The FCC issued Report and Order 04-165 on July 12, 2004. The Smart Antenna System (SAS), capable of forming multiple antenna beams, forged a pathway to frequency reuse and ultimately powers the unlicensed air-to-ground networks being developed by SmartSky Networks and Gogo. A review of the FCC 15-247 offers an understanding of an overall system architecture and performance estimates.
From the Report and Order:
Systems employing advanced antenna designs such as sectorized antennas and phased array adaptive antennas are now being used, or contemplated for use, as a part of wide area network systems operating in the 2.4 GHz band.
Sectorized antenna systems take a traditional omnidirectional coverage area and subdivide it into fixed sectors that are each covered using a single beam or antenna element to transmit desired information to all devices in the sector.
A smart antenna system must be capable of multiple transmit beams. Under 15.247, the SAS is restricted to the 2.4 GHz band. Mobile devices are allowed. Different information is sent to different receivers.
A phased array antenna system has directional gain along one or more beams. Typically all the antenna elements are applied to create the beam.
A sectorized antenna system is an aggregation of several antenna systems, each dedicated to cover a specific sector. A sectorized antenna system uses only the elements applicable to a given sector.
Spatial multiplexing (multiple in - multiple out, MIMO) is another system that uses parallel spatial streams (2x2, 3x3, 4x4) offering higher throughput in the presence of multipath and with multiple receive antennas. This technology is not utilized with long-range comms, where one high gain beam is used to reach a given device.
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| from Emission Test Procedures for 802.11 Devices, FCC OET, 2005 |
Beams from a SAS are presumed to operate without overlap. Any areas of overlap may require a reduction in power to ensure collective emissions are compliant. In the figure below, two beams are formed to support two different users. The sidelobes from each beam overlap, but that combination may not result in excessive interference, unlike when the boresite beams overlap. Typically six beams are provided for any given frequency channel, with the potential of hundreds of beams.
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from Smart Antenna for Cellular Mobile Communication, RK Jain, Sumit Katiyar and NK Agrawal, 2011
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Transmit power must be decreased one dB for every three dB above a standard 6 dBi antenna when using a SAS. The baseline limit is 0 dBW (one Watt) using a six dBi antenna. When using a 30 dBi antenna, power would be limited to -8 dBW (or about 0.16 Watts).
A single SAS may have as many as six independent beams each operating at "full power" for a given frequency. Any additional beams will require overall reductions in aggregate carriers to maintain a constant collective transmit output power eight dB above a full power single beam.
15.247 (e) expresses a conducted power spectral density limitation of 8 dBm in 3 kHz. That equates to about one Watt at 450 kHz and has no limitation in the candidate applications.
Harris Air-to-Ground System
Read more about the Harris FCC grants
TAKE NOTE
THE FOLLOWING DISCUSSION IS BASED ON LIMITED INFORMATION AND MAY NOT ACCURATELY REPRESENT THIS SYSTEM.
Harris has been awarded a grant for a remote radio head (RRH) to be used with a smart antenna system, in this case an eight element phased array with a boresite gain of about 29 dBi.
Harris also has been awarded a grant for an airborne radio (ABR). The ABR uses multiple antennas with gain up to 6 dBi (Tx, Rx) and one Rx antenna with a gain estimated to be about 13 dBi.
SmartSky Networks plans to deploy the Harris Air-to-Ground system to service aircraft. A set of RRH installations would create a cellular-like network. A given aircraft may be in range of one or more RRH installations.
Each RRH can operate channels using one of four modulations. The bandwidth on the forward channel (to the aircraft) is about 28 MHz. Two 9 MHz channels take 20 MHz. Alternatively, five 4.5 MHz channels take 24.5 MHz. The remainder and unused channel space is used for either 540 or 900 kHz channels.
QPSK modulation provides spectral efficiency of 1-2. Assuming QPSK is best case. A link budget would confirm - Harris did not have to provide one. LTE waveform performance may vary considerable from what is currently envisioned.
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| Remote Radio Head (RRH) Test Report 1 Harris |
From QPSK, assuming spectral efficiency of 1.5, then:
- 9 MHz channel yields about 14 Mbps
- 4.5 MHz channel yields about 7 Mbps
- 900 kHz channel yields about 1.4 Mbps
- 540 kHz channel yields about 0.8 Mbps
Running the possible combinations of channels to fill the entire 28 MHz allocation yields about 40 Mbps.
Taking it up six-fold (per RRH) would yield about 240 Mbps, spread over 6-300 beams, each from 0.8-28 Mbps.
Operating unfavorably (more noise, farther away) may diminish throughputs by a factor of two or more.
If assuming a composite of 100-250 Mbps per RRH, 200 access points would aggregate 20-50 Gbps (forward channel). Assuming a similar amount from the return channel would create a total of about 40-100 Gbps (200 RRH for CONUS).
