A Prototype Adaptive Fade Countermeasure System for TDMA Operation at Ka Band Dieter Kreuer+, Ansgar Radermacher+, Sven Martin*, Thomas Frach+ +Institute of Computer Science IV *Institute of Computer Science VI Aachen University of Technology D-52056 Aachen, Germany
[email protected] Abstract: Ka band, the 20/30 GHz frequency band for satellite communications, has gained much interest in recent times. It is well suited for applications with small sized earth stations. The major disadvantage of Ka band, however, is its sensitivity to rain fading. Several satellite experiments are being carried out at the moment studying so called adaptive fade countermeasures, the purpose of which is to improve link quality in presence of rain. In this paper, the design and early laboratory experiments of a prototype fade countermeasure system implemented on a TDMA controller are described. This system is used for an experiment carried out by German PTT, Swiss PTT and DLR to investigate the significance of adaptive fade countermeasures for the availabilty of a Ka band satellite link. First results show, that an effective improvement of the link is possible with simple fade countermeasures. 1. INTRODUCTION Operational satellites using the 20 GHz downlink / 30 GHz uplink frequency range, called Ka band, have only recently been launched. At Ka band frequencies, a particular antenna size offers a higher gain than at lower bands. This makes Ka band satellites well suited for networks with small earth stations, such as VSATs. However, Ka band has one major disadvantage. While at clear sky conditions, atmospheric attenuation is less than 0.5 dB, the situation is different under conditions of rain. For instance, to obtain an annual channel availability of 99.95%, fades of 5 dB on the downlink and 10 dB on the uplink have to be counteracted. A link budget with fixed margins of 10 dB, however, would be a waste of resources. An adaptive approach is much more feasible. There are several methods to adaptively counteract rain fading on digital satellite channels, known as adaptive fade countermeasures (AFCM). They have been considered in numerous recent papers ([1,2,3,4,5,6,7]). The most simple method is uplink power control (ULPC) [7] taking advantage of a surplus in transmitter
power (typically 10 dB) which is put into action only when uplink fades occur. A control system intends to keep the flux density at the satellite at a constant value. If the power is too high, the transponder is pushed beyond saturation causing interferences. For the same reason, power control cannot be employed for downlink fades. Slowing down the transmission rate is another method to improve link quality. A 50 % reduction of the transmission rate yields a gain of 3 dB in bit energie to noise density Eb/N0, thereby reducing the bit error rate. The most effective fade countermeasure, however, is forward error correction coding. Punctured convolutional codes [8,9] allow to implement several coding rates with different gains on a single coder/decoder (codec). The idea is to generate a low rate code and systematicly to delete bits in the encoded data stream. If e.g. 7 data bits are encoded as 14 by a 1/2 code, deleting 6 bits out of 14 yields a 7/8 code. The decoder has to stuff dummy bits at the appropriate positions to reconstruct the original data. The coding gain depends on the number of deleted bits. The gain of a suitable (n-1)/n punctured code may be as high as that of a good (n-1)/n standard convolutional code [8]. A research group consisting of researchers from German PTT Telekom (Forschungs- und Technologiezentrum FTZ), German Aerospace Research Establishment (Deutsche Versuchsanstalt für Luft- und Raumfahrt, DLR), and Swiss PTT is conducting two experiments concerned with propagation measurements and adaptive fade countermeasures [10]. For the experiments, a prototype TDMA controller (cf. [11]) was ordered from GEC Marconi, Chelmsford, UK. The system is capable of different code and symbol rates, as well as transmission power control via software, but otherwise offers only basic layer 2 functionalities, namely channel access and user data transmission. Fade countermeasures have to be controlled via console. Our institute was asked to implement routines which measure fades in up- and downlink, exchange the fade information between communicating stations, and automatically select the appropriate coderate, symbolrate, and transmission power.
