1、Rec. ITU-R S.1425 1 RECOMMENDATION ITU-R S.1425 TRANSMISSION CONSLDERATIONS FOR DIGITAL CARRLERS USNG HIGHER LEVELS OF MODULATION ON SATELLITE CIRCUITS* (Questions ITU-R 73/4 and ITU-R 230/4) (2000) The ITU Radiocommunication Assembly, considering a that the radio spectrum and geostationary orbit ar
2、e limited resources; b) that there is spectrum congestion in certain frequency bands allocated to the FSS; c becoming increasingly difficult; that due to the rapid development of different radio services, sharing conditions in many frequency bands are d) such as 8-PSK and 16-QAM as compared to QPSK;
3、 that the spectral efficiency of satellite networks can be increased through the use of higher level modulations e techniques; that the power efficiency of satellite networks can be improved through the use of appropriate encoding f) that there is a continuous need to update the information availabl
4、e on the characteristics of satellite transmissions and their related performance; g) that different transmission techniques can be implemented to transport the same information stream, taking into account a sensitive to linear and non-linear distortion than those transmitting QPSK signals; that sat
5、ellite circuits transmitting higher level modulations and in particular QAM such as 16-QAM are more b) phase noise than those transmitting in QPSK format; that satellite circuits transmitting higher level modulations such as 8-PSK and 16-QAM are more sensitive to c of more transmitter power or highe
6、r earth station G/T and greater earth station e.i.r.p., that use of higher level modulation techniques improve the bandwidth efficiency for a given Eb/No at the cost recommends 1 satellite circuits using higher level modulations such as 8-PSK and 16-QAM. that the information contained in Annex 1 be
7、used as a guide for implementation of transmission quality for * This Recommendation should be brought to the attention of Radiocommunication Working Parties 4 SNG and 10-11s. 2 Rec. ITU-R S.1425 ANNEX 1 Transmission considerations for carriers using higher levels of modulation on satellite circuits
8、 1 Introductiodpurpose This Annex provides information regarding the development of new technologies intended for use on, but not limited to, FSS networks. The initial purpose of these developments are to increase the throughput capacity of the available spectrum that is allocated to that service. T
9、his could be achieved by utilizing more efficient modulation processes for the transmission of information. 2 Background 2.1 Purposdobjective With the increased satellite transponder power and concatenated error correction techniques, higher modulation schemes such as 8-PSK and 16-QAM have become fe
10、asible for implementation in geostationary satellite orbit (GSO) satellite communication systems. These high order modulations significantly improve the transponder bandwidth efficiency which suits the needs of broadband format structures and applications, such as asynchronous transfer mode (ATM), f
11、rame-relay and HDTV. In order to achieve acceptable quality of service as characterized by low BER and higher data rates with the higher order modulations, there is a requirement for not only a higher CIN ratio at the input of the demodulator. but also a need to control interference and limit distor
12、tion within the satellite transmission circuit. This Recommendation describes the nature of the higher order modulations considered applicable to geostationary- satellite networks and details the reasons for their susceptibility and increased sensitivity to transmission impairments. It will also des
13、cribe the tests and results of 8-PSK and 16-QAM circuits implemented to determine their operational requirements in a typical satellite network. 2.2 8-PSK and 16-QAM modulations In 8-PSK the transmitted symbol is a sinusoidal carrier with eight equally spaced phases. The signal-space diagram is show
14、n in Fig. 1. Every 8-PSK modulated symbol represents three user bit occupancies, as opposed to 2 bit occupancies for QPSK modulation, thereby improving bandwidth utilization by 50%. On the other hand, each symbol is 45“ closer to each other in phase compared to the QPSK symbols which introduces more
15、 difficulty to the decision-making processes of the receiver. Theoretical calculations shows that without any error correction techniques, 8-PSK requires 7 dB more energy per bit than QPSK in order to achieve 1 x lop8 BER. Figure 2 is an example of a 4 x 4 QAM (16-QAM) signal-space. Each symbol repr
16、esents four information bits and requires that the receiver distinguish between the different combinations of both the phase and amplitude modulation process. As shown in Fig. 2, the receiver must distinguish between the possible 12 phases and four amplitudes in order to make a decision. As the sepa
17、ration between the signal states are even closer together than for 8-PSK, both amplitude noise and phase noise will have a more pronounced affect in the decision-making process. However, although more vulnerable to transmission impairments than 8-PSK, the bandwidth utilization is improved by 100% co
