David:
You want to know what NSATF engineers say about the Ka band. You want to know some of their names. I found this site hidden within the norsat website after doing a web search. Best to just access the site, but for now read on...
norsat.com
Ka-band RF Front Ends: Requirements and Design Approach
J. L. Fikart and M. Schefter
IMT Communications Systems Inc.
Burnaby, B.C. Canada
1. Introduction
In the last few years, there has been a significant surge of interest in the utilisation (both current and planned) of Ka-band for communications purposes. Technical work in this band has been going on for considerably longer but it is only relatively recently that a stronger application and commercial push has been created. This is to a large degree due to the large number of applications to FCC for satellite and frequency slots by many U.S. companies who envision a large global market particularly for small residential user terminals of the interactive type. In addition to ground terminals for communications over satellite, the LMDS (US) and LMCS (Canada) market is beginning to develop. In both cases, the main challenge on the equipment side is the front end part of the terminal since it must meet demanding technical specifications while ultimately also being inexpensive to manufacture. At this time, the latter requirement is still very far from reality as most of the work on front ends done to date had to focus on difficult technical issues first and the market is still rather nebulous.
In this paper, typical requirements, design approaches and cost considerations are discussed for Ka-band front ends in different terminal types. A pattern is shown from "demonstration" types through more "cost conscious, commercial equipment" approaches to designs for low-cost, consumer-type hardware. Examples of such front ends for satellite ground stations already developed, or currently under development at IMT Comsys, are described. These are:
ú 1.8/1.2/0.6m, 70 MHz - 20/30 GHz front ends for experiments with the ACTS satellite, designed and built from commercially available subassemblies. The design is narrowband around a specified frequency and has been successfully tested with a T1 CDMA signal spread into a 10 MHz band.
ú The "Picoterminal" - a portable briefcase terminal complying with IATA size regulations. The front end featues a 35 cm offset parabolic antenna that folds into the briefcase. The terminal, designed for 4.8 kb/s data rates with spectrum spreading, has been successfully tested over the Kopernikus satellite in Europe.
ú A 1.5m, 20/30 GHz front end primarily for VSATs, featuring standard L-band indoor/outdoor interface and designed for Tx data rates up to E1 (2Mb/s). The transceiver has been designed "from scratch" with extensive use of surface mount technology for low cost in medium volumes.
ú A 20/30 GHz RF front end for multimedia home terminals with a 70 cm antenna, and an L-band indoor/outdoor interface. This front end, currently under development, is being designed for very low cost in large volumes.
ú 28 GHz LMCS front end.
2. "Rack and Stack" Front Ends for ACTS Terminals
IMT Comsys built a 20/30 front end from commercially available subassemblies in response to a request for fast delivery of such equipment for use with the ACTS satellite. This front end was based on a CRC design [1] and was developed under license. This approach was chosen due to the short schedule and proven record of the CRC design.
The front end consists of three major subassemblies, namely the converter, SSPA/LNA subassembly and the antenna. The converter is enclosed in a 20"x20"x8"weather-proof steel box (see Fig. 1) mounted on the back of the antenna. The SSPA/LNA subassembly is integrated with the feedhorn/OMT (also Fig. 1) and mounted on the end of the boom in a "cradle" to facilitate polarisation plane adjustment. Three versions were built, with the same converter components but with the mechanical packaging of the SSPA/LNA subassembly customised to interface with different sized antennas; specifically, 1.8 m, 1.2m and 0.6 m offset-fed, single reflector assemblies were used.
The RF front end accepts a 70 MHz IF signal from a modem and upconverts it to 4.6 GHz first and then to 29 GHz. The upconverted signal is then amplified, applied to the horn via an OMT/diplexer and transmitted to the satellite. The 19 GHz signal received from the satellite is passed through the horn/OMT to the LNA, amplified and downconverted first to 4.5 GHz and then to a 70 MHz IF. This process is narrowband with a bandwidth of 40 MHz since it utilises fixed-frequency LOs in the conversion chains with the LO frequencies derived form ovenised XTAL oscillators at approx. 100 MHz. Two uplink and corresponding downlink channels are available by manually changing the LOs. The front end uses linear orthogonal polarisation that is manually set for the location dependent angular deviation from the vertical (or horizontal) plane, and for the correct Tx/Rx polarisation mix. Table 1 below summarises the RF front end performance for the key parameters.
