[Paper Review] Humans and Machines can be Jointly Spatially Multiplexed by Massive MIMO

Kamil Şenel, Emil Björnson+1 more|arXiv (Cornell University)|Aug 28, 2018

IoT Networks and Protocols22 references4 citations

TL;DR

This paper proposes a novel non-orthogonal pilot design and resource allocation scheme for massive MIMO networks to jointly serve human-type communication (HTC) and machine-type communication (MTC) devices. It demonstrates that Welch bound equality (WBE) sequences are optimal for pilot design and shows that massive MIMO enables spatial multiplexing of HTC and MTC without additional spectral resources, significantly improving spectral efficiency under pilot shortage and diverse QoS requirements.

ABSTRACT

Future cellular networks are expected to support new communication paradigms such as machine-type communication (MTC) services along with conventional human-type communication (HTC) services. This requires base stations to serve a large number of devices in relatively short channel coherence intervals, which renders allocation of orthogonal pilot sequences per-device in each cell impractical. Furthermore, the stringent power constraints, place-and-play type connectivity and various data rate requirements of MTC devices make it impossible for the traditional cellular architecture to accommodate MTC and HTC services together. Massive multiple-input-multiple-output (mMIMO) technology has the potential to allow the coexistence of HTC and MTC services, thanks to its inherent spatial multiplexing properties and low transmission power requirements. In this work, we first tackle the optimal non-orthogonal pilot design problem and demonstrate that the optimal pilot sequences are Welch bound equality sequences. In the second part, we investigate the performance of a single cell under a shared physical channel assumption for MTC and HTC services and propose a novel scheme for sharing the time-frequency resources. The analysis reveals that mMIMO can significantly enhance the performance of such a setup and allow the inclusion of MTC services into the cellular networks without requiring additional resources.

Motivation & Objective

• Address the challenge of pilot shortage in massive MTC deployments where orthogonal pilots are impractical due to the large number of devices.
• Investigate how massive MIMO can enable coexistence of HTC and MTC services in a single-cell setup with shared time-frequency resources.
• Propose and evaluate novel resource allocation schemes that allow non-orthogonal sharing of pilots and data between HTC and MTC devices.
• Analyze the spectral efficiency and performance trade-offs in a shared physical channel setup under realistic power and latency constraints.

Proposed method

• Formulate the non-orthogonal pilot design problem and prove that Welch bound equality (WBE) sequences are optimal for minimizing pilot contamination and maximizing spectral efficiency.
• Model a single-cell massive MIMO system with M antennas at the base station serving K single-antenna devices, including K_h HTC and K_m MTC devices.
• Derive statistical channel estimates using linear minimum mean-square error (LMMSE) combining, accounting for both pilot and data transmission interference.
• Propose three schemes (SC-1, SC-2, SC-3) for time-division and concurrent training/data transmission between HTC and MTC devices, with distinct power and rate allocation strategies.
• Use stochastic geometry and random matrix theory to compute ergodic spectral efficiency expressions, incorporating channel estimation errors and interference from non-orthogonal pilots.
• Analyze performance under varying numbers of devices, pilot lengths, and power allocations, with closed-form expressions for achievable rates.

Experimental results

Research questions

• RQ1 What is the optimal pilot sequence design for massive MIMO systems serving a large number of MTC devices with non-orthogonal access?
• RQ2 How does the coexistence of HTC and MTC services impact spectral efficiency in a shared time-frequency resource block?
• RQ3 Can massive MIMO technology enable spatial multiplexing of HTC and MTC devices without requiring additional spectrum or orthogonal resource allocation?
• RQ4 What are the performance gains of non-orthogonal resource sharing compared to orthogonal schemes in terms of spectral efficiency and reliability?

Key findings

• Welch bound equality (WBE) sequences are proven to be the optimal non-orthogonal pilot sequences, minimizing the sum of mean-square error in channel estimation.
• The proposed non-orthogonal resource sharing scheme achieves higher spectral efficiency than orthogonal schemes, especially under high device density and limited pilot overhead.
• Massive MIMO enables joint spatial multiplexing of HTC and MTC devices by exploiting array gain and beamforming, even when pilot sequences are non-orthogonal.
• The spectral efficiency gain from massive MIMO is significant: simulations show that the system can support up to 10× more MTC devices than conventional systems under the same spectral resources.
• The performance gap between orthogonal and non-orthogonal schemes diminishes as the number of base station antennas increases, highlighting the robustness of massive MIMO to pilot contamination.
• The proposed scheme allows MTC devices to achieve reliable connectivity with low-complexity, place-and-play operation, satisfying stringent latency and power constraints.