18、mpared to QPSK. Rec. ITU-R S.1425 3 FIGURE 1 Signal-space for 8-PSK I Decision lines FIGURE 2 Signal-space for 16-QAM (0110) (0101) (0011) Q I +I (0000) Q (1 100) Es : Energy per symbol (0010) (O00 1) , ( l1 11 Decision Q - ,/ lines J (1110) Q 3 Transmission test methods and measurements 3.1 Descrip
19、tion of test facility Tests were conducted to investigate the capability and circuit limitations necessary for 16-QAM broadband transmissions in a satellite network. To that purpose a flexible test configuration was implemented which also allowed the determination and optimization of intermediate tr
20、ansmission level points associated with the non-linear transmission elements in the network. Figure 3 presents a block diagram representation of the test configuration used. The switch points indicated in this Figure are shown to convey the variety of the test configurations used. 4 Rec. ITU-R S.142
21、5 FIGURE 3 Test facility for data transmission tests Modem 1 I Group delay equalizer 11 , Satellite AL under test interference test set converter HPA: LNA: TWTA: high power amplifier low noise amplifier travelling wave tube amplifier 1425-03 The transmission bandwidth of any of the network configura
22、tions was limited to that of a 36 MHz transponder. In all tests the digital transmission rate was chosen to be 45 Mbitls (DS-3). The spectral bandwidths of the 8-PSK and 16-QAM data modems used in these tests were determined to be about 26 MHz and 15 MHz. This allowed multicarrier transmission in a
23、single transponder for 16-QAM modulation. All modems, for all rates, used in the tests depicted here incorporated FEC rate 3/4 and concatenated 188,204 Reed-Solomon (RS) outer coding. BER measurements made during these tests were actually made with respect to Eb/N ratios. For presentation purposes h
24、ere the results were converted to a C/N format. Conversion between the two formats can be accomplished with the following formula: where: p = 2forQPSK = 3 for 8-PSK = 4 for 16-QAM FEC: FEC code rate (e.g. 3/4) RS: Reed-Solomon code rate (e.g. (188,204) 1.2 : bandwidtWsymbo1 rate ratio. 3.2 Test conf
25、igurations and results 3.2.1 Baseline F loopback test An initial test configuration was established to test the modems in an F loopback facility. This was achieved by figuratively closing switch point 2 and opening switch point 1 of the facility shown in Fig. 3, resulting in the test configuration s
26、hown in Fig. 4. The purpose of the tests was to achieve a baseline for the following RF test measurements. Measurements were performed for QPSK, 8-PSK and 16-QAM modulations. The resulting BER performance for each modulation scheme is shown in Fig. 5. Rec. ITU-R S.1425 5 FIGURE 4 Block diagram confi
27、guration for the IF loopback test tester I BER under “Z Noise and interference test set i Spectrum Plotter El FIGURE 5 If loopback test results 104 5 2 1 o- 5 2 10-6 5 Y m2 1 5 2 10-8 5 2 1 O 2 4 6 8 10 12 14 CIN (dB) QPSK . 8.PSK _ 16-QAM 1425-05 It is seen from Fig. 5 that for a BER of 1 x lop8 th
28、e required circuit CIN for QPSK, 8-PSK and 16-QAM modulated transmissions are 5.5, 8.7 and 12.2 dl3 respectively. 6 Rec. ITU-R S.1425 3.2.2 RF baseline test In order to quantify the effects of the earth station HPA non-linearity, the test facility was configured (figuratively by closing switch point
29、s 3, 4, 6 and 7) so as to provide a path that excluded the space components. Tests were conducted to determine the linearity distortion effects on 16-QAM modulation by backing off the signal input from the HPA saturation level until an acceptable operating point was obtained. A series of tests indic
30、ated that between 14 and 16 dB of input backoff from the saturation level was required before acceptable non-linearity levels could be achieved. Figure 6 presents a graph depicting performance impairments due to the HPA non-linearity as compared to the IF loopback case. The results show that operati
31、ng the HPA at a point equal to 14 to 16 dB input back-off increases the required CIN by 0.4 to 0.6 dB at a 1 x lop8 BER point. FIGURE 6 Earth station HPA linearity 16-QAM carrier input back-off 104 5 2 10-5 5 2 10-6 5 2 10-7 5 2 10-8 5 2 10-9 5 2 10-10 . . , 11.5 12 12.5 CIN (dB) 13 13.5 3.2.3 Satel
32、lite transponder linearity In order to determine an acceptable operating point to the satellite TWTA, the network facility was configured to include an RF path component (figuratively by closing switch points 3, 4, 7, 10, and 12). This routed the RF signal through the TWTA. The test consisted of var
33、ying the level of a single 16-QAM carrier input to the TWTA while input to the HPA was maintained at a 16 dB backoff with a constant CIN ratio of about 14 dB. Figure 7 shows the 16-QAM BER performance at the TWTA output versus signal input back-off from its saturation point. Table 1 relates the tran
