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A Unified Linear Precoding Design for Multi-User MIMO Systems

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A Unified Linear Precoding Design for Multi-User MIMO Systems MD. ABDUL LATIF SARKER, CHONBUK NATIONAL UNIVERSITY, KOREA A Unified Linear Precoding Design for Multi-user MIMO Systems Md. Abdul Latif Sarker algorithms. In [2], Authors considered low-complexity hybrid Abstract—We address the problem of the bit-error-rate (BER) precoding in the massive multi-user MIMO systems and performance gap between the sub-optimal and optimal linear proposed the full-complexity ZF linear precoding to enhance precoder (LP) for a multi-user (MU) multiple-input and the spectral efficiency of the systems. In [6-7], authors shown a multiple-output (MIMO) broadcast systems in this paper. two-tier hybrid precoder scheme to approach the performance Particularly, mobile users suffer noise enhancement effect due to a of the traditional LZF precoder in a multi-user massive MIMO sub-optimal LP that can be suppressed by an optimal LP matrix. system. There are many papers on LZF and LMMSE precoding A sub-optimal LP matrix such as a linear zero-forcing (LZF) precoder performs in high signal-to –noise-ratio (SNR) regime focusing on different design criteria in [9-12]. Authors in [8], only, in contrast, an optimal precoder for instance a linear also shown a two-tier precoder for block diagonalization of a minimum mean-square-error (LMMSE) precoder outperforms in multiuser MIMO channel including other-cell interference. both low and high SNR scenarios. These kinds of precoder All of the above related works have considered the illustrates the BER gap distance at least 0.1 when it is used in itself traditional LZF and LMMSE based precoding. Usually, the in a MU-MIMO systems. Thus, we propose and design a unified conventional LZF precoder achieves the performance close to linear precoding (ULP) matrix using a precoding selection the sum-capacity when the system interference is limited or the technique that combines the sub-optimal and optimal LP matrix number of users become large, otherwise, it requires significant for a multi-user MIMO systems to ensure zero BER performance feedback overhead with respect to SNR while an imperfect CSI gap in this paper. The numerical results show that our proposed ULP technique offers significant performance in both low and at the transmitter that provides the significant throughput loss high SNR scenarios. due to residual multiuser interference. In addition, the conventional LMMSE precoder does not work properly when the user is qualify large in order to a multi-user interference environment. However, we thus far notice that the BER Index Terms—Multiuser MIMO, precoding technique, performance gap between the traditional LZF and LMMSE sub-optimal and optimal precoding, L-ZF and L-MMSE precoder is still high as in [1-4, 6-13].Thus, we propose and precoding, the BER performance gap, a ULP technique design a unified linear precoding (ULP) scheme that overcome the BER performance gap completely in this paper. I. INTRODUCTION This paper is organized as follows: RECODING is an important technique to explore the First, we investigate the system model and problem formulation P significant performance in terms of bit-error rate (BER) as in Section II. Then we design a ULP in Section III. Finally, well as the achievable sum-rates for MU-MIMO downlink numerical results and conclusions are presented in Section IV transmission [1-2]. The most common linear precoding scheme and Section V. such as a LZF precoding in [3-4], a LMMSE precoding in [1, 4] and the nonlinear precoding like dirty-paper coding (DPC) based precoding in [4-5]. Recently, a hybrid precoding scheme has proposed in [1-2, 6-8]. In [1], authors shown a multi-stage robust hybrid linear precoding in a multi-user MIMO systems and proposed the several kind of MMSE precoding based two efficient iterative Fig. 1. Linear Precoding schemes in [4]. II. SYSTEM MODEL AND PROBLEM FORMULATION System Model Md. Abdul Latif Sarker is with the Chonbuk National University, Jeonju, We consider a multiuser MIMO system with downlink 54896 Korea (e-mail: [email protected]). KMt Rk, × M T channel H ∈ whereas the total Kt users each employing M Rk, receive antennas for k− th (kK= 1,2,..., ) receivers and receiving their own data streams are precoded MT ×1 transmit symbol vector x ∈ for Kt users that can be MD. ABDUL LATIF SARKER, CHONBUK NATIONAL UNIVERSITY, KOREA − expressed as x= Fx at the BS with M transmitting antennas HH2 1 T Fu= βσ uu H( HH uu+ m I) where is the MM× LP matrix and is the original symbol −1 F TT x HH FLZFP= β LZFP H() HH ,if u= 0; m = 0; vector for