MIMO Modes in LTE

Introduction

Multiple input multiple output (MIMO) technology is a key technology for fourth generation (4G), and beyond, wireless communications. MIMO exploits the space diversity to improve performance as well as augment its capacity. Long Term Evolution (LTE), the dominant 4G standard, operates in diverse MIMO modes (Zarrinkoub, 2014). Figure 1 illustrates the key MIMO modes in LTE. The single input single output (SISO) mode has a single link and offers no diversity; hence, the link reliability is compromised in SISO mode. In the single input multiple output (SIMO) mode, a single antenna is present at the transmit end, while multiple antennas are placed at the receiving end. In this mode, receiver diversity is exploited. The key techniques employed at the receiver are selection combining (SC) and maximal ratio combining (MRC). The multiple input single output (MISO) mode exploits transmit diversity and has multiple antennas at the transmit end, while a single antenna is placed at the receiving end. It provides the same diversity order as the MRC and does not require any bandwidth expansion or any feedback from the receiver to the transmitter (Alamouti, 1998). Thus, the prime advantage of this technique is that it does not require channel state information (CSI).

Figure 1.

Key MIMO modes in LTE

In MIMO mode, multiple antennas are employed at both the transmit end, as well as the receiving end. The degree of freedom is dictated by the product of transmit and receive antennas, provided they are not correlated. Figure 2 illustrates the key benefits of MIMO technology in spatial multiplexing, spatial diversity, and beam-forming techniques (Mietzner et al., 2009). A sublime trade-off exists between performance and the capacity advantage regarding multiplexing and diversity gain. To improve link reliability, diversity gain is enhanced, which may undermine the multiplexing gain. The beam-forming technique is oriented to combine the advantages of spatial diversity and spatial multiplexing by focusing the transmit energy towards the desired user. Orthogonal frequency division multiplexing (OFDM) offers immunity against multipath fading and combats inter-symbol interference (ISI). Blended with OFDM, MIMO technology offers a suitable air interface for 4G, and beyond, wireless standards.

Figure 2.

Key Benefits of MIMO (Adapted from Mietzner et al., 2009)

Key Contributions Of The Article

This study provides an analysis of the SIMO and MIMO modes using the MRC and SC techniques for exploiting receive diversity. The following sections analyze the different configurations of the MIMO under different fading environments and different numbers of transmit and receive antennas. Earlier works by Hasna and Alouini (2003) outlined the outage probability in Nakagami-m channel, while Goldsmith and Varaiya (1997) derived the closed form expression for Capacity with CSI. The key contributions of the paper are:

• 1.

  Develop a mathematical model for analyzing SIMO and MIMO modes in different fading environments.

• 2.

  Derive a closed form expression for bit error rate (BER), ergodic capacity, average signal-to-noise ratio (SNR), and outage probability (P_out).

• 3.

  Analysis of performance metrics with different number of transmit and receive antennas for a given number of channel taps and under different fading environment.

System Analysis

SISO Analysis

In their analysis, the authors have considered the SISO mode for a Rayleigh channel. Figure 3 plots (1), the BER depicting a SISO mode for a Rayleigh channel for binary phase shift keying (BPSK) modulation.

Figure 3.

BER for a fading channel

The channel is defined as (1).

(1)

where are received signal, channel, transmit signal, and Gaussian noise, respectively.