Proof of Coverage
The Helium blockchain uses a novel work algorithm called “Proof of Coverage” (PoC) to verify that Hotspots are located where they claim. Put another way, PoC tries to verify, on an ongoing basis, that Hotspots are honestly representing their location and the wireless network coverage they are creating from that location.
See Proof of Coverage in Action
The Helium Network Explorer is the best resources for viewing POC-related data. Before you dive into the POC internals, here are a few POC Challenges to help you visualize how this works:
Why Proof of Coverage?
The Helium Network is a physical wireless network that succeeds based on the amount of reliable coverage it can create for users deploying connected devices on it. As such, it required a work algorithm that was built for this use case. Proof-of-Coverage takes advantage of the unique, undeniable properties of radio frequency (RF) to produce proofs that are meaningful to the Helium Network and its participants. Specifically, PoC relies on the following characteristics:
- RF has limited physical propagation and, therefore, distance;
- The strength of a received RF signal is inversely proportional to the square of the distance from the transmitter; and
- RF travels at the speed of light with (effectively) no latency;
Using these properties, the blockchain is constantly interrogating Hotspots using a mechanism known as a “PoC Challenge”. The ultimate power of Proof-of-Coverage lies in the fact that the data generated by the ongoing proofs and stored in the Helium blockchain is definitive verification of the wireless coverage provided by Hotspots on the Network.
Proof of Coverage Challenges
The "challenge" is the discrete unit of work for Proof of Coverage. To date, there have been 10s of millions of challenges issued and processed by the Helium blockchain. With each new challenge, the blockchain records more data about the quality of the network. Let's take a look at how challenges actually happen.
POC Challenges involve three distinct roles:
- Challenger - The Hotspot that constructs and issues the POC Challenge. Hotspots issue challenges approximately once per every 300 blocks.**
- Transmitter - Sometimes called "Challengee". This Hotspot is the target of the POC challenge and is responsible for transmitting (or "beaconing") challenge packets to potentially be witnessed by geographically proximate Hotspots.
- Witness - Hotspots that are geographically proximate to the Transmitter and report the existence of the challenge packet after it has been transmitted.
Challenge Proof Construction and Target Selection
As noted above, Hotspots are currently allowed to submit a challenge proof roughly once per 300 blocks. They do this reliably to earn the portion of HNT rewards allocated to challengers. The Challenger first generates an ephemeral public/private key pair to be used in the challenge. A SHA256 digest of the public key and the SHA256 digest of the private key are both submitted, along with the current block hash, as a PoC request. If the request is valid and accepted by the blockchain, the hash of the block that the receipt appears in is combined with the hash of the ephemeral public key and the challenger's identity to generate verifiable entropy. A uniform random number generated via this entropy is then used to select the target from all Hotspots on the Network.
After the the challenge packet is created, it is then delivered to the transmitter via the Helium Peer-to-Peer Network. The target receives the challenge packet, decrypts the outermost layer using its private key and the ephemeral public key for this challenge (this ephemeral public key appears in the PoC packet and the receiving Hotspot can inspect the blockchain for an active PoC receipt with the corresponding SHA256 of the ephemeral key), and immediately transmits the resultant packet to the Helium Network. Any number of proximate geographic Hotspots will hear it and will Witness the packet.
As of HIP 15, Proof of Coverage relies entirely on beaconing. A beacon is a single transmission witnessed by any Hotspot. Previous versions used multi-hop challenge paths that didn't accurately test Hotspots on what they are built to do: capture RF packets.
After a Transmitter receives the challenge, it will beacon the challenge packet. There is no intended recipient of this packet, and any Hotspot that is geographically proximate to the transmitter is eligible to witness it and report back to the blockchain. After a Hotspots witnesses a beacon from a transmitter, it will submit this as part of the POC Challenge receipt that gets assembled and submitted to the blockchain by the POC Challenger.
POC Reward Scaling
For every epoch, each reward type is split amongst Hotspots who had a role in that reward pool. For example, if your Hotspot was challenged during an epoch, it will be eligible to a portion of the 5.31% of rewards that go to PoC Transmitters. A practical way of thinking about this is that a Hotspot might earn a "reward unit" for succeeding at a challenge. If five additional Hotspots succeeded at a challenge during the epoch and each of them also earned a "reward unit", then each Hotspot gets 1/6th of the 5.31% of rewards in that epoch.
- The number of witnesses, detailed in HIP15
- The number of Hotspots in the hex tile of the transmitter, detailed in HIP17
The HIPs themselves provide a rich explanation of these mechanisms, so you're encouraged to read those, but they can be summarized as follows:
- For the Transmitters, the more witnesses, the more the Transmitter earns;
- For the Witness, each additional witness past a total of four reduces what is earned by each witness in that challenge.
- The Witness earns less if the number of Hotspots in the area of the Transmitter exceeds the "target density". Target density varies by hex resolution, as detailed in the HIP and defined in several chain variables.
Verifying the Proof
Once the Challenger has the complete set of receipts from the POC Witnesses and Transmitter, or the elapsed time since the challenge was issued has passed the upper time bound, the POC Challenge is considered complete. At this point, the Challenger then submits the proof receipt as a transaction to the blockchain to be verified by the current consensus group. Because the steps taken by the Challenger to construct and complete the proof are deterministic and easily reproduced, members of the consensus group can verify the legitimacy of the proof. Specifically the Challenger reveals the secret ephemeral key it used for both obtaining the original PoC request and for encrypting each layer of the challenge packet. This crucial information, which has been hidden until the receipt is published, allows the re-creation of the deterministic entropy.