Terminology » Distributed MIMO (D-MIMO) Vs Coordinated Multi-AP (CoMP):Key differences

Distributed MIMO (D-MIMO) Vs Coordinated Multi-AP (CoMP):Key differences

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Introduction : In the quest for faster, more reliable wireless communication, two advanced technologies have emerged as key enablers of next-generation networks: Coordinated Multi-Access Point (Multi-AP) and Distributed Multiple-Input Multiple-Output (MIMO). While both aim to enhance wireless performance by utilizing multiple transmission and reception points, they operate on different principles and are tailored for distinct scenarios. This article delves into the intricacies of each technology and presents a clear comparison to delineate their unique characteristics.

Distributed MIMO (D-MIMO)

Distributed MIMO, often associated with concepts like cell free massive MIMO, represents paradigm shift in wireless network architecture.

In such a system, rather than a single AP or base station with many collocated antennas, you have large number of geographically separated antennas, often referred to as access points or radio units, cooperatively serve a user.

They coordinate (in the PHY layer) to transmit/receive jointly (e.g. via shared channel state information, joint precoding) and thereby exploit spatial multiplexing, diversity and reduce inter node interference.

The core principle of Distributed MIMO is to transform the entire network of distributed antennas into a massive, virtual MIMO system. This offers several significant advantages as follows.

Enhanced Signal Strength and Coverage
Increased Spectral Efficiency
Improved Reliability

Distributed MIMO systems require a high capacity fronthaul network to connect the distributed antennas to a central processing unit where the complex signal processing and coordination take place.

Use cases: In cellular networks, Nokia Bell Labs conducted research on “Distributed Massive MIMO” for 5G Advanced uplink capacity. Some work is being explored for its use for WiFi access points acting in joint mode.

Coordinated Multi-AP

Coordinated Multi-AP (in WiFi context) or Coordinated Multi-Point (CoMP) in cellular networks refers to a technique where multiple network nodes (e.g., access points or base stations) coordinate their transmissions and/or receptions to minimize interference, improve link reliability, and boost performance. Several key coordination strategies are employed in Coordinated Multi-AP systems as follows.

Coordinated Scheduling (using c-TDMA) : This involves synchronizing the transmissions of different APs to avoid collisions. It uses technique called coordinated Time Division Multiple Access (c-TDMA), in which APs take turns transmitting on the same channel, preventing them from interfering with each other.
Coordinated Spatial Reuse (c-SR) : This technique allows multiple APs to transmit simultaneously on the same channel by intelligently managing the spatial separation between them and the client devices.
Coordinated Beamforming (Co-BF) : In this approach, multiple APs work in concert to focus their radio signals towards a specific client device. This not only enhances the signal strength for the intended recipient device but also minimizes interference for other nearby devices.
Dynamic Frequency Selection and Power Control : APs can be configured to use non-overlapping channels to reduce interference and adjust their transmission power based on their proximity to other APs.

Use case: The upcoming Wi-Fi 8 (IEEE 802.11bn) standard is expected to heavily feature Multi-AP coordination to deliver the high reliability and ultra low latency required for industrial applications and high density environments.

Key differences

Feature	Coordinated Multi-AP (CoMP)	Distributed MIMO
Primary goal	Minimize interference and optimize resource utilization in WLANs	Maximize signal quality, spectral efficiency, and reliability by creating a virtual, large scale MIMO system
Operational domain	Primarily within Wireless Local Area Networks (WLANs), such as Wi-Fi	Primarily in cellular networks and future wireless systems
System architecture	A network of independent APs that coordinate their actions through signaling	A large number of geographically separated antennas connected to a central processing unit.
Signal Processing	Coordination decisions are often made at the AP level or by a centralized controller	Joint and centralized signal processing of data from multiple antennas is a key feature
Level of coordination	Focuses on scheduling, channel selection and power control to avoid interference	Involves coherent joint transmission and reception, treating distributed antennas as a single entity.
Complexity and cost	Lower (comparatively): coordination overhead is less, simpler to implement in existing infrastructure.	Higher complexity: more coordination, stringent hardware/sync requirements, higher cost/backhaul

Summary:

For WiFi: The upcoming WiFi 8 (IEEE 802.11bn) emphasises multi-AP coordination (MAPC) as a key enabler for ultra-high reliability (UHR) - letting APs coordinate time, spatial reuse and joint scheduling.

For cellular wireless: Distributed MIMO and CoMP are both relevant. For example, CoMP (coordinated multi-point) is already part of LTE/5G strategies for improving cell edge throughput and interference management.