The front end was successfully tested over the ACTS satellite early in the spring of 1997. The satellite was configured to loop back the UL signal using the East Sector Scan beam.An EF data modem was used with a data rate of 2.045 Mb/s, QPSK modulation and rate 1/2 convolutional encoding. A spread spectrum test using a Sigtek CDMA modem applying T1 signal with no coding spread into 10 MHz was also successful.
Table 1: Key Parameters of Front Ends for ACTS
Parameter
Performance
Parameter
Performance
Tx Frequency
29.125/29.225 GHz
Tx Polarisation
Vertical/Horizontal
Rx Frequency
19.405/19.505 GHz
Rx Polarisation
Horizontal/Vertical
Antenna size
1.8/1.2/0.6 m
Antenna Tx gain
53/49/44 dBi
Antenna Rx gain
49/46/39 dBi
Tx Power @ 1 dB C.P.
26 dBm
EIRP @ 1 dB C.P.
49/45/39 dBW
Rx Noise Figure
2.0 dB
G/T
26/23/16 dB/K
Tx IF
70 +/- 20 MHz
Rx IF
70 +/- 20 MHz
Power Consumption
120 W
Fig. 1: Photograph of the ACTS Front End TransceiverComponents
3. Picoterminal Front End
The Picoterminal was developed to be compatible with the existing CODE network architecture in Europe. Although originally part of the Olympus Utilisation program, the Picoterminal final design was re-directed to be able to operate over the 20/30 GHz transponders of the DFS Kopernikus and Italsat satellites. For the RF front end, the latter requirement resulted in the necessity to take into account the different Tx/Rx polarisation mix on the two satellites: vertical (Tx)/horizontal (Rx) for DFS Kopernikus, and vertical/ vertical on the Italsat. Thus the Kopernikus application required an OrthoMode Transducer (OMT) whereas Italsat needed a diplexer.
The Picoterminal system link budget determined the two main electrical parameters of the terminal, namely the EIRP and G/T. The physical size was dictated by the requirement of adhering to IATA regulation briefcase. This resulted in significant constraints on the size of the antenna as well as that of the RF units. The antenna size determined the required noise figure of the LNA and the output power from the SSPA. While the former did not present a particularly difficult problem, the latter, combined with the size limitations on the SSPA, was a challenge.
Physically, the RF front end itself is composed of an 34 x 35 cm offset fed parabolic antenna with a detachable boom equipped with the RF head containing the horn and the high frequency parts of the transceiver, namely the OMT/diplexer, SSPA and LNA. The up-and downconverter and the transceiver power supply are located inside the briefcase and connected to the RF head by good quality EHF cables. This arrangement is reflected in the block diagram in Fig. 2 where the RF head is referred to as "EHF Unit". A photograph of the Picoterminal set up for action is shown in Fig. 3.
Fig. 2: Picoterminal Transceiver Configuration
Fig. 3: Photograph of the Picoterminal
The "EHF Unit/horn" has been designed to be outwardly physically identical for both the co-polar and cross-polar use. This unit and the detachable boom can be stored in the Picoterminal briefcase with the antenna folding down into it. The briefcase contains other components such as the modem with an L-band interface to the front end, a Notebook PC and other supporting equipment. The whole package is powered from a small 12 V battery pack.
Electrically, the main parameters characterising the front end are shown in Table 2 .
Table 2: Main Picoterminal Front End Parameters
Parameter
Performance
Parameter
Performance
Parameter
Performance
Tx Freq.
29.5-30 GHz
Tx Polarisation
Vertical
Rx Freq.
19.2-19.7 GHz
Rx Polarisation
Horiz/vertical
comment: Kopernikus/Italsat
Antenna
single offset
Ant. size
34 x 35 cm
Tx/Rx Gain
38.6/35.2 dB
EIRP
35.6 dBW
Tx Power
27 dBm
Comment:
@ 1 dB C.P.