The actual satellite experiments, after the premature outage of Olympus conducted on the German DFS-2 satellite, took place until July 1994 and results will soon be published by the GECO group. In this paper, we will describe the control software and give some results obtained in a laboratory configuration. 2. BASIC TDMA HARDWARE AND SOFTWARE Hardware Components
3. EXTENSIONS TO THE MARCONI SYSTEM As the processor load of the controller was high running the Marconi TDMA routines already, a decision was made to add a second MVME 147 processor board, called TxC, to the rack of the transmitting unit. On the TxC, our automatic fade countermeasure control routines were implemented under the real time operating system pSOS+. In our software design, 4 different task groups were created:
The Marconi TDMA station [11] has been developed for a number of advanced communication experiments. The station consisting of a processor based TDMA controller and a multi-rate modem, allows individual data packets to be transmitted at bit rates in the range of 5128192 kbit/s. Each subburst can be protected by FEC encoding. Coding rates of 1/2, 2/3, 4/5, and 1/1 are implemented employing a soft decision codec for punctured codes. Channel quality is monitored by taking advantage of soft quantized decoding (see below). TDMA functions are implemented in software whereever possible yielding a flexible system easily adaptable to other TDMA schemes. Fig. 1 gives an overview of the TDMA system. The controller is implemented in two VME bus racks, one for the transmitting (Tx) and one for the receiving (Rx) unit. Communication between the two is conducted via a SCSI bus. Each unit is based on a Motorola 68030 processor card (MVME147) fitted with 4 MB RAM. Using the VME bus, the CPU communicates with several other cards in the same rack. Tx and Rx each provide four serial ports and one Ethernet interface for data I/O and system monitoring. TDMA Frame Structure Within the TDMA frame information is sent in short bursts by each station. The Reference Burst (RB) is transmitted by the master station and marks the beginning of a frame. It is destined for all other stations, called slaves, and used for communicating timing and other information. In response, each slave sends a Housekeeping Burst (HB) to the master station containing local timing information. Each frame, another station is permitted to transmit its HB at the end of the frame. User data are transmitted via Data Bursts (DB), composed of Data Subbursts (DSB), one for each destined station, and a preceeding Control Subburst (CSB) for timing and routing of the subsequent DSBs. The contents, composition, and transmitting parameter setup of the frame is achieved solely by software.
Figure 1: Structure and functionality of the TDMA components.
Statistics: This group, consisting of a single task, computes the statistics on signal level and channel quality for each received burst over the last 10 seconds. Fade Countermeasures: This is also a single task group. Here, the actual fade countermeasures are implemented. User Interaction: A collection of three tasks, one for user input, one for user requested terminal screen output, and one for system messages terminal screen output. System Maintenance: Several tasks for system upkeep: communication with the Marconi Tx board software, system power-up initialisation, processor load measuring, status report generation, and data dump via serial interface . 4. CHANNEL QUALITY ESTIMATION The Marconi hardware provides a mechanism to determine the quality of the channel [11]. Received bits are demodulated in analog: +0.5 V is output for a detected "1", and -0.5 V for a detected "0". Additive white Gaussian noise causes two bell shaped distributions (one for each bit parity) of the output voltage centered around +0.5 V and -0.5 V. For error correction decoding, this voltage is
4 bit A/D-converted, yielding 4 soft decision bits for every received bit. The most significant bit is "1" for positive and "0" for negative voltage while the other 3, termed soft decision level, indicate the absolute voltage (000 to 111 for 0 to 0.5 V). The statistical distribution of the soft decision level is closely tied to the Eb/N0 value [12]. When signal quality is high, the modem voltage scatters tightly around the peak voltages, so that only high soft decision levels occur. If, on the other hand, the noise level is high, the voltage will scatter more widely and yield lower soft decision levels as well. From the distribution of the soft decision level, the corresponding Eb/N0 can be determined. The Marconi hardware records soft decision levels below 101. The count over one subburst is conveyed to the software as channel quality word. The counter is realized as a state machine in an EPROM which we have changed to implement an adder for soft decision levels. Dividing the level sum by the number of received bits yields the
Marconi modem could not keep the modem voltage stable anymore, leading to false results. For the satellite experiment, a low received signal level was planned, with which the Eb/N0 determination method would not have worked. 5. THE CONTROL ALGORITHMS The fade countermeasures are divided into algorithms to control the power of the transmitting station (ULPC) and measures to select code and symbol rates. ULPC is intended for uplink fades only, whereas code and symbol rate switching may be applied to both up- and downlink. Input to the algorithms are the fade depths in uplink and downlink. A station can determine the total fade monitoring the difference between TSL and RSL (transmit and received signal levels) of its own bursts. If a satellit beacon signal is available, the downlink fade can be observed directly, thus yielding uplink attenuation from the total fade. Otherwise, up- and downlink fades must be estimated from the total fade using a fixed frequency scaling factor: rain fading at 30 GHz is roughly two times stronger than at 20 GHz [13]. This method is less exact than using a beacon, as the actual factor varies with weather conditions. Fade depths in uplink and downlink are determined locally by each station and then distributed to the other stations via housekeeping bursts. Each sending station can then take the appropriate measures to counter its own uplink fade and the downlink fades of remote stations. The first two algorithms are ULPC methods based on the RSL at the satellite, which can be estimated knowing the local uplink fade. The functionality of the first algorithm is simple: if the satellite RSL is too weak (strong), the TSL is raised (lowered) in fixed steps, until the estimated satellite RSL is close enough to the desired target.
