by
The above method includes FPGA board implemented Accumulators, Multipliers, COordinate Rotation DIgital Computer (CORDIC) for cosine of angle, exponential, and division calculation and comparator with memory blocks to find arg max.
According to another aspect in the present invention there is provided a receiver for implementing the method for estimating channel for communication signal in wireless communications involving mmWave systems with non-ideal power amplifier comprising
In the above receiver, the channel estimation unit is configured to detect presence of a pilot in the delay-Doppler domain for channel estimation based on significant distinction in the power of the distorted pilot and distorted guard and data.
As stated hereinbefore, the present invention provides a new pilot patterns and channel estimation scheme for mmWave systems in high mobility scenarios with non-ideal power amplifier, which is applicable in the fifth generation (5G) and beyond networks. Wireless applications in high mobility scenarios have become evident with the upcoming intelligent digital society. It includes many applications like intra and inter-vehicular communications in high-speed vehicles, high-speed trains, autonomous driving vehicles, etc. This high mobility induces Doppler effect where the wireless channels exhibit time selective fading. Moreover, the presence of multiple scatterers gives rise to the frequency selectivity of the channel. The currently used modulation scheme of fourth generation (4G), orthogonal frequency-division multiplexing (OFDM), can handle the frequency selectivity but is inexpedient in high mobility scenarios as it is inherently sensitive to the high Doppler effects. Recently, a new waveform design, orthogonal time-frequency space (OTFS), has been proposed in the literature to deal with the high mobility issues and is a promising upcoming modulation design.
Furthermore, mmWave frequency bands with a massive chunk of the available spectrum are a promising candidate for 5G. However, attributed to the high frequency and large bandwidth in mmWaves, the integrated circuits (ICs) in mmWaves exhibit nonlinear response and introduce nonlinear distortions in the system. One crucial component that causes nonlinearity in the communication system is the power amplifier. The nonlinear distortion distorts the pilot and guard region in OTFS systems, which results in incorrect channel estimation. Hence, to realize the benefits of fast communication, high reliability, and improved quality of services (QoS) at mmWave in high mobility applications, it is mandatory to design a proper pilot pattern that can handle the effect of nonlinear distortion for accurate channel estimation. The acquisition of channel state information is very crucial in order to retrieve the transmitted symbols in the receiver.
The present invention discloses two different pilot patterns in the delay-Doppler domain to combat the effect of ISI from the nonlinear impairment of power amplifier for the accurate channel estimation. Furthermore, the nonlinear distortion will also distort the pilot symbol, which should be incorporated in the channel estimation scheme. Hence, the present invention also discloses a new channel estimation method with the newly designed pilot pattern, which accurately estimates the high mobility channel in the delay-Doppler domain in the presence of nonlinearity. Moreover, present invention discloses about employing a receive analog beamforming and establishing an input-output relation in the delay-Doppler domain with nonlinearity, which aids in the efficient design of the channel estimator. The beamforming associated with proposed pilot patterns reduces the allocated pilot power to bring energy efficiency to the systems. The beamforming also aids in mitigating the huge path loss of mmWaves.
Let an OTFS frame X∈CM×N consists of a pilot, guard, and complex data symbols arranged in a two-dimensional M×N delay-Doppler grid. M and N denote the number of delay samples and Doppler samples, respectively. Let A is the payload size and vs is the speed of the vehicle. The total frame duration and bandwidth of the transmitted OTFS frame are Tf=NT and B=MΔf, respectively, where T and Δf represent the sampling interval of time and frequency axis, respectively. We consider the case where TΔf=1, i.e., the OTFS signal is critically sampled.
The different proposed pilot pattern designs in the delay-Doppler domain, which consider the nonlinearity effect are shown in
Where d0 is the pilot, d(l, k) is the information-bearing symbol, and zero is used as the guard. The pilot position is confined within the grid l0∈[lmax, M−1−lmax], k0∈[2kmax, N−1−2kmax] for ease of representation. Here, lmax and kmax denote the channel's maximum delay and Doppler index. Also l=0,1, . . . , M−1 and k=0,1, . . . , N−1. Similarly, the expression for pilot pattern of
The distorted pilot in the receive delay-Doppler domain will be composed of multiplicative distortion and ISI due to the nonlinearity. This ISI depends on the information data which is unknown at the receiver. Subsequently, the distorted pilot at the receiver will become unknown. However, with the wise placement of guards on all the Doppler grids corresponding to pilot delay grid, will remove the ISI. Now, there will be only multiplicative distortion in the pilot. This multiplicative distortion depends on the pilot magnitude and power amplifier parameters, which is known at the receiver. Hence, the amplitude and phase of the distorted pilot can be evaluated at the receiver end to carry out the channel estimation. Further, the ISI from the nonlinear distortion distorts the guard region as well. The pilot patterns A and B after experiencing nonlinear distortion will resemble the patterns shown in
The transmitted nonlinearly distorted OTFS sample (n) in nth time instant is represented as
(n)=G (|s(n)|)exp(jΦs(n)+jΨ)(|s(n)|))
Where s(n) is the time-domain OTFS sample at nth instant;
the amplitude distortion defined by amplitude modulation-amplitude modulation (AM/AM) and Ψ(|s(n)|)=κ|s(n)|{tilde over (q)}
where pi is input signal power to power amplifier and psat is saturation power. Lower IBO constitutes a higher nonlinear distortion from the power amplifier.
