Wireless Protocols

CR 4/8

Q: How much extra airtime does CR 4/8 add compared to CR 4/5?

LoRa coding rate only affects the bits in the payload portion of the packet, and the relationship is proportional to the denominator. CR 4/5 transmits 5 coded bits per 4 data bits (1.25x), while CR 4/8 transmits 8 coded bits per 4 data bits (2.0x), so the coded payload itself is 1.6 times longer at 4/8 than at 4/5. Because the LoRa preamble and header are sent at a fixed overhead, the total time-on-air increase is usually 20 to 40 percent rather than the full 60 percent, depending on payload length and spreading factor. At SF12 with a 51-byte payload, moving from CR 4/5 to CR 4/8 typically raises time-on-air from roughly 2.8 seconds to about 3.6 seconds, which matters directly for duty-cycle compliance in the EU868 band.

Q: What is the effective code rate and coding gain of CR 4/8?

The LoRa coding-rate notation 4/n means 4 data bits are encoded into n total bits, so CR 4/8 has an effective code rate of 4/8 = 0.5, the lowest of the four options. The redundant 4 parity bits per nibble implement an extended Hamming(8,4) code that can correct a single bit error and detect a double bit error within each block. The practical coding gain over uncoded transmission is on the order of 1 to 2 dB of additional link margin compared with CR 4/5, which can translate to a meaningful range extension in fading or interference-limited environments. The gain is modest because LoRa already derives most of its robustness from chirp spread spectrum processing gain governed by the spreading factor.

Q: When should I choose CR 4/8 instead of leaving the default 4/5?

CR 4/8 is worth selecting when the link is interference-limited rather than purely noise-limited, for example in dense urban deployments with bursty co-channel collisions, or on long fixed point-to-point backhaul links where the extra 1 to 2 dB of correction capability buys reliability and you control the duty cycle. It is a poor choice for high-density LoRaWAN networks because the longer time-on-air increases collision probability and consumes more of the regulatory duty-cycle budget. Most LoRaWAN networks default to CR 4/5 and let adaptive data rate manage the spreading factor instead. Note that the coding rate of uplinks is signaled in the explicit header, so a gateway decodes the payload correctly regardless of which rate a node selects.

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The strongest of the four selectable LoRa coding rates, written 4/8, in which the Hamming forward error correction encoder maps every 4 payload data bits onto 8 transmitted bits. The 4 added parity bits give an effective code rate of 1/2, the lowest LoRa offers, and let the receiver correct a single bit error per block while detecting double errors. This buys roughly 1 to 2 dB of extra link margin over the default 4/5 setting at the price of 1.6× longer coded payloads and correspondingly more on-air time. Because LoRa already derives most of its sensitivity from chirp spread-spectrum processing gain set by the spreading factor, CR 4/8 is reserved for interference-limited or high-reliability links where duty cycle is not the binding constraint.

Category: Wireless Protocols

Effective code rate: 1/2 (0.5)

FEC block: Hamming(8,4)

How LoRa Coding Rate 4/8 Trades Airtime for Robustness

LoRa modulation lets the system designer pick one of four forward-error-correction coding rates, expressed as 4/5, 4/6, 4/7, and 4/8. The numerator is always 4, representing the 4 information bits per coding nibble, and the denominator is the total number of bits actually transmitted after parity is added. CR 4/8 therefore sends 8 bits for every 4 bits of real payload data, the maximum redundancy LoRa supports. The encoding is built on an interleaved extended Hamming structure: the 4 parity bits per block implement a Hamming(8,4) code with an added overall parity bit, which corrects any single bit flip within the block and flags blocks corrupted by two errors.

The benefit of this redundancy is additional decoder resilience when the channel is dominated by short-burst interference or fading rather than by a steady noise floor. In a noise-limited link, LoRa sensitivity is set overwhelmingly by the spreading-factor processing gain and bandwidth, so the demodulator either resolves the chirp or it does not, and the coding rate adds only a small increment. In an interference-limited link, where collisions and impulsive noise corrupt scattered bits, the stronger Hamming code at 4/8 recovers packets that a 4/5 link would drop. Field measurements typically place the coding gain of 4/8 over 4/5 at 1 to 2 dB of effective link margin.

The cost is on-air time. The coding rate scales only the payload portion of the frame, not the fixed preamble and synchronization, so a longer payload pays a larger penalty. At a high spreading factor such as SF12, where each symbol already lasts tens of milliseconds, moving from 4/5 to 4/8 can extend a multi-second transmission by hundreds of milliseconds. That extra airtime consumes the regulatory duty-cycle budget in bands like EU868 and raises the collision probability in dense LoRaWAN networks, which is why 4/5 remains the protocol default.

Coding Rate and Effective Bit Rate

Effective code rate:
R_c = 4 / (4 + n) = 4 / 8 = 0.5 (n = 4 parity bits)

LoRa symbol rate and chip rate:
R_s = BW / 2^SF R_chip = BW

Effective payload bit rate:
R_b = SF × (BW / 2^SF) × R_c

Airtime scaling vs. CR 4/5:
payload_4/8 / payload_4/5 = (4+4) / (4+1) = 8/5 = 1.6×

Example: SF7, BW = 125 kHz, CR 4/8 → R_b ≈ 7 × (125000 / 128) × 0.5 ≈ 3.42 kbps, versus ≈ 5.47 kbps at CR 4/5.

LoRa Coding Rate Comparison

Coding Rate	Parity bits (n)	Coded bits / 4 data	Effective R_c	Payload airtime (rel. 4/5)	Typical use
CR 4/5	1	5	0.80	1.00× (baseline)	Default; dense LoRaWAN
CR 4/6	2	6	0.67	1.20×	Mild interference margin
CR 4/7	3	7	0.57	1.40×	Moderate burst noise
CR 4/8	4	8	0.50	1.60×	Max robustness, fixed links

Common Questions

Frequently Asked Questions

How much extra airtime does CR 4/8 add compared to CR 4/5?

The coding rate scales only the coded payload, by the ratio of denominators: 8/5 = 1.6× longer payload at 4/8 versus 4/5. Because the preamble and header carry fixed overhead, the total time-on-air rise is usually 20 to 40 percent. At SF12 with a 51-byte payload, that is roughly 2.8 s climbing to about 3.6 s, which directly eats into the EU868 duty-cycle budget.

What is the effective code rate and coding gain of CR 4/8?

The 4/n notation means 4 data bits map to n total bits, so 4/8 gives an effective code rate of 0.5, the lowest LoRa option. The 4 parity bits per nibble form an extended Hamming(8,4) code that corrects one bit error and detects two per block. Practical coding gain over 4/5 is about 1 to 2 dB, modest because chirp spread-spectrum processing gain already dominates LoRa sensitivity.

When should I choose CR 4/8 instead of leaving the default 4/5?

Pick 4/8 on interference-limited links, such as dense urban deployments with co-channel collisions, or controlled point-to-point backhaul where the extra 1 to 2 dB buys reliability and duty cycle is not binding. Avoid it in high-density LoRaWAN, where the longer airtime raises collision probability. The explicit header signals the coding rate, so a gateway decodes any node's selection correctly.

Related Terms

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