34、sfer characteristics for a typical TWTA. Rec. ITU-R S.1425 7 Y F4 1 5 2 104 5 2 10-5 5 2 10-6 5 2 1 5 2 10-8 6 FIGURE 7 Satellite transponder linearity Earth station HPA-LBO = 16 dB, TWTA input CIN = 13.8 dB 8 10 12 Transponder IBO (dB) 14 BER 1425-07 TABLE 1 Typical TWTA transfer characteristics Ou
35、tput back-off (dB) 0.0 o. 1 0.3 0.6 1.0 1.4 1.8 2.4 3.0 3.8 4.6 5.5 6.5 7.6 3.2.4 Full system tests Figure 8 presents the BER performance for single QPSK, 8-PSK and 16-QAM carriers through a complete network. Test results are shown for two levels of input to the satellite receiver corresponding to a
36、 13 dl3 and 10 dl3 input level back-off 8 Rec. ITU-R S.1425 operating point to the satellite TWTA. The operating point of the earth station HPA was kept at a constant back-off of 16 dB. A comparison of Figs. 5 and 8 shows that there is a degradation of BER performance of the complete network from th
37、e IF loopback configuration of about 1.1 to 2.5 dB depending on the back-off input to the satellite TWTA. FIGURE 8 Full system HPA/TWA IBO = 16/10 and 16/13 dB 1 o- 5 2 10-6 5 2 10-7 5 2 Y F4 10-8 5 2 10-9 5 QPSK 8-PSK 1 6-QAM/ 10 1 6-QAM/ 13 . - - - - - - - - - - . - . - . - . - . - . - 3.2.5 Multi
38、carrier configuration and test results A multicarrier test facility was configured by adding a second modem into the network (closing switch points 3, 4, 6 and 8). Tests were performed on an RF path configured with two 16-QAM carriers operating at 45 Mbitls loaded in a single transponder. Centre fre
39、quencies of the carriers were adjusted for 17 MHz separation when centred in the 36 MHz transponder. The total input power into the HPADWTA was maintained at a 16/13 dB backoff point. Figure 9 compares the BER results of the IF loopback case with the one and two-carrier situation. The results show t
40、hat at a BER of 1 x lop8 the per channel performance of two 16-QAM 45 Mbitls carriers operating in a single transponder is degraded by only 1.7 dB when compared to the IF loopback baseline configuration. 3.3 Summary of performance results BER performance tests for single 45 Mbitls carriers were cond
41、ucted on a satellite circuit for QPSK, 8-PSK and 16-QAM modulations. Each carrier used an inner convolutional code of rate 3/4 and a RS outer code of 188,204. The BER performance results for the three modulations are shown in Fig. 8. Figure 9 compares one and two 16-QAM carrier BER performance. Tabl
42、e 2 summarizes the C/NO requirements at the receiver demodulator for a BER of 1 x lop8 for all three schemes. Rec. ITU-R S.1425 2 9 I I I FIGURE 9 Multiple 16-QAM carriers TWTA IBO = 13 dB total FEC + RS IF loop (dB) 2 1 o- 5 2 10-6 5 2 10-7 5 2 10-8 5 2 314 + 188,204 314 + 188,204 314 + 188,204 5.7
43、 8.7 12.2 One carrier, HPA/TWTA IBO = 16/13 (dB) Two carrier, HPATWTA IBO = 16/13 (dB) _ 11 5.7 8.8 13.2 13.6 12 13 14 15 CIN (dB) Loop . One carrier - Two carriers TABLE 2 CIN required for 1 x lo4 BER for QPSK, 8-PSK and 16-QAM carriers Configuration I QPSK I 8-PSK I 16-QAM I 3.4 Effects of co-chan
44、nel interference In order to investigate the effects of Co-channel interference, a digitally modulated interfering signal was introduced into the test configuration when implemented in the IF loopback test set-up mode per Fig. 2. Interference was adjusted for C/Z levels of 20 and 24 dB. BER was meas
45、ured for various C/NO levels in the presence and without the presence of the interfering signal. Figure 10 shows the BER performance for a single 16-QAM carrier for the various interference scenarios. It is apparent that interference into a 16-QAM circuit must be carefully controlled. 10 2 10-5 5 2
46、Rec. ITU-R S.1425 - . . =. FIGURE 10 16-QAM interference sensivity loopback test 10-6 5 2 10-7 5 2 10-8 5 2 2 I l I 11 11.5 12 12.5 13 10-10 CIN (dB) 4 Conclusions From the above results it is apparent that higher order modulations such as 16-QAM improve bandwidth efficiency for a given Eb/No at the
47、 cost of more transponder power, greater earth station e.i.r.p. Besides the major contribution of rain losses in 14/11 GHz and 30/20 GHz bands, the non-linear characteristics of the satellite network plays an important role in the received signal distortion at the receiver demodulator. One of the im
48、portant considerations of applying higher order modulations for satellite operation is a requirement to control the input level into the satellite TWTA or the resulting distortion emanating from it. This might suggest the possible need for earth station power control. It is also noted that neither a
49、mplitude or phase equalization was employed during any of the above tests suggesting that some further improved performance may be possible through the use of linearizers and equalizers, either on the spacecraft or in the uplink path. The test results indicate that all three carrier modulation schemes are, in a relative sense, equally affected to the changes in operating C/N level. For example, for all three situations, if the BER operating point is at 1 x loplo, it will drop to 1 x lop3 if the network C/N drops by less than 2 dB. F
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