Radio engineering concepts for the License-free 5GHz Outdoor RLAN and ISM Bands
A common consideration is whether to use an ITU-R “Licensed band” or a 5GHz license-free band such as RLAN or ISM for a radio backhaul link.
A key question is: can a radio link solution in 5GHz allow guaranteed, constant data throughput, low and constant latency?
Licensed bands use FDD. 5GHz solutions typically use TDD (but sometimes use FDD in the ISM band). Licensed band radios have unimpeded, independent transmit and receive paths. 5GHz TDD link RLAN and ISM band radios must share a contiguous portion of bandwidth for both transmit and receive (this creates higher and variable latency). Radios in the ISM band work using either TDD or FDD.
So, what are the considerations for use of 5GHz bands? Is it possible to achieve the aforementioned goals of “guaranteed, constant data throughput, low and constant latency”?
The 5GHz band has two key bands for outdoor use: the 5470-5725MHz RLAN and 5725-5850MHz ISM bands.
Ideally, in the RLAN band, only one radio works at a time while the other radios wait their turn – this means “access” to each radio channel must be “shared”.
In the ISM band, many radios may all work at the same time – according to the ITU, for ISM bands “radiocommunication services operating within these bands must accept harmful interference, which may be caused by these applications”.
The 5GHz RLAN and ISM bands have different rules which have different effects on radio data links – the impact for the radio links is on:
- data throughput,
- possibly bit errors
Radio Interference in the 5GHz band can cause communication quality degradation:
RLAN Band 5.47-5.725 GHz:
- a reduction in radio link data throughput speeds and variable data throughput speeds.
- an increase in the radio link’s latency and variable latency vs time.
The above deleterious effects are due to mandatory “band-sharing” specified for the RLAN technology.
ISM Band 5.725-5.850 GHz:
- the can be radio channel bit errors that must be corrected by the radio’s error-correction technology (e.g. FEC – Forward Error Correction)
The challenge in the license-free bands for Outdoor Radio communications is as follows:
5.47-5.725 GHz (RLAN Band)
This is a “sharing” band. Ideally, only one radio transmits at a time. These radios must “detect” the presence of another radio or radios trying to use the same channel. If there are other radios close-by, a radio MUST back-off and share access to the radio channel.
Sharing is not optional. The RLAN radio technology MUST share access to the band. Sharing is a regulatory Type Approval requirement for the RLAN band. Sharing is determined by specification clauses listed in the ETSI specification EN 301 893.
Consider a test radio link scenario. When a nearby, non-link radio starts transmitting, the aggregate data throughput speed of the test link WILL reduce. As more radios are detected, the time that’s available to access to the channel reduces. “Listen-before-Talk” is the band-sharing mechanism listed in ETSI specification EN 301 893.
RLAN compliant radios must have “conformance test results” from accredited test laboratories. The tests show the radios are compliant with the band-sharing requirements listed in ETSI EN 301 893. Successful tests results are needed before radio Type Approval certification.
A RLAN radio with “marketing claims” of high data throughput speeds may need to reduce its aggregate data throughput speed. The data speed reduction arises due to “channel sharing”.
Some radios use techniques such as beam-forming and GPS coordination. Even so, it is still necessary to share the access to the channel with other vendor’s products. Some vendor’s radios will not necessarily be GPS-synchronous with other vendor’s radios. Some radios will not “cooperate” with other vendor radios that use proprietary band-access protocols.
The EIRP limit for the RLAN band is also restrictive – 1W. So, RLAN is best suited for short distance links. RLAN radios can use high-order modulation such as 64QAM (802.11n) or 256QAM (802.11ac). These modulations allow a high-speed transfer of data in a short time-period.
Beware of other radios, “hidden nodes”, that didn’t detect the transmissions of a radio. “Hidden nodes” transmit when they shouldn’t. “Hidden nodes” cause interference and reduce the likelihood of a radio using 64QAM or 256QAM. “Hidden-nodes” may force a radio to use QPSK instead of 64QAM or 256QAM modulation. The result is slower, higher-and-variable latency links.
5.725-5.850 GHz (ISM Band)
For this band, the radios must accept any deleterious effects due interference. So, higher order modulations such as 64QAM or 256QAM will not always work well. Higher-order modulation needs a higher Carrier-to-Interference C/I ratio to ensure error-free data transmission. Luckily, in some regulatory domains, an EIRP of 200W applies for this band.
Consider an urban deployment scenario, bearing in mind the C/I constraint. Despite the lower data speeds, QPSK becomes more attractive than 16QAM, 64QAM, 128QAM or 256QAM for the ISM band.
For the ISM band, engineer maximum power and minimize spurious-source interference at the receiver. Do this by taking advantage of a 200W EIRP limit, if specified in the regulatory domain. Use larger, hence higher-gain antennas that have narrower beamwidth. Narrower antenna beamwidth assists by attenuating spurious interference energy that is not directly incident in the direction of the antenna’s maximum gain lobe.
QPSK modulation is the most robust for Carrier-to-Interference if considering QPSK vs 16QAM vs 64QAM. Considering the ISM band: what happens if, instead of QPSK, one tries to work with higher order modulations such as 16QAM or 64QAM? Radios using these higher-order modulation types will be more affected by interference than QPSK. The effect of interference is significant: 9dB for QPSK vs 16QAM and 16dB for QPSK vs 64QAM. In conclusion, QPSK is the best choice to ensure error-free data transfer across the radio link in the ISM Band.
One can mitigate the effects of radio interference:
- Make sound choices using radio-engineering rationale.
- Using a SGT-LPN58V with 200W EIRP and QPSK is the most robust solution choice for the 5.8GHz ISM Band.
- The choice to use QPSK instead of 64QAM (or a higher order modulation) requires a trade-off. The trade-off is between “throughput speed” and “susceptibility to interference”.
Use QPSK in the ISM Band, which is more robust than other higher-order-modulation methods. There will be a lower data throughput speed instead of having an interference-affected link.
Let’s get back to our original question about data throughput speed and latency. Given the above observations for RLAN and ISM bands, there are definite challenges to using either of the bands to achieve guaranteed constant data throughput, low and constant latency.
Challenges arise when attempting to use the RLAN and ISM technologies to try guarantee high data-throughput speeds.
High and variable latency can also be problem for RLAN radios.
There is no getting away from interference induced link errors (ISM) or band-sharing requirements (RLAN) in an urban deployment environment, either using ISM or RLAN bands.