Figure 2: Linear relation between estimated Eb/N0 and actual fade depth.
mean soft decision level, from which the Eb/N0 value can be estimated much more accurately. The theoretical background for the estimation is presented in detail in [12]. Fig. 2 shows the fade depth vs. the estimated Eb/N0 for different symbol rates. Error bars are 99%. The estimation is least accurate for high Eb/N0, when the voltage distribution is extremely narrow. For the important range from 4 to 12 dB, however, the estimation is very precise. The different transmission speeds yield parallel straight lines, separated by 3 dB, as expected for rate doubling. However, the experimentators decided not to apply this method to drive any fade countermeasures, because below a certain input level, the automatic gain control of the
Simple ULPC do forever begin diff = measured_rsl - target_rsl; if |diff| > cutoff then begin (are we close enough ? ) if diff>0 then tsl_set = tsl_set+ step; if diff cutoff then (ignore small changes ) tsl_set = tsl_set + α·diff; if |tsl_set - tsl_inuse| > tolerance then SetSignalLevel(tsl_set); end
The third ULPC method directly determines the TSL from the uplink attenuation via a table. The target TSL is thus achieved in one step. This method was implemented, because the table can be freely adjusted by the operator. A hysteresis is used to reduce unsteady fluctuations of the TSL. Power is increased only if the fade exceeds the switching threshold given in the table by at least x dB, where x is the hysteresis. Switching to a lower TSL is done if the fade depth becomes less than the threshold minus x dB. Tabular ULPC do forever begin if (fade_uplink current index-range + hysteresis) then begin tsl_set=LookupTabTsl(fade_uplink); SetSignalLevel(tsl_set); end end
A single algorithm was developed to control both codeand symbolrate. The algorithm works similar to the tabular ULPC algorithm. However, as a single data burst may contain subbursts destined for several stations with separate downlinks, individual subbursts must be controled independently. The driving value here is the total fade on up- and downlink minus the gain of ULPC. Tabular code & symbol rate switching do forever begin for station =1 to max. number of stations do begin fade_remain = fade_downlink (station) + fade_uplink - ulpc_gain; if (fade_remain current index-range + hysteresis) then begin cdr=LookupTabCdr (fade_remain); syr=LookupTabSyr (fade_remain); SetCodeAndSymbolRate (station, cdr, syr); end end end
6. PERFORMANCE TESTING Fig. 3 shows the setup used to evaluate the performance of the fade countermeasure system. The TDMA system is operated in IF loop configuration, i.e. the 70 MHz modem output is looped back to the modem
input. Fades are simulated by a channel simulator developed by DLR playing back recorded Ka band events. Behind the channel simulator noise of -20 dBm (70 ± 20 MHz) is added. A data logger is connected to the TxC unit recording measured fades, BER, and actions of the fade countermeasures. As uplink and downlink are not physically separated, fading is assumed to be part on the downlink and part on the uplink, as with a real satellite loopback transmission. The uplink-downlink-relation for fade determination was set to 2/1 . The TDMA system generates 48 frames per second, each one containing a single data burst. The burst contains 8192 bits, generating a stream of 393216 bits per second, the bit error rate of which is measured. To get more accurate results for low error rates a moving average over ten seconds is applied to the data. All three ULPCs were investigated, but they show only minor differences. Only the results for the simple ULPC are presented here. ULPC tries to keep the RSL at the satellite at -20 ±1 dBm. If this range is exceeded, the TSL is increased or decreased in steps of 0.5 dB. 16 frames (0.3 s) for a satellite propagation delay are allowed to pass prior to checking the power level again. The TSL may be varied from -10 dBm ... 0 dBm, offering a 10 dB power margin. The table for code- and symbolrate looks like this: fade depth [dB] 0 2 5 6.5 8 10
coderate 1/1 4/5 2/3 1/2 1/2 1/2
symbolrate [Mbd] 4 4 4 4 2 1
A higher encoding yields more gain than a lower symbolrate while consuming less frame time, so that the symbolrate is switched only when the highest encoding has been chosen already. A 0.5 dB hysteresis and a check delay of 16 frames (0.3 s) is used. A total of about 4 hours of events was played back to provide a large enough statistical basis. Fig. 4 shows a 35 minute event. The lower graph represents the total fade..
can eliminate uplink fading, the BER is not significant but shoots up to very high values otherwise. Employing the code-/symbolrate control reduces periods of high BER to
Figure 3: Configuration used for the experiments.
