Microwave Radio Solutions – An Introduction

Microwave Radio Solutions: Licensed band and License-free bands (regulatory overview – EIRP constraints for different bands, Transmit Receive duplex spacing, frequency band options etc.)

Below, we look at regulatory considerations for ‘Licensed Band’ and ‘License-free Bands’.   We consider EIRP constraints for license-free bands, transmit/receive (T/R) duplex spacing, frequency band options etc.

Follow these links to quickly get to a specific area of interest

T/R Spacings for Licensed Bands
EIRP considerations for License-exempt Bands
17GHz considerations
‘Adaptivity’ and 100mW considerations for 2.4, 17 and 24GHz
60GHz considerations
Summary for the 5GHz Band

Licensed Band Radios

FDD backhaul radio links in these ITU-R defined bands are point-to-point or point-to-multipoint.  These links are deployed in ‘Carrier-grade’ voice & data networks.

Below, we consider 3.5MHz, 7MHz, 14MHz, 28MHz or 56MHz channel point-to-point microwave radios links.  For example, a 7MHz FDD link will use 7MHz bandwidth for transmit and 7MHz bandwidth for receive.  Another description for 7MHz FDD is 7MHz/7MHz.

Sagittar LCS
Sagittar LCS Series Microwave Radios


Sagittar manufactures Microwave Radios for many ITU-R frequency bands.  The list below includes common ‘licensed bands’ from 4GHz to 42GHz.  Additionally, licensed band radios also operate in the 2.3~2.5GHz and 70/80GHz bands (‘E-Band’).

For licensed bands, ITU-R regulations specify the ‘duplex’ transmit/receive (T/R) spacing.  T/R is the spacing between the centres of transmit and receive channel frequencies.  ITU-R specification documents for each band list the channel transmit and receive frequency pairs.

The table below lists common T/R spacing options for microwave radio links.

Microwave Radio TR Options

Click on the picture above to see more details

Note the many T/R spacing options for the different frequency bands.  The number of T/R spacing options is between 1 and 5, depending on the band.

Each radio’s IF circuitry, RF synthesizer(s) and duplexers must take the T/R spacing into account.

Now, let’s consider practical throughput options for 1+0 (unprotected) microwave radio links.  Data throughput speed varies with Ethernet frame size and modulation.

Please follow this link to see Typical throughput and latency for Sagittar LCS microwave radios.

Using this link: Consider a 28MHz channel transmitting 64 Byte frames.  As modulation increases from 2 to 10 symbols [ 4 (22) to 1024 (210) ], throughput increases by a factor of 5 from 52Mbps to 263Mbps.

License-exempt Band Radios

Common license-exempt frequency bands include: 2.4GHz, 5GHz, 17GHz, 24GHz and 60GHz (‘V-Band’).

IP Radios regulations specify:

  • maximum-allowed EIRP (Effective Isotropic Radiated Power),
  • radio-access sharing mechanisms (using terms including “Adaptivity”, LBT “listen before talk” and DAA “Detect And Avoid”)
  • the frequency channel extent (defined by minimum and maximum frequency bounds)
  • maximum-permissible radiated power outside the allowed frequency bands

FDD or TDD are duplex options within the 5.8GHz ISM band.  Furthermore, regulations mandate Safety and EMC requirements for the radio terminal assemblies.

If the ISM Band technology is FDD, the equipment designers can choose the T/R spacing.  Small T/R spacing means that duplexer cost and complexity are overriding and limiting considerations.  When the T/R spacing is too small, technical requirements for the duplexer are demanding.

So, what are the pitfalls of transferring deterministic services (voice, video or mission-critical data) using license-exempt frequency bands?

Shared use of frequency channels is an overriding consideration because it affects the amount and variation of latency and link capacity.

Shared use enforces channel-sharing access mechanisms and makes consideration of C/I (Carrier-to-Interference) important.

In the license-free bands, there are no guarantees that a channel will be free from interference.  Interference arises when:

  • the channel must be shared and devices automatically back-off to allow sharing,
  • when a “non-sharing” device purposely ignores regulatory requirements (usually by implementing a non-legal air-access protocol) and illegally operates in a “sharing channel” or
  • when adjacent channel power from another transmitter “spills” into the working channel

A radio link that works today may degrade at ANY time in the future due to co-location of interfering radios.  Interference sources include CSMA/CA radios, non-CSMA/CA co-channel interference radios, radios on adjacent channels, antenna sidelobe spill-over etc.

You achieve maximum throughput and stability by minimizing interference.

One must engineer the solution to maximize carrier signal energy at the receiver.  The design goal is to achieve “highest possible C/I” at the receiver.  The ‘Carrier’ signal energy (C) must be significantly higher than the interference power level (I).   “High C/I” ensures that the modem works with the highest possible “higher-order modulation” method.