G/T
10 dB/K
Noise Temp.
330 K*
Rx Noise Fig.
2.9 dB
Tx & Rx IF
878+/- 10 MHz
DC Power
36 W
Briefcase
25x35x50cm
* Includes sky temperatute, clear sky atmospheric attenuation and effect of entire receive chain.
The antenna gain includes the horn and OMT loss. On the receive side, the G/T figure includes small degradation to the overall noise temperature caused by sky and atmospheric noise.
Because of "design from scratch", the production cost would be more reasonable than in the ACTS terminals but far from optimised. In essence, this was still another "demonstration" type of terminal in which the emphasis was on meeting relatively tough specifications. For example, the usual method of polarisation adjustment, i.e. turning the feed assembly around the horn axis, has been replaced by turning the entire dish together with the boom containing the feed assembly (EHF Unit with horn) about an axis perpendicular to the antenna plane. In this way, the mutual position of the feedhorn on the boom, and the dish remains constant. This was deemed necessary in order to meet the stringent cross-polarisation specifications at all polarisation settings. The relatively small dish diameter and the required performance necessitated the use of very accurate manufacturing processes. The reflector is machined from one slab of metal (aluminum for light weight) on a CNC machine, to meet the required surface smoothness and dimensional accuracy.
The technology used in the transceiver is the relatively expensive "classical" thin film and MHMICs mounted on metal carriers. The use of softboards with SMTs would have also been possible up to 20 GHz. While cheaper, it would have resulted in somewhat larger size. HEMTs in the LNA and MMICs in the SSPA driver and power stages have been used. The power stage utilises on-chip combining of MESFETs resulting in >28 dBm from one 2x4 mm chip.
The transceiver performance was measured on the bench and the EIRP and G/T parameters of the front end as a whole were confirmed in tests via the Kopernikus satellite in Graz, Austria, using a hub station located there. More details on the Picoterminal are available in Refs. [2] and [3].
4. Front End for 20/30 GHz VSATs
This section describes the main features and components of a Ka-band RF front end designed for the European Space Agency to be initially used for VSAT- type data communications via Kopernikus and Italsat and/or future Ka-band satellites with similar parameters. The outdoor part of the front end uses block conversion between 29.5-30 GHz/19.7-20.2 GHz and the standard "low" L-band (950-1450 MHz), and can therefore interface with standard QPSK modems at 70 or 140 MHz via commercially available L-band Converter Units (LCU) providing fine frequency agility. Its EIRP, G/T, spectral purity and instantaneous bandwidth are high enough to allow two-way communications with sufficient margin, using the above satellites, for single Tx channel data rates from 9.6 kb/s to 2 Mb/s with the Rx bandwidth constrained only by the LCU.
The RF front end is composed of an outdoor and indoor installation interconnected with an Interfacility Link (IFL). The physical makeup is fairly standard and is shown in Fig. 4. The outdoor installation consists of an offset-fed parabolic antenna and a Ka/L-band transceiver split into two units, one of which is integrated with the feedhorn and the other placed on the back of the dish. The indoor installation features an LCU, 70 or 140 MHz modem and a power supply/monitor, alarm & control unit all mounted in a 19" cabinet.
Fig. 4: Indoor/Outdoor Installation (VSAT)
The feed unit on the antenna boom (EHFU in Fig. 4) contains the horn and OMT/Diplexer integrated with only the EHF part of the transceiver, namely the SSPC (SSPA + upconverter) and Low-Noise Block converter (LNB). A DC/DC converter and additional auxiliary circuitry are placed into the Power Supply and Interface Unit (PSIU in Fig. 4) which is physically located on the back of the dish. This mechanical division has been done to have a reasonable size feed unit while still allowing for a direct, no-loss connection between the SSPC and the OMT (eliminating the flexible waveguide typical in installations with the SSPC and other Tx parts separated from the feedhorn and mounted lower down on the boom). The interconnecting RF and DC cables between the EHFU and PSIU run inside the boom. The Interfacility Link (IFL), connected to the PSIU, consists of the Tx and Rx L-band coaxial cables, a DC power cable and a twisted pair link for alarm/control. A monitor port is provided on the PSIU to assist with the antenna adjustment.
