Physical Layer Impairments in DVB-S2 Receivers

Transcription

1 Physical Layer Impairments in DVB-S Receivers Elias Nemer Advanced Technology Office. Consumer Electronics Group. Intel Corporation 350 E. Plumeria Drive San Jose, CA U.S.A Abstract Digital Video Broadcasting via satellite is being widely deployed worldwide. The first generation European standard (DVB-S) is a QPSK based system that supports up to 40 Msymb/s transmission rates. The second generation DVB-S specification recently finalized incorporates higher order modulations from 8PSK to 3 APSK. Physical layer impairments on the transmitreceive link include those introduced by the channel, such as reflections and interference, as well as those induced by various components in the system, such as tuner I/Q imbalance or amplifier non-linearity. These impairments degrade the received and may in some cases affect the convergence behavior of various loops in the receiver. The paper provides a qualitative and quantitative description of the system and channel impairments and their effects on the received cluster. and any further required processing, such as error correction and de-multiplexing, to generate the final MPEG packets. The Carrier to noise ratio in db-hz at the IF output of an earth terminal is function of the transmitted power, back off levels, total signal path loss and system temperature [16] and given by: C N 0 G = P max B 0 + βp 1 + κ ζ L T Eq 1. Keywords: DVB-S, DVB-S, Channel Impairments. I. INTRODUCTION Digital video broadcasting delivers high fidelity pictures and sound, creating a true home-theatre experience for the end user. A number of standards for terrestrial [1][4]satellite [6][9]and cable broadcasts [10