License-exempt Band EIRP Considerations

There are signification EIRP limitations for some of the license-exempt frequency bands.  Take South Africa as an example, which follows ETSI guidelines.   2.4GHz, 17GHz, 24GHz are limited to 100mW EIRP (GAZETTE, G. (2008). 29 JULY 2008, No. 31290. SOUTH AFRICA GOVERNMENT GAZETTE) in ITU Region 1 regions).

While the EIRP limit in America is 4W EIRP for 2.4GHz, the EIRP limit in many ITU Region 1 ETSI regulatory domains is 100mW EIRP!

  • 2400-2483.5MHz 100mW EIRP (83.5MHz), No duty cycle, no channel spacing, EN 300 328, EN 301 489-1,17, EN60950, CEPT/ERC/REC 70-03
  • 17.1-17.3GHz 100mW EIRP (200MHz), EN 300 440, EN 301 489-1,3, EN60950, CEPT/ERC/REC 70-03
  • 24.05-24.25GHz 100mW EIRP (FDDA – 200MHz), No duty cycle, no channel spacing, EN 300 440, EN 301 489-1,3, EN60950, CEPT/ERC/REC 70-03

License-exempt 5GHz bands allow more EIRP flexibility:

1W (+30dBm) EIRP from 5470-5725MHz (255MHz)

200W (+53dBm) EIRP for PtP links 5725-5850MHz (125MHz)

  • 5150-5350MHz (GAZETTE, G. (2008). 29 JULY 2008, No. 31290. SOUTH AFRICA GOVERNMENT GAZETTE) (Indoor Use Only – channels 34 to 64) 200mW (23dBm) EIRP, Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) are obligatory, EN 300 893, EN 301 489-1,17, EN60950, ITU-R.1625
  • 5470-5725MHz (GAZETTE, G. (2008). 29 JULY 2008, No. 31290. SOUTH AFRICA GOVERNMENT GAZETTE) (channels 100 to 140), 1W (30dBm) EIRP, Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) are obligatory, EN 300 893, EN 301 489-1,17, EN60950, ITU-R.1625
  • 5725-5850MHz (GAZETTE, 2 DECEMBER 2009, No. 32769, 2009), ISM Band: 4W (36dBm) PtMP EIRP, 200W (53dBm) PtP EIRP (digital modulation only and the nominal bandwidths of transmission must not be less than 1MHz)

Taking the above 100mW (+20dBm) EIRP limits into account, this means that receive levels at 100m from a ‘base station radio’ are -60dBm, -77dBm and -80dBm respectively for 2.4GHz, 17GHz and 24GHz.  These low power levels at short distances are the consequence of “short-range-device” 2.4GHz, 17GHz and 24GHz equipment classifications.

What is the concern about using license-free bands such as 2.4GHz, 17GHz or 24GHz for backhaul?  Let’s consider 17GHz as an example.  

Why is such a low power of 0.1W EIRP specified by regulators for 17GHz?

Answer: The mandatory short-range low transmit power of 0.1W EIRP is because these systems are not allowed to affect Radio Location Radar or Land Movement Detection (earthquake) equipment working in this band.  17GHz radar equipment has ultra-sensitive receivers that allow detection of return signals from as far away as 300 nautical miles.  Receiving signals from 556km range means that other 17GHz equipment must NOT cause interference (the legal implications of affecting 17GHz-based mission critical systems cannot be ignored).   So, illegal use of 17GHz can result in significant legal repercussions by the Regulator.

Furthermore, earthquake movement detection, landslide monitoring, static and dynamic load analyses of constructions like bridges and buildings, and urban area subsidence detection equipment also works in the 17GHz.

Why is 17GHz Radio NOT considered suitable as CARRIER-GRADE Backhaul?

Answer: License-free band 17GHz equipment is mandated to implement LBT (Listen before talk) and DAA (Detection and Avoid).  So, if there is interference, this equipment MUST back-off and equitably share access of the bandwidth.  So, typical “marketed” 100Mbps of this technology MUST therefore automatically reduce.  This means that the UNPROTECTED (NON-REDUNDANT) backbone throughput speed is NOT GUARANTEED.

Illegal use of high transmit power levels in 17GHz can affect mission-critical systems and have significant legal repercussions.  Using a 17GHz radio fitted with a 0.6m diameter antennas with a transmit power level higher than 16 micro watts is illegal: 16 micro Watts is 16 million’th of a watt.

Ask yourself:  are you using a 17GHz 0.6m antenna?  Can you set your radio’s transmit power as low as -18dBm (16 microWatts) to ensure your EIRP is limited to 100mW EIRP?  If not, you are operating illegally in a +20dBm EIRP-regulated environment.

