ML Detection in Phase Noise Impaired SIMO Channels With Uplink Training
Summary (3 min read)
I. INTRODUCTION
- The demand on wireless data services is expected to increase significantly over the next decade.
- Hence, the study of the impact of hardware impairments is of particular importance and relevance in Massive MIMO systems.
- This happens when the variance of the phase noise innovations and the coherence interval of the channel fading are large [7, Section IV.C].
- Finally, the authors of [11] observe that the phase noise number (i.e., the second term in the high-SNR capacity expansion) is higher in the case of independent phase noise sources.
- Motivated by that, the authors rigorously derive the optimal detector in phase noise impaired SIMO systems with uplink training for various cases of interest.
II. SYSTEM MODEL
- A single-antenna user communicates with a BS equipped with M antenna elements, which are impaired with phase noise.
- Two different cases are treated with respect to the knowledge of the wireless channel.
- Namely, in the first case, termed as constant channel (CC), the channel is assumed deterministic and known at the receiver [25] , [26] .
- Hence, the transmitted symbol is observed in the presence of only additive noise and multiplicative phase noise.
- The authors start with the description of the CC for simplicity and subsequently they describe the extension to the FC.
A. Constant Channel (CC)
- The Fourier expansion in (3) can represent any pdf in [−π, π) that is continuous, differentiable, unimodal, even and has zero mean.
- For oscillators equipped with a PLL, the phase noise increment is well modeled by a random variable from a von Mises (or, equivalently, Tikhonov) distribution [11] .
- This models a practical distributed antenna deployment where the use of a separate oscillator per BS antenna is required.
- Hybrid topologies are also possible, however, they are less interesting to analyze since their SER is expected to lie between the SER of the S and NS operations.
- Hence, they are not considered in this work.
B. Fading Channel (FC)
- During the second channel use, the signal received at the m-th BS antenna is given by EQUATION.
- The fading coefficients h m remain constant over both channel uses.
Proposition 1:
- The result follows trivially from Proposition 1 by taking the natural logarithm of (44), ln (p(x, y|s)), and dropping the terms that are independent of s.
- The results in Corollary 1 hold for arbitrary constellations.
- In the following the authors particularize Corollary 1 for the case of phase shift keying (N-PSK) constellations.
Proposition 2:
- For the S operation, the calculation of the optimal detectors requires the computation of inner products between vectors of size M. For the NS operation, the computational complexity scales also linearly with M. Hence, the complexity of the optimal detectors scales in all cases linearly with the number of BS antennas.
- From (15) and ( 25), the ML detectors involve the calculation of infinite series, where the l-th term is a function of the modified Bessel functions, I l .
- Since the series converge very fast, the truncation error can be made negligible.
- The following lemma is useful to demonstrate this claim.
A. High SNR Analysis for the Synchronous Operation
- Hence, distinguishing two symbols that have different amplitude is trivial at high-SNR.
- Distinguishing two symbols that have different phase is much more challenging.
- Constellation points that are at the same radius belong to the same sub-constellation and their pairwise error probability can be calculated in a manner similar to PSK constellations.
- The authors proceed by deriving the asymptotic SER at high-SNR for PSK constellations.
- For the CC-S case, let φ be zero mean random variable with pdf p Φ (φ) as in (3) , where the distribution is unimodal and symmetric around the mean, also known as Proposition 4.
The observation vector
- The derivation of the optimal detection rule even in this regime appears to be mathematically intractable.
- The suboptimal decision rule that the authors use is the minimum Euclidean distance from a scaled N-PSK, i.e., ŝ = arg min EQUATION ) where S is the N-PSK alphabet.
- Hence, the SER floor for the FC-NS case can be made arbitrarily small as M → ∞.
- Similar conclusions were also drawn in [11] for the uplink case.
- Similar conclusions have already been drawn in prior work, such as [12] , where it is shown that in the NS operation the phase noise impairments at the BS average out but not the ones at the user terminals.
V. NUMERICAL EXAMPLES
- In this section the authors present numerical examples that verify the validity of the analytical results presented in Sections III and IV.
- As κ increases the distribution becomes more concentrated around the mean.
- In the medium SNR regime (≈ 0 − 10 [dB]) the non-synchronous operation has a clear advantage over the synchronous operation.
- The theoretical SER floor for the CC-S and FC-S cases is also given by the dotted line.
- This implies that the additional randomness due to fading in the FC-NS case has a direct impact on the SER performance.
A. Extension to Longer Data Intervals
- In practice more than one channel uses are spent for data transmission.
- For this purpose, the authors consider a setup where the data interval is extended to T channel uses.
- Hence, the approach can be summarized as follows.
- If a suboptimal tractable detector for the NS operation performs better than a genieaided, i.e. better-than-optimal, tractable detector for the S operation, then the same will hold for the corresponding optimal detectors.
- As noted, this detector is suboptimal but implementable.
ŝt = arg max
- This detector performs better than the actual causal ML detector for the synchronous operation, since it has the additional knowledge of the true prior symbols and the evolution of the phase noise process up to t − 1.
- This corresponds to the practical scenario of free-running oscillators.
- It is clear that the suboptimal FC-NS detector performs better than the 'better-than-optimal' FC-S detector, which establishes the claim that the results of the previous sections are valid in more complex setups.
VI. CONCLUSIONS
- The problem of ML detection in a training-assisted single-user SIMO channel with phase noise impairments and M BS antennas was studied.
- For both assumptions on the channel gains two operations were investigated, i.e., the synchronous and non-synchronous operations.
- Closed-form expressions of the optimal detectors were given for a general parameterization of the phase noise increments.
- SER floors were observed for all cases under study.
- For the synchronous operation the SER floors were independent of M for both the constant and fading channel case.