Capacity is dependent on line-of-site. With a honeycomb coverage in place, peak density capacity can be augmented through "cell-splitting" by adding additional RRH.
Looking a few years into the future, one user will consume an average forward throughput of one Mbps. A single airplane may have 50-100 users if a free service, with a demand in the 25-75 Mbps range.
The most a single RRH can provide would to one airplane would be about 28 Mbps, assuming two 9 MHz channels are dedicated. One option to go beyond 28 Mbps would be to dedicate a channel from a second RRH to one aircraft allowing for a doubling of capacity. Another option is to occupy five 4.5 MHz channels, which might deliver 34 Mbps from one RRH beam.
One RRH could support six aircraft, each with 28-34 Mbps.
200 RRHs would support 1200 aircraft, each with up to 34 Mbps, assuming the aircraft are evenly distributed across the coverage area. A 10,000 feet above ground level service limit is assumed. Minimal enroute coverage RRH placement may not offer enough capacity in congested areas. I have counted more than 50 American Airlines airplanes operating nearby DFW, for example.
case 1: 1 Mbps per airplane: 30 planes per beam, 180 planes per RRH
case 2: 5 Mbps per airplane: 6 planes per beam, 36 planes per RRH
case 3: 30 Mbps per airplane: 1 plane per beam, 6 planes per RRH
case 4: 100 Mbps per airplane: 1 beam from 3 RRH per plane 2 planes per RRH
Forward channel spectral efficiency is maximized by the addition of the airborne receiver high gain antenna. The high gain antenna would increase sensitivity to the noise floor from the area surrounding the access point. Distance between the ABR and RRH is a factor as well. Best performance is the ABR operating nearby a RRH installed in a desolate location. RRHs installed near populated areas, or ABRs operating farther away from an RRH, will diminish performance.
Return channel (from the airplane) spectral efficiency benefits from the RRH high gain smart antenna system. The smart antenna system receiver would be sensitive to noise from the areas surrounding the RRH. Careful RRH placement may result in both a lower noise floor and higher effective gain, in comparison to the forward channel.
Link budgets fall victim to assumptions. While the above discussion suggests QPSK is supported (assumed under favorable conditions), it is likely that actual performance will be unfavorable, depending on separation to the device, and proximity to nearby noisy communities. Placement of RRHs that offer a buffer zone from interference is a crucial undertaking.
15.247 (c)
Code of Federal Regulations,Title 47 → Chapter I → Subchapter A → Part 15 → Subpart C
§15.247 Operation within the bands 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz
(2) In addition to the provisions in paragraphs (b)(1), (b)(3), (b)(4) and (c)(1)(i) of this section, transmitters operating in the 2400–2483.5 MHz band that emit multiple directional beams, simultaneously or sequentially, for the purpose of directing signals to individual receivers or to groups of receivers provided the emissions comply with the following:
(i) Different information must be transmitted to each receiver.
(ii) If the transmitter employs an an- tenna system that emits multiple directional beams but does not do emit multiple directional beams simultaneously, the total output power conducted to the array or arrays that comprise the device, i.e., the sum of the power supplied to all antennas, antenna elements, staves, etc. and summed across all carriers or frequency channels, shall not exceed the limit specified in paragraph (b)(1) or (b)(3) of this section, as applicable.
However, the total conducted output power shall be reduced by 1 dB below the specified limits for each 3 dB that the directional gain of the antenna/antenna array exceeds 6 dBi.
The directional antenna gain shall be computed as follows:
(iii) If a transmitter employs an antenna that operates simultaneously on multiple directional beams using the same or different frequency channels, the power supplied to each emission beam is subject to the power limit specified in paragraph (c)(2)(ii) of this section.(A) The directional gain shall be calculated as the sum of 10 log (number of array elements or staves) plus the directional gain of the element or stave having the highest gain.(B) A lower value for the directional gain than that calculated in paragraph (c)(2)(ii)(A) of this section will be accepted if sufficient evidence is presented, e.g., due to shading of the array or coherence loss in the beamforming.
If transmitted beams overlap, the power shall be reduced to ensure that their aggregate power does not exceed the limit specified in paragraph (c)(2)(ii) of this section.
In addition, the aggregate power transmitted simultaneously on all beams shall not exceed the limit specified in paragraph (c)(2)(ii) of this section by more than 8 dB.(iv) Transmitters that emit a single directional beam shall operate under the provisions of paragraph (c)(1) of this section.
Stay tuned!
Peter Lemme
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Peter Lemme has been a leader in avionics engineering for 35 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chairs the SAE-ITC AEEC Ku/Ka-band satcom subcommittee developing PP848, ARINC 791, and PP792 standards and characteristics.
Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).
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