The received time-domain signal after beamforming for each identified path, {tilde over (r)}θ
Here, (l, k) is the nonlinearly distorted OTFS sample in the delay-Doppler domain which can expressed as
Where X(l, k) is the OTFS sample in lth delay index and kth Doppler index, and
represents the multiplicative distortion, where
denotes the ISI from the nonlinear power amplifier. Note that, additive white Gaussian noise (AWGN) that is independent and identically distributed (i.i.d) complex random variable with zero mean and variance σv2 is omitted in (2) for brevity.
The proposed channel estimation 640 will determine the Doppler shift, the delay, and the fading coefficient in the presence of nonlinear distortions. Firstly, the existence of a path is determined by thresholding the received signal after beamforming. Then, the estimation of Doppler, delay shift, and fading coefficient will be established.
Following (3), the expression of the nonlinearly distorted pilot can be written as
From (4), one can observe that the distorted pilot contains additional multiplicative distortion and ISI which is unknown (since these terms depend upon the data symbols). Hence, the distorted pilot is unknown to the receiver, and the channel estimation could not proceed. However, with the proposed pilot patterns A and B, where zeros are placed in all the Doppler grids corresponding to pilot position delay index l0, the ISI does not exist, and the amplitude and phase of the distorted pilot can be calculated at the receiver.
Note that in pilot pattern A, due to the placement of all zeros from k=0: N−1, k≠k0, at delay index l0, the power of the received nonlinearly distorted pilot in the delay-Doppler domain will be high as compared to the power of the distorted data symbols or distorted guard symbols. The received distorted pilot will not have any ISI with pilot pattern A and the level of severity in multiplicative distortion will also be less. Hence, the power amplifier will mainly boost the power of the received pilot. In contrast, the data and guard will experience multiplicative distortions and ISI due to the nonlinear power amplifier. These multiplicative distortions and ISI lead to a reduction in the power of the nonlinearly distorted data and distorted guard as ISI may result in destructive interference. Hence, a significant distinction in the power of the distorted pilot and distorted guard and data can be observed. This distinction in the power level can be utilized to establish the presence of a pilot in the delay-Doppler domain for channel estimation, even when low power is allocated to the pilot. Similar arguments follow for the pilot pattern B. Moreover, the guard symbols of ideal pilot pattern B do not get affected due to the nonlinear power amplifier. Hence, the proposed pilot pattern and channel estimation in the presence of nonlinearity can achieve reliable communication.
The determination of the Doppler shift corresponding to an identified path can be given by
{circumflex over (k)}
θ
=NTv
s cos (θi)/λ. . . 5
The estimation of the delay can be accomplished by scanning the following region
θ
={(l, k0+{circumflex over (k)}θ
and the delay index estimate is obtained by
Finally, the estimate of the channel coefficient incorporates the nonlinearly distorted pilot and can be given for the estimated delay and Doppler index by
The estimated channel can be used for equalization 650 in order to retrieve the information symbols.
For testing, OTFS symbols of N=128 and subcarriers M=128 is considered. The carrier frequency is centered at 28 GHz and subcarrier spacing is 200 kHz. Further, we adopted the Urban Microcell (UMi) street canyon channel model, simulated according to the tapped delay line (TDL)-B model of 3GPP [3GPP TR 38.900: Study on channel model for frequency spectrum above 6 GHz.” Tech. Rep] in the NLOS scenario with a delay spread of 66 ns. The nonlinear power amplifier model proposed by IEEE 802.11ad TG [E. Perahia et al., “IEEE P802.11 Wireless LANs TGad Evaluation Methodology,” IEEE, vol. 802, pp. 3-5, 2010] is considered to model the nonlinearity of mmWave power amplifier. We define the signal-to-noise ratio (SNR) of the data symbols as
and SNR of the pilot as
The performance of the proposed channel estimator with the different proposed pilot patterns at 0 dB IBO over the mmWave NLOS UMi channel is evaluated in
The advantages of the present invention can be summarized as hereunder:
Number | Date | Country | Kind |
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202231028390 | May 2022 | IN | national |