Now, let’s consider C/I (Carrier-to-Interference) for the 17GHz example:

Considering -77dBm at 100m for a 17GHz link, QPSK link errors will arise from -86dBm interference due to multipath reflections or spurious energy of other transmitters.  (Ref: European Communications Office (ECO), C. (January 2010). SEAMCAT (Spectrum Engineering Advanced Monte Carlo Analysis Tool) Handbook. CEPT).

-96dBm (C/I ~19dB) interference levels will affect 16QAM (Ref: Dieter Scherer Lucent Technologies,Wireless Broadband Networks. (n.d.).  Optimizing Frequency Re-use in Point-to-Multipoint Deployments).  -103dBm interference levels will affect 64QAM reception.

What the impact of “Adaptivity” requirements and 100mW EIRP for 2.4GHz, 17GHz or 24GHz

Enforced shared-access mechanisms (e.g. ‘adaptivity’) mean the throughput of radio links MUST reduce as more radios attempt to share the frequency resource.

For 2.4GHz, ETSI EN 300 328 discusses ‘adaptivity’ and terms such as LBT (Listen before Talk) and DAA (Detect And Avoid) as mechanisms that result in shared channel usage.  However, these adaptivity mechanism can result in variations of a link’s capacity and latency.

For 17GHz and 24GHz, EN 300 440 discusses the implementation of Detect And Avoid (DAA) and LBT (Listen before Talk) techniques.  These ‘adaptivity’ mechanisms result in variations of capacity and latency.

The 100mW (+20dBm) EIRP limit in ITU Region 1 (which includes Europe, Africa and the Middle East west of the Persian Gulf) force short range applications.  This means that one cannot consider 2.4GHz, 17GHz or 24GHz for useful (practical) long-distance backhaul.  Interference at any time in the future can render such links inoperable (or severely degraded).  There is a high risk if one attempts to deploy backbone carrier-grade solutions using these frequencies, especially with such low EIRP limits.

Considering the use of the 60GHz (‘V-Band’) for South Africa:

Firstly, in May 2014, South Africa’s ICASA distanced itself from licensing the extremely high-frequency radio spectrum bands in the 50GHz – 80GHz, saying that it has no plans to do so in its 2014/15 annual performance plan.

Secondly, as of May 2014, ‘ICASA currently does not have a regime under which these technologies can be type-approved and allowed to be used in South Africa.’

Finally, taking practical power limits into consideration, for the USA, PTP distance is limited by the power density rule – 15.255(b)(1).  The power density rule effectively limits power to 0-10 dBm.  So, with a 12” antenna, distance is limited to ~700 meters (Gregg Levin, BridgeWave Communications, 2005) (typical for most U.S. cities). Again, apart from interference considerations, this shows that 60GHz is not practical for long-distance backhaul considerations.

Summary for the 5GHz Band

Above, we have excluded consideration of the 2.4GHz, 17GHz, 24GHz and 60GHz license-exempt bands as options for long-distance backhaul.  Now, let’s consider the 5GHz band, where higher EIRP levels are permitted.

For ITU Region 1 areas (e.g. Africa, Middle East west of the Persian Gulf), decisions must be based upon EIRP constraints.  For 5GHz it is practical to:

  • first consider 5725-5850MHz bands for outdoor ‘transmission usage’ (higher EIRP limits of 4W and 200W for PtMP and PtP respectively),
  • second, consider 5470-5725MHz with the 1W (+30dBm) EIRP PtMP constraint.

Prudent design considerations for license-exempt band solutions include:

  • Maximizing the receive level relative to other unwanted or spurious receive signals (carrier-to-interference susceptibility gets progressively worse for increasing modulation order e.g. QPSK (9dB), 16QAM (19dB), 64QAM (26dB)).
  • Consideration that for 5725-5850MHz, where 200W EIRP (PtP) is allowed, more robust modulations (in interference environments) such as QPSK and 16QAM allow:
    • 6 x 7MHz channels of 8Mbps (QPSK) or 17Mbps (16QAM)
    • 3 x 14MHz channels of 17Mbps (QPSK) or 34Mbps (16QAM)
  • The requirement for ‘adaptivity’ specified within EN 301 893 means that shared-access is mandatory.   ‘Adaptivity is an automatic channel access mechanism by which a device avoids transmissions in a channel in the presence of transmissions from other RLAN systems in that channel.’ (ETSI. (2012). ETSI EN 301 893 V1.7.1 (2012-06) Broadband Radio Access Networks (BRAN);5 GHz high performance RLAN; Harmonized EN covering the essential requirements of article 3.2 of the R&TTE Directive. ETSI.)
  • Managing and coordinating license-free band frequency usage on masts (if indeed possible!)
  • Using higher gain (larger) antennas to minimize antenna beamwidth.
  • Using screens (also termed ‘shields’ or ‘collars’) with solid parabolic antennas to minimize antenna side-lobe reception.

[1] also considering the 200W EIRP PtP benefit of the 5.8GHz ISM Band for South Africa

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