Non-Coherent Massive MIMO-OFDM Down-Link Based on Differential Modulation

Abstract

Orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) are wireless radio technologies adopted by the new Fifth Generation (5G) of mobile communications. A very large number of antennas (massive MIMO) is used to perform the beam-forming of the transmitted signal, either to reduce the multi-user interference (MUI), when spatially multiplexing several users, or to compensate the path-loss when higher frequencies than microwave are used, such as the millimeter-waves (mm-Waves). Usually, a coherent demodulation scheme (CDS) is used in order to exploit MIMO-OFDM, where the channel estimation and the pre/post-equalization processes are complex and time consuming operations, which require a considerable pilot overhead and also increase the latency of the system. As an alternative, non-coherent techniques based on a differential modulation scheme have been proposed for the up-link (UL). However, it is not straightforward to extend these proposals to the down-link (DL) due to the (usually) reduced number of antennas at the receiver side. In this paper we overcome this problem, and assuming that each user equipment (UE) is only equipped with one single antenna, we propose the combination of beam-forming with a differential modulation scheme for the DL, enhanced by the frequency diversity. The new transmission and reception schemes are described, and the signal-to-interference-plus-noise ratio (SINR) and the complexity are analysed. The numerical results verify the accuracy of the analysis and show that our proposal outperforms the existing CDS with a significant lower complexity.

Non-coherent massive MIMO-OFDM for communications in high mobility scenarios

“…Examples of application are Bluetooth [25], with a non-coherent frequency shift keying (FSK) receiver or Zigbee [26], using DPSK. Differential modulation is one of the most frequently used techniques for NCDS [12] - [19]. In this case the information is encoded by computing the phase difference between the current complex data symbol and the previously transmitted symbol.…”

“…Typically, only two and four transmit antennas are taken into account [27] - [31]. More recently, the combination of beamforming and NCDS has been proposed in order to exploit the high number of antennas through compensating the path loss and enhancing the quality of the link, and spatially multiplexing the different UEs [18], [19]. In these cases a certain channel knowledge is needed to point the beam towards the UE through the beam-management procedure, and the signal is processed non-coherently in each beam afterwards.…”

“…The mapping schemes and multi-user constellations proposed for the UL are still valid, while we need a few more ingredients to make these schemes suitable for the DL. We explain in the following these key ingredients for a combination of NCDS with beamforming based on [19], where the BS is simultaneously transmitting the data streams through its antennas, while the UEs are receiving with their single-antenna device.…”

“…Reference [18] proposed a similar idea assuming, however, an ideal case where the MUI is completely mitigated by the beamforming. Meanwhile, the combination of NCDS with a practical beamforming technique is proposed in [19], taking into account the residual MUI.…”

“…Furthermore, the complexity of the signal processing techniques need to be bounded to reduce the cost of the devices and the delay of the required operations. As an alternative to classical coherent demodulation schemes (CDS), non-coherent demodulation schemes (NCDS) [7][8][9] have been proposed recently to be combined with massive MIMO systems [10] - [19]. They are capable of avoiding the overhead produced by the reference signals due to the fact that the transmitted symbols can be recovered without the knowledge of channel state information (CSI).…”