Geo-Spatial TDMA Backbone Overlay: Scalable Long-Distance Trunking

[ttadegraaff]

[RFC] Geo-Spatial TDMA Backbone Overlay: Scalable Long-Distance Trunking for MeshCore

Executive Summary (TL;DR)

As MeshCore networks grow, standard CSMA/CA flood routing faces severe airtime congestion, packet collisions, and rapid depletion of Hop Limit across long distances.

This RFC proposes a Geo-Spatial TDMA Backbone Overlay—a self-healing, zero-configuration infrastructure layer that isolates long-distance transit from local neighborhood traffic. By coupling user-designated backbone nodes with a fixed-cadence Geographic TDMA schedule (leveraging node GPS/location and TCXO precision clocks), the network achieves zero-collision long-distance routing and virtualized single-hop transit, without requiring complex over-the-air schedule negotiations.

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1. Problem Statement

Currently, MeshCore handles all communication (local telemetry, short chats, and inter-city messages) on a single contention-based layer (CSMA/CA).

• Airtime Congestion: High-elevation repeaters are frequently overwhelmed by localized, low-priority broadcasts.
• Collisions & Hidden Nodes: As distance and hop counts increase, uncoordinated transmissions inevitably collide at intermediate nodes.
• Hop Limit Depletion: Inter-city packets consume their maximum hop quota navigating dense local clusters before reaching their target region.

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2. Proposed Architecture

The proposed solution introduces a two-tiered network topology: a Local Client Layer and a Geo-TDMA Overlay Layer.

A. Dedicated Backbone Pool & Dynamic Role Assignment

• User-Opted Isolation (Backbone Node = TRUE): Nodes opt into the backbone layer via a configuration toggle, but eligibility is automatically validated by the firmware. A node must maintain a minimum RSSI/SNR link quality threshold with at least one neighboring geographic tile. Nodes failing to reach adjacent tiles are automatically restricted to standard client mode to prevent gaps in the trunking chain.
• Per-Packet Dynamic Roles (Zero-Config): Nodes are not static entry/exit points; roles are dynamically assigned per packet based on the destination's GPS coordinates:

1. Ingress Node: Captures a local packet and resolves the destination tile using either the target's Geohash-encoded Node ID prefix or the local NodeDB position cache.
2. Transit Node: Forwards the encapsulated frame strictly via the Overlay schedule.
3. Egress Node: Detects that the destination coordinate lies within its local geographic tile, decapsulates the packet, increments the Hop Limit by exactly 1, and broadcasts it locally.

B. Geo-Spatial TDMA & Fixed Cadence Clockwork

To eliminate collisions across long distances, the Overlay operates on a mathematically deterministic time-location schedule:

1. Geographic Grid Partitioning: The global map is partitioned into a fixed grid of geographic tiles. A node's coordinates deterministically map it to a specific time slot in a repeating multi-slot time structure.
2. Spatial Reuse: Identical time slots are reused by tiles separated by sufficient physical distance, preventing co-channel interference while maximizing spectral efficiency.

For example:

 0 1 2 3 │ 0 1 2 3 │ 0 1 2 3 
 4 5 6 7 │ 4 5 6 7 │ 4 5 6 7 
 8 9 A B │ 8 9 A B │ 8 9 A B 
 C D E F │ C D E F │ C D E F 
─────────┼─────────┼─────────
 0 1 2 3 │ 0 1 2 3 │ 0 1 2 3 
 4 5 6 7 │ 4 5 6 7 │ 4 5 6 7 
 8 9 A B │ 8 9 A B │ 8 9 A B 
 C D E F │ C D E F │ C D E F
 
 [Each character 10 km Tile | Grid repeats every 40 km | Min distance between identical slots = 30 km]

3. Fixed Cadence: The TDMA schedule maintains a strict, unyielding tick-tock cadence. Because the time schedule is fixed and synchronized via precise onboard timing (TCXO / GPS / NTP sync), complex multi-branch network topology mesh lines never desynchronize.
4. Universal Pre-transmission Listen-Before-Talk: Every node performs a rapid Channel Activity Detection (CAD) scan at the very start of its assigned time slot. If a node detects strong local RF activity or noise, it immediately yields its slot and skips the current cycle. The packet remains buffered for it's next cycle, completely preventing receiver blinding.
5. Inter-Slot Guard Intervals: Each time slot enforces a mandatory Guard Interval at the end. This absorbs minor clock jitter, compensates for speed-of-light propagation delays across tiles, and provides transceivers adequate time to switch between RX and TX modes.
6. Intra-Tile Cycle Interleaving: When multiple backbone nodes reside within the same geographic tile, they avoid local contention by sorting active co-tile Node IDs (discovered via position telemetry) in numerical order. Nodes deterministically take turns across consecutive frame cycles in a round-robin rotation.
7. Micro-Sleep Energy Management: If a node does not detect an incoming transmission preamble from a neighboring tile during the initial CAD scan phase, it immediately enters a low-power micro-sleep for the remainder of that time slot, waking up precisely for the next assigned slot.

       [ Client A ]
            │ (Local CSMA)
            ▼
┌────────────────────────┐
│  Backbone Node (Tile 1)│ ──> [Ingress] Encapsulates payload & freezes Hop Count
└────────────────────────┘
            │
            ▼ (Fixed-Cadence Geo-TDMA Overlay - Slot 1 -> Slot 2)
┌────────────────────────┐
│  Backbone Node (Tile 2)│ ──> [Transit] Forwards via deterministic spatial slot
└────────────────────────┘
            │
            ▼ (Fixed-Cadence Geo-TDMA Overlay - Slot 2 -> Slot 3)
┌────────────────────────┐
│  Backbone Node (Tile 3)│ ──> [Egress] Decapsulates & emits locally (+1 Hop)
└────────────────────────┘
            │
            ▼ (Local CSMA)
       [ Client B ]

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3. Key Technical Advantages

• Zero Collisions on Long Routes: Deterministic geographic time-slotting mathematically eliminates packet collisions across intermediate backbone nodes.
• Virtual Single-Hop Transport: The entire backbone transit counts as a single logical hop to the end user, dramatically expanding effective network reach without altering standard hop limits.
• Branching & Topology Resilience: Operating on an absolute, location-based clock rather than dynamic over-the-air slot negotiation prevents "slot drift" and breakdown across intersecting or branching mesh pathways.
• Zero Configuration Burden: No central network controller or manual routing tables required. Nodes independently resolve their slot, ingress, transit, and egress behavior using onboard location data and target coordinates.

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4. Discussion & Next Steps

We invite open feedback, technical critiques, and implementation ideas from the core development team and the broader MeshCore community.

Looking forward to hearing your thoughts and refining this RFC together!