Macrodiversity
Definition: Macrodiversity
Macrodiversity
Macrodiversity (also called site diversity) combats large-scale fading (shadowing) by receiving or transmitting the same signal from multiple geographically separated base stations or access points.
While microdiversity (antenna diversity at a single site) combats small-scale multipath fading, macrodiversity addresses the correlated shadow fading that occurs when a large obstacle (building, hill) blocks the path between a user and a single base station.
With macrodiversity, the user connects to base stations simultaneously. Since the stations are separated by hundreds of metres to kilometres, the shadow fading to each station is approximately independent, providing macrodiversity order .
The separation needed for independent shadow fading (decorrelation distance) is typically 50-100 metres in urban environments β much larger than the half-wavelength spacing for microdiversity.
Definition: Soft Handover
Soft Handover
In soft handover (also called soft handoff), a mobile user simultaneously maintains radio links with two or more base stations in the active set. The signals from all base stations are combined at the mobile (downlink) or at the network (uplink), providing macrodiversity gain.
In contrast, hard handover switches abruptly from one base station to another, causing a brief interruption and no diversity benefit during the handover.
Soft handover was a defining feature of CDMA systems (IS-95, UMTS), where all base stations share the same frequency band and the mobile can receive and combine signals from multiple cells. In OFDM-based systems (LTE, 5G), inter-cell interference makes classical soft handover impractical, leading to CoMP techniques.
Definition: Coordinated Multipoint (CoMP)
Coordinated Multipoint (CoMP)
Coordinated Multipoint (CoMP) is a framework in LTE-Advanced and 5G NR where multiple geographically separated transmission/ reception points (TRPs) coordinate their transmissions to a user equipment (UE).
CoMP techniques include:
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Joint Transmission (JT): Multiple TRPs simultaneously transmit the same data to the UE, providing macrodiversity. The signals combine coherently at the UE, similar to soft handover.
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Dynamic Point Selection (DPS): The network dynamically selects the best TRP for each transmission time interval based on instantaneous channel quality.
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Coordinated Scheduling/Beamforming (CS/CB): TRPs coordinate their scheduling and beamforming to reduce inter-cell interference, without jointly transmitting data.
CoMP requires backhaul connectivity between TRPs with low latency and high capacity to share data and CSI.
Example: Macrodiversity Gain in Cellular
A mobile user at the cell edge receives signals from two base stations. The shadow fading to each station follows a log-normal distribution with dB (standard deviation) and the fading is independent between stations.
(a) Without macrodiversity, what is the outage probability (probability that the received power falls more than 10 dB below the median)?
(b) With 2-station macrodiversity (selection of the stronger link), what is the outage probability?
(c) What is the macrodiversity gain?
Single-link outage
Outage margin: dB below median.
About 10.6% outage probability.
Macrodiversity with selection
An outage occurs only if both links simultaneously fall below the threshold. With independent shadow fading:
About 1.1% outage probability.
Macrodiversity gain
The outage probability is reduced from 10.6% to 1.1% β nearly a 10-fold improvement. Equivalently, to achieve 1% outage with a single link, the fade margin would need to be dB. With macrodiversity, only 10 dB is needed.
The macrodiversity gain is dB in terms of fade margin at 1% outage.
Quick Check
What is the fundamental difference between macrodiversity and microdiversity?
Macrodiversity uses more antennas than microdiversity
Macrodiversity combats shadowing via geographically separated sites; microdiversity combats multipath fading via co-located antennas
Macrodiversity only works in 5G; microdiversity works in all generations
Macrodiversity provides higher diversity order
Microdiversity addresses small-scale (multipath) fading using closely spaced antennas at one site. Macrodiversity addresses large-scale (shadow) fading using widely separated base stations, where the shadowing is independent.
CoMP in LTE-Advanced and 5G NR
CoMP was introduced in 3GPP Release 11 (LTE-Advanced) and refined in subsequent releases. In 5G NR, multi-TRP operation is a core feature:
- Release 15/16: Basic multi-TRP with non-coherent joint transmission (each TRP sends independent layers)
- Release 17: Coherent joint transmission with phase synchronisation between TRPs
- Release 18: Enhanced multi-TRP for uplink, enabling macrodiversity on the UL
The performance of CoMP is fundamentally limited by backhaul capacity and latency. Ideal CoMP (with infinite backhaul) converts inter-cell interference into useful signal; practical CoMP with finite backhaul achieves a fraction of this gain.
Common Mistake: Macrodiversity Requires Backhaul Coordination
Mistake:
Assuming that macrodiversity provides free diversity gain without considering the infrastructure requirements.
Correction:
Joint transmission CoMP requires that all cooperating TRPs have access to the same user data before the transmission time. This demands:
- Low-latency backhaul: data must arrive at all TRPs within a fraction of the TTI (e.g., s for 5G NR)
- High-capacity backhaul: user data must be duplicated to each cooperating TRP
- Tight synchronisation: for coherent JT, TRPs must be phase-synchronised to within a fraction of a wavelength
In practice, fibre-connected TRPs can meet these requirements; wireless backhaul typically cannot, limiting CoMP to fibre-connected deployments (small cells, C-RAN architecture).
Why This Matters: CoMP in 5G NR
In 5G NR, multi-TRP operation enables macrodiversity in the downlink and uplink:
- Downlink multi-TRP: the UE receives PDSCH from multiple TRPs, either via space-frequency diversity (SFBC across TRPs) or spatial multiplexing (different layers from different TRPs)
- Uplink multi-TRP: multiple TRPs receive the PUSCH and combine at the CU (centralised unit) β uplink macrodiversity
The Cloud RAN (C-RAN) architecture, where baseband processing is centralised and TRPs are simple remote radio heads connected via fronthaul, is the ideal deployment for CoMP. In C-RAN, the centralised baseband unit has direct access to all TRP signals, enabling optimal joint processing.
See full treatment in Interference Alignment
Key Takeaway
Macrodiversity combats shadowing, the large-scale fading caused by obstacles between the user and the base station. By connecting to multiple geographically separated base stations, the probability that all links are simultaneously blocked drops dramatically. Modern implementations (CoMP in LTE-A/5G NR) extend this concept with coordinated transmission, but the benefit is fundamentally limited by backhaul capacity and synchronisation requirements.
Macrodiversity
Diversity obtained from geographically separated base stations or access points, combating large-scale fading (shadowing). Requires backhaul coordination between sites.
Related: Coordinated Multipoint (CoMP), Soft Handover, Shadow Fading
Coordinated Multipoint (CoMP)
A framework where multiple transmission/reception points coordinate their transmissions to provide macrodiversity and/or interference mitigation. Includes joint transmission, dynamic point selection, and coordinated scheduling.
Related: Macrodiversity, Multi Trp, C Ran
Soft Handover
A handover mechanism where the mobile maintains simultaneous links with multiple base stations, combining their signals for macrodiversity. Characteristic of CDMA systems (IS-95, UMTS).
Related: Macrodiversity, CDMA, Active Set