transmission, respectively in Fig.1. Then, the −1 received signal vector y for all user is given by =βσHH +=>2 (7) FLMMSEP LMMSEP H( HHm I) , if u 0; m 0; = − y= HFx + z (1) HH1 FULZFP=β UZFP H u() HH u u ,if u>= 1; m 0 × KMt Rk, 1 where z ∈ is an additive white Gaussian noise −1 =βσHH+2 >> (AWGN) vector with noise covariance matrix FULMMSEP UMMSEPH u( HH u u m I) , if u 0; m 0; H H {}zz= N0 IKM , the operator ()⋅ is called Hermitian, N0 where βu is an estimated unified constant as in (3), that is t Rk, is the noise variance, and I is the KM× KM identity M KMt Rk, t Rk,, t Rk β = T u H . (8) matrix, respectively. Tr ()FFuu However, to compensate for the effect of amplification by a Problem Formulation factor of βu at the transmitter, the received signal must be Let, the traditional LP matrix, F is an optimal LP matrix, divided by βu via automatic gain control (AGC) at the receiver that is given by [4, 1-2, 13-14] ˆ as depicted in Fig.1. Thus, the estimated signal x u after a −1 F= βσ HHH( HH+ m 2 I) unified precoding for all users is given by −−11 −1 ˆ ββ= + = β HH = (2) xu = u y u u( HFx uuu z u) FLZFP LZFP H() HH ,if m 0; − = −12HH 1 −1 =ββ ++ σ , (9) =βσHH +>2 uH uuu H( HH uu m I) xu zu FLMMSEP LMMSEP H( HHm I) , if m 0. T T −1 where m ≥ 0 is a constant that indicates the channel inversion where yyu = []0 , xxu = []0 , zz u= β uu or the regularized channel inversion, and β is a constant to T zzxu =[] −c , respectively. meet that the total transmitted power constraint after precoding and it is given as IV. NUMERICAL RESULTS M β = T We numerically compare our designed ULP scheme in Fig.2 H . (3) Tr ()FF to Fig.4 against the conventional LP scheme for a multiuser MIMO broadcast systems. In computer simulations, we Thus, the estimated signal xˆ after a conventional LP for all user consider = , = , = and assume the total is given by MT 8 M Rk, 1 Kt 20 ˆ −−11 x=ββ y=() HFx+ z (4) active users, KMat= T = 8 in which Kat = 8 users with the highest norm values are selected out of K = 20 . The In reality, we observe that in (2), if m=0 or m > 0 , the LP t matrix F indicates the characteristic of the LZF precoding or quadrature phase shift keying (QPSK) modulation scheme is the LMMSE precoding and the BER performance demonstrates used for the symbol normalizing at 8 transmit antennas with 10 a very high gap between LZF and LMMSE precoding schemes. frames. In the independent and identically distributed (i.i.d) Thus, we design a unified linear precoding (ULP) to mitigate Rayleigh fading channel environment, we treat 1,000 times of this problem in next Section III. Monte Carlo channel realizations in the computer simulations. In this paper, Fig.2 to Fig.4 shows the BER performance gap III. PROPOSED ULP MATRIX DESIGN between the conventional LP and proposed ULP schemes in at several SNR values such as 14, 20 and 30 [dBs] SNR values. We consider a unified channel matrix H is u The measured performance BER gap is 1.1× 10−1 in Table I T Hu = [] HIu (5) between the LZF and LMMSE precoding at 14 [dBs] SNR where u ≥ 0 is a constant that indicates individual or unified values when m=0 and m=1 is applied in (2) which has been precoding, and the pseudo-inverse of a unified channel matrix shown in Fig.2. In contrast, we observe that, the zero BER gap between the LZF and LMMSE precoding based on the H is given by [14] u proposed ULP scheme when u=0, m=0 and u=0, m>0 is applied − † HH1 Hu= H u() HH uu . (6) in (7). The proposed ULP schemes outperform the slightly BER gap between ULZF and ULMMSE precoding at 14 [dBs] SNR By setting (5) in (2) and design a ULP to combine the LZF and cases only if u=1, m=0 and u=1, m=1 is applied in (7) and Table LMMSE precoding is as follows: I. Similarly, in Fig.3 to Fig.4 illustrates 4.7× 10−2 and MD. ABDUL LATIF SARKER, CHONBUK NATIONAL UNIVERSITY, KOREA × −3 5.3 10 BER performance gap between the traditional LZF 0 10 and LMMSE precoding at 20 [dBs] and 30 [dBs] SNR values respectively. Looking in Fig.2 to Fig.4, we note that the channel -1 BER gap gains of almost 1.99, 3.24 and 3.63[dBs] are lost using the 10 traditional LP schemes at 14, 20 and 30 [dBs] SNR scenarios, respectively. -2 10 We also observe in Table I, there is no BER performance gap of LZF, LMMSE, ULZF and ULMMSE while a unified BER -3 precoder select u=0, m=0 and u=0, m=1 at high SNR scenarios 10 but the low SNR scenario illustrates a negligible BER gap such −3 LZFP,when m=0 with c-ch.(2-4) as 1.5× 10 BER gap (in Table I) between the designed ULZF -4 10 LMMSEP, When m>0 with c-ch.(2-4) and ULMMSE precoding when a ULP is selected u=1, m=0 LZFP when u=0,m=0 with u-ch.(7-9) LMMSEP when u=0,m>0 with u-ch.(7-9) and u=1, m=1, respectively.
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