Abstract
In this paper, we investigate the downlink achievable ergodic spectral efficiency (SE) of a single-cell multi-user millimeter wave system, in which a uniform rectangular array is used at the base station (BS) to serve multiple single-antenna users. We adopt a three-dimensional channel model by considering both the azimuth and elevation dimensions under single-path propagation. We derive the achievable ergodic SE for this system in with maximum ratio transmission precoding. This analytical expression enables the accurate and quantitative evaluation of the effect of the number of BS antennas, signal-to-noise ratio (SNR), and the crosstalk (squared inner product between different steering vectors) which is a function of the angles of departure (AoD) of users and the inter-antenna spacing. Results show that the achievable ergodic SE logarithmically increases with the number of BS antennas and converges to a value in the high SNR regime. To improve the achievable ergodic SE, we also propose a user scheduling scheme based on feedback of users’ AoD information and obtain the maximum achievable ergodic SE. Furthermore, we consider a dense user scenario where every user’s AoD becomes nearly identical and then derive the system’s minimum achievable SE.
Highlights
With the proliferation of smart wireless services such as high-definition video streaming, cell broadcasting, and mobile TV, mobile data traffic is envisioned to grow 1000fold by 2020 [1]
We see that for a fixed number of base station (BS) antennas and inter-antenna spacing, the total achievable spectral efficiency (SE) for ZF precoding increases with the signal-to-noise ratio (SNR) for the reason that the ZF precoding scheme is able to completely cancel out inter-user interference, whereas the total achievable SE for maximum ratio transmission (MRT) precoding without scheduling method converges to a saturated value in the high SNR regime because the inter-user interference becomes dominant as the SNR increases
This observation implies that massive antenna arrays contribute substantially toward improving the total achievable SE, which is especially appealing for millimeter wave (mmWave) multiple-input multipleoutput (MIMO) systems
Summary
With the proliferation of smart wireless services such as high-definition video streaming, cell broadcasting, and mobile TV, mobile data traffic is envisioned to grow 1000fold by 2020 [1]. From Lemma 1, it is evident that the optimal beamforming vector wokpt is obtained by adopting MRT precoding This is because when the number of BS antennas grows without bound, the channel steering vectors between different users become asymptotically orthogonal to each other. Based on Theorem 2, we observe that in high-SNR regime, the achievable ergodic SE is a function of the number of BS antennas, the number of users and the crosstalk, which converges to a constant. The reason is both the signal power and the interference power increase as the SNR. In order to proceed, substituting (4) into (21), the crosstalk coefficients aj for URA configuration can be expressed as aj
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