Figure 5: Selected coderate and symbolrate for the fade of figure 4.
Figure 4: Effect of ULPC on a sample fade.
2/3 of the fade depth are interpreted as uplink fade and countered by the simple ULPC method. The maximum gain is 10 dB, resulting in the clipped topmost graph. The compensated uplink is a fairly straight line with only some single data points outside the ±1 dB range, except for the section where the ULPC is at maximum. Fig. 5 shows the code- and symbolrate countering the remaining fade, over the same period of time. Moderate encoding suffices as long as ULPC can compensate the uplink fade. Then, first the coderate and later the symbolrate are switched to higher gains. For some moments, the highest available gain is required, indicating a remaining fade of at least 10 dB. Also shown is the required relative bandwidth, that means, by how much the duration of the burst has to be increased to allow for the required code-/symbolrates. The contribution by the coderates is merely 1.25 to 2 times the clear sky bandwidth. However, slowing down the transmission rate to 1 Mbd in combination with 1/2 rate encoding requires 8 times the clear sky bandwidth. Fortunately, only deep fades which saturate the ULPC cause short periods of high bandwidth consumption. Fig. 6 shows the resulting bit error rates for the same fade and countermeasures. With no active countermeasures the BER is high most of the time. For periods, where the BER exceeds 10-3, the satellite link can be considered unavailable for practical applications. ULPC alone already achieves a huge improvement. For periods, when ULPC
Figure 6: resulting BER for the fade countermeasures of figs. 4 and 5.
some short peaks, when the station loses complete bursts. Burst headers are safeguarded by 1/2 rate encoding and 0.5 Mbd transmission. During the deepest fade, however, even this is not enough to reliably protect the header. Figure 7 shows BER vs. fade depth as measured over all played back events. Also shown are 99 percentiles. The same events have been used for three runs, one with no active fade countermeasures, one with just the simple ULPC active, and one with ULPC as well as code- and symbolrate control. The TDMA station ceases to operate at a fade depth of 14.5 dB when no countermeasures are active. The improvement by the different countermeasures is quite obvious. ULPC cannot compensate for the total fade, but it reduces the fade depth. As expected, the maximum gain of ULPC is 10 dB. For instance, at 15 dB total attenuation, the BER is equal to 5 dB attenuation with no countermeasures. When all countermeasures are active, there is a significant improvement over ULPC alone beyond fades of 4.5 dB. Then, the downlink fade being 1/3 of the total fade exceeds 1.5 dB, which is within the 0.5 dB hyteresis around the threshold for coderate 4/5. The achieved gain does not show up as a sharp drop in BER, because of the hysteresis and as a moving average
was run over the BER values to determine low error rates. The 99 percentile drops no earlier than at 7 dB, when the hysteresis zone is passed. The BER then stays negligible up to a total fade of
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[10] Figure 7: BER vs. attenuation for different countermeasures. [11]
total fade of 18.5 dB. In static tests, with slow changes in fade depth, a total gain of 20 dB was achieved. In the dynamic case shown here, steep fade slopes obviously cause the fade countermeasure system to sometimes react too late. Here, the 0.3 s delay between adjustment and verification of the achieved gain becomes significant. Some short moments of high BER can already lead to a sharp rise in the mean BER, since the BER responds very sensitively to varying signal strength. 7. CONCLUSION Our measurements have shown that adaptive fade countermeasures can very effectively increase the availability of a Ka band satellite link. The achieved gain is somewhat less than theoretically expected for the applied countermeasures due to the inertia inherent to any controller acting on a very long propagation delay link. ULPC alone, which does not require any additional frame time can improve the availability of the link quite significantly. However, if some excess bandwidth is available, coderate and symbolrate switching can increase the availability further still . Extremely high bandwidth requirements occur only rarely. This bandwidth could be taken from other stations. We are investigating adaptive resource sharing strategies, the purpose of which is to distribute a limited reserve to those stations which momentarily require more bandwidth to counter a deep fade. In future experiments, some of these algorithms may become implemented in the Marconi or a similar TDMA system.
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