The basic block diagram of the transceiver is shown in Fig. 5. As shown, the PSIU contains a DC/DC converter for the EHFU (in fact only for its Tx part), the Tx and Rx Los for the SSPC and LNB, variable gain L-band amplifiers used for compensating IFL loss and a MAC unit for remote monitoring of Tx power, SSPA temperature, LO alarms and for control of IFL loss compensation and Rx polarisation. The PSIU also contains diplexers to separate incoming 100 MHz reference (from the LCU) from the Tx L-band signal and for delivering a DC voltage (from the LCU) on the Rx L-band cable to the LNB. This voltage is switched between 13V and 18V for polarisation setting as is currently done in IRDs powering universal LNBs in Ku-band DTH TV installations. In the EHFU, the SSPA is connected to the Tx port of the OMT while the LNB has dual input from two orthogonal ports on the OMT, selecting either vertical or horizontal polarisation.
Electrically, the main parameters characterising this RF front end are shown in Table 3.
Fig. 5: Basic Front End Block Diagram
Table 3: Main Parameters of the VSAT Front End
Parameter
Performance
Parameter
Performance
Parameter
Performance
Tx Freq.
29.5-30 GHz
Tx Polarisation
Vertical
Rx-Freq
19.7-20.2 GHz
Rx Polarisation
Horiz/Vertical
Comment: Kopernikus/Italsat
Antenna
single offset
Ant. size
1.5 m
Tx/Rx Gain
51/48 dB
EIRP
53 dBW
Tx Power
32 dBm
Comment:
@ 1 dB C.P.
G/T
24 dB/K
Noise Temp.
250 K*
LNB NF
2.0 dB
Tx IF (at IFL)
950-1450 MHz
Rx IF (at IFL)
950-1450 MHz
Input IF
70/140 MHz
Output IF
70/140 MHz
Inst. BW
36/72 MHz
* Includes sky temperatute, clear sky atmospheric attenuation and effect of entire receive chain.
The transceiver features remote switching between co-linear and orthogonal Rx polarisations for a set Tx polarisation, or between equal and opposite circular Rx polarisations for a set Tx polarisation. The latter is an option achieved by inserting a dual-band circular polariser between the horn and the OMT/diplexer in the feed.
The reflector and horn of the antenna have also been designed to meet ETSI specifications for sidelobe levels, cross-polarisation performance, as well as wind load, safety and other ETSI requirements. The feed assembly is a dual-band scalar rings horn that provides symmetric radiation patterns with low cross-polarization characteristics. This type of horn is suitable for casting and has therefore potential for low cost.
As stated before, the transceiver (designed and made at IMT Comsys) is divided into the "EHF Unit" (EHFU), mounted on the boom, and the PSIU mounted on the back of the reflector. A picture of the EHFU shown in Fig. 6 (For size comparisons, the horn is approx. 51 mm in diameter). More information on this unit and the entire front end is available in Ref. [4].
Fig. 6: Photograph of the EHFU
The technology used in the realisation of microwave circuits for all units except the SSPA is packaged and mostly discrete components on softboard. This SMT technique is routinely used by various Ku-band LNB manufacturers and has been proven to be very cost-effective. We have extended this even to the 1/30 GHz upconverter. In the SSPA, the classic "chip and wire" approach is used with alumina substrates and semiconductor chips (at 30 GHz, the softboard alternative is still hard to realise mainly because of packaging problems with the semiconductors.)
5. Front End for Residential Terminals
In future residential terminals for multimedia applications, the price will be a dominant issue. While production costs will be reduced just by application of the economies of scale (large volumes expected), the technical requirements will still be difficult to reconcile with the currently envisioned (by marketeers) "acceptable" purchase price (< $1000 or so). This becomes clear when considering the fact that the basic block diagram for VSATs (Fig. 5) still applies (albeit with some simplications, see below) and the Tx output power will remain somewhere around 1-2W particularly since a smaller antenna of about 70 cm diameter is envisioned for practical home installations.