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Citations
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Cites background from "ML Detection in Phase Noise Impaire..."
...In other words, the amplitude of phase noise is determined by the imperfections of local oscillators [66]....
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Cites background from "ML Detection in Phase Noise Impaire..."
...Due to the impairments of local oscillators, phase noise is generated during the up-conversion of baseband signal to bandpass and vice versa [14]....
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...B = 2πc B Ts, where both c [m] X and c [n] B are constants and represent the one-sided 3-dB bandwidth of the Lorentzian spectrum of the oscillators at the m transmit and n receive antennas, respectively [14], [15], [27]....
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...Phase noise is generated at both transmitter and receiver sides during the up-conversion of baseband signal to bandpass and vice versa due to the impairments of local oscillator [14], [15], [27]....
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...According to [14]–[16], phase noise innovation variances are closely relevant to physical properties of oscillators, i....
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...According to [14]–[16], phase noise is determined by the quality of the oscillators being used at the transmitter and receiver....
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18 citations
References
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"ML Detection in Phase Noise Impaire..." refers background in this paper
...Recently, it has been shown that massive multiple-input multiple-output (Massive MIMO) can provide substantial gains in spectral efficiency and radiated energy efficiency [1]–[3]....
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6,184 citations
"ML Detection in Phase Noise Impaire..." refers background in this paper
...Ratio of modified Bessel functions Iμ(x) I1(x) and the bound from Lemma 1 for μ = [2, 3, 4] as a function of x ....
[...]
...Recently, it has been shown that massive multiple-input multiple-output (Massive MIMO) can provide substantial gains in spectral efficiency and radiated energy efficiency [1]–[3]....
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Frequently Asked Questions (14)
Q2. What is the phase noise of a free-running oscillator?
For free-running oscillators phase noise is often modeled as a discrete-time Wiener process, where the increments are i.i.d. wrapped Gaussian increments [30].
Q3. What is the effect of the phase noise increment on the input symbol?
The authors note that due to the symmetry of the von Mises distribution around its mean and the uniform priors on the input symbols, the conditioning on any particular input symbol does not affect the result.
Q4. What is the likelihood function of the observed phases?
The likelihood function of the observed phases ψ ∆ = [ψ1, . . . , ψM ] T , given the transmitted phase arg(s), can be expressed aspΨ|S(ψ|s) = M∏m=1pΨm|s(ψm|s) = eκ∑M m=1 cos(ψm−arg(s))(2πI0(κ))
Q5. What is the common way to get a VM(0, ) random?
To get a mathematically tractable expression, the authors consider that the increments φm are independent VM(0, κ) random variables for m = 1, . . . ,M .
Q6. What is the m-th component of the additive white Gaussian noise vector?
During the first channel use, the received complex baseband symbol, xm, at the m-th BSantenna is given by7 xm = √ ρgme jθm + wm, m = 1, . . . ,M, (1)where wm is the m-th component of the additive white Gaussian noise (AWGN) vector, w, distributed as a circularly symmetric complex Gaussian random vector, CN (0, IM), θm is the unknown initial phase reference uniformly distributed in the interval [−π, π).
Q7. What is the ML detector in the CC-S case?
From (15) and (25), the ML detectors involve the calculation of infinite series, where the l-th term is a function of the modified Bessel functions, Il(·).
Q8. What is the corresponding ML decision rule for symbols selected from some alphabet S?
Mand the corresponding ML decision rule for symbols selected from some alphabet S is given byŝ = argmax s∈S pΨ|S(ψ|s) = argmax s∈SM∑m=1cos (ψm − arg(s)) .
Q9. What is the SER floor for the CC-S case?
(32)Corollary 5: From (32) the authors observe that there is a non-zero SER floor for the CC-S case, which depends only on the statistics of the phase noise increment and the PSK constellation density, N , but is independent of the number of receive antennas, M .Remark 1: The preceding analysis is also true for FC-S by defining x̃ ∆= h and ỹ ∆= hejφs.
Q10. What is the SER floor for equiprobable N-PSK symbols?
Then the SER floor at the high-SNR for equiprobable N-PSK symbols is given byPr {ǫ} ∆= 1− ∫ π N− π NpΦ(φ)dφ = 1− α0 N− ∞∑l=12αl πl sin ( l π N ) .
Q11. What is the optimum estimate of the transmitted information symbol?
The BS uses the received vectors x and y jointly, to derive the optimum estimate, ŝ, of the transmitted information symbol, s, i.e.,ŝ ∆ = argmax s∈S p(x,y|s).
Q12. What is the SER for equiprobable input symbols?
(33)Proposition 5: The decision metric, µn, for the symbol ej 2πn N from an N-PSK constellationbased on (33) is given byµn ∆ =M∑m=1(cos(ψm − 2πnN) − cos (ψm) ) = M∑m=1sin (πnN) sin ( ψm − πnN). (34)17If the authors denote by ǫ the error event, i.e., the case where the detected symbol ŝ is different from the transmitted symbol s, then the SER for equiprobable input symbols is given byPr {ǫ} = Pr { N−1⋃n=1{µn >
Q13. What is the ML detection rule for the CC-NS case?
The detector for the S operation is given by Corollary 3.Corollary 3: For a discrete constellation, S, the ML symbol, ŝ, for the S operation isŝ = argmax s∈S LSs = argmax s∈S B + ln(β0 + 2 ∞∑l=1βl cos (lζ)).
Q14. What is the ML detection rule for a CC-NS case?
Proposition 2: The pdf of the received vectors (x,y) given a symbol s for the S operationis given byp(x,y|s) = A (β0 + 2∞∑l=1βl cos (lζ))(20)where for the CC-S case the authors have A as in (10),βl = αlIl(2 √ ρ|s∗gTy|)