An interesting development at this time is the European initiative of starting the process towards interactive terminals by combining existing Ku-band DTH Rx-only assemblies with a Ka-band (30 GHz) return link, rather than with a fully Ka-band equipped (i.e 20 GHz Rx and 30 GHz Tx) front end. Because of the large DTH market in Europe, this approach may indeed be the first one to be exploited commerciallly.
Whether Ku/Ka or Ka/Ka, we can start by looking at some of the differences when compared with the VSAT mode. First, "traffic" in this application is expected to be characterised by Tx data rates higher than about 144 kb/s and perhaps as high as 2 Mb/s, while on the Rx side, higher data rates in excess of 40Mb/s, rather than individual E1 or T1 channels, would be typical. Residential installations will not require long IFLs which affects the choice of cables and simplifies the PSIU. On the other hand, both Tx and Rx polarisation will probably have to be remotely switched. In the Ku/Ka case, polarisation will likely be linear whereas in the Ka/Ka front ends, circular polarisation is expected in most systems. This influences the changes to the EHFU and PSIU as follows (see block diagram in Fig. 7):
ú DRO-equipped LNB similar to existing Ku-band LNBs (no phase-locking)
ú "Bare-bones" PSIU (no Rx LO, no Rx variable gain, demultiplexer etc.) for reduction in cost and size
ú Due to the relatively short length of the IFL in a typical home, an inexpensive multi-purpose TVRO cable could be used to replace all the coaxial, DC and comlink cables needed in the VSAT application.
ú A dual output SSPA and dual-Tx port/dual-Rx-port OMT may be needed to switch polarisation.
Fig. 8 shows a possible realisation of the outdoor assembly, Ka/Ka version, with a 60 cm antenna and circular polariser included (inserted between horn and OMT instead of a waveguide section). Here, the current VSAT version of the EHFU with a 2W SSPA is used and it is seen that some reduction in size would be welcome.
Electrically, the main parameters characterising this RF front end are as shown in Table 4.
Fig. 7: Proposed Block Diagram of Residential UT Front End
Fig. 8: Possible realisation of Residential UT Front End
Table 4: Main Parameters of a Residential UT Front End
Parameter
Performance
Parameter
Performance
Parameter
Performance
Tx Freq.
29.5-30 GHz
Rx-Freq (Ka)
19.7-20.2 GHz
Rx Freq (Ku)
10.7-12.75GHz
Tx Polarisation
Linear
Rx Polaris. (Ka)
Rx Polaris.(Ku)
Linear
circular
circular
Antenna
single offset
Ant. size
55 x 60 cm
Tx/KaRx/KuRx
42/39/35 dB
EIRP
42 dBW typ.
Tx Power
30 dBm typ.
Note: EIRP, TxP @ 1 dB C.P.
G/T (Ka/Ku)
16/15 dB/K
Ka/Ku Noise T
200/100 K*
Ka/Ku NF
2.0/1.0 dB
Tx IF
950-1450 MHz
Rx IF (Ka)
950-1450 MHz
Rx IF (Ku)
950-1950 MHz
1100-2150 MHz
* Includes sky temperatute, clear sky atmospheric attenuation and effect of entire receive chain.
6. LMCS/LMDS Front Ends
LMCS (Canada) or LMDS (US) is a terrestrial system not unlike microwave point-to-point radio except that instead of the linear arrangement of terminal-repeaters-terminal, a star configuration with a hub is the prevalent scenario. These systems are being designed for relatively small distances (5 miles or so) because of the propagation characteristics at 30 GHz and the desire to keep antennas small. The frequency range assigned is between 24 and 31 Ghz with most North American systems expected at 27.5-28.5 GHz. The equipment on customer?s premises may be either Rx only (expected first) or both Rx and Tx. The difference between these and the satellite systems is the Rx frequency range being shared with, or close to, that of the transmitter and of course the pointing of the antenna.
IMT Comsys has done extensive system studies as well as first conceptual designs of the full Tx/Rx assembly. In general, the preferred concept is almost identical to that of the satcom residential front end in Fig. 7, with an L-band indoor/outdoor interface and a DRO-equipped Rx LO in the LNB (however, it is expected that in some applications, the Rx LO may be locked). The LNA input is simplified (no dual input) as the polarisation choice (if linear) is expected to be done by 90ø rotation of the relatively small antenna with the receiver integrated into it. The LNA is also expected to have lower gain than in the satcom case. On the Tx side, about +20 dBm may be expected for Tx power requirement which makes it possible to simplify the "satcom" SSPA since this kind of output power is achievable from a single "driver" device and no combining of power devices is needed.
The antenna gain ranges from 10 dBi to 35 dBi depending on the system and whether the antenna is meant for the subscriber unit or the hub, transmitter or receiver. This in turn means realisations from planar arrays to small 10 -15 cm parabolic dishes with dipole feeds, to a 40 cm dish with a feedhorn. Furthermore, if the Rx and Tx frequency ranges are shared in such a way that no diplexing is possible, a reasonable Tx/Rx isolation could be achieved only by using either orthogonal polarisation with a single antenna, or by separate Tx and Rx antennas adequately spaced apart.
In terms of production costs, the subscriber transmitter will be cheaper by some $150 or so (in large volumes) than the residential satcom transmitter due to the lower Tx power; on the other hand the receivers may be somewhat more expensive due to the impossibility of using packaged devices on softboard (frequency too high). Overall, with the generally smaller antennas, the LMCS subscriber units should be cheaper than the residential satcom front ends.
7. Summary
In this paper, typical requirements, design approaches and implementations of Ka-band front ends for ground stations have been discussed in conjunction with examples of hardware built and tested at IMT Comsys as well as that being currently designed for VSAT and residential user terminal applications. It has been demonstrated that satisfactory performance in existing or planned satellite systems can be obtained in relatively small RF heads suitable for mounting into focal points of single-offset parabolic dishes as small as 60 cm. Different packaging is likely to be used in transceiver/antenna integration in LMCS front end due to smaller antenna size and, possibly, use of planar antennas. For cost reduction, surface mount technology on softboard can be used in all parts of the front end except for the 30 GHz SSPAs. However, to achieve the cost targets for consumer acceptance, much additional work is needed both in conceptual design of systems, subsystem and circuits, and in technology refinements, particularly in 30 GHz transmitters.
8. Acknowledgements
The author wishes to thank the European Space Agency (ESA) and Canadian Space Agency (CSA) for the financial and technical support of the Picoterminal and VSATprojects, and particularly Mr. C. Hughes, Mr. J. Horle and Mr. F. Feliciani of ESA, and Mr. A. Bastikar of CSA, for their contributions to the design process in their roles as project managers and/or scientific authorities. Furthermore, the cooperation of our colleagues at Joanneum Research in Graz, Austria and at PML and GMTL in England is gratefully acknowledged.
Finally the author wishes to thank his colleagues on the IMT Comsys management team for their encouragement and to IMT?s members of the technical staff involved in the Ka-band projects for extraordinary efforts in their very challenging assignments.
References
[1] C. Pike, P. Tardiff, W. Janssen, E. Chan and G. Rivens, "Ka-band (29/19 GHz) Suitcase Satellite Terminal", Communications Research Centre (CRC), Industry Canada.
[2] T. Lentsch, C. Netzberger and O. Koudelka, "Picoterminal - a Portable Ka-band System", Proceedings of 2nd Ka-band Utilisation Conference, pp. 121-127, Florence, Italy, Sept. 1996
[3] J. Fikart, "RF Front End for a 20/30 GHz Briefcase Terminal", Proceedings of 2nd Ka-band Utilisation Conference, pp. 141-148, Florence, Italy, Sept. 1996
[4] J. Fikart, "Versatile RF Front End for Small Ka-band Ground Terminals", 3nd Ka-band Utilisation Conference, Sorrento, Italy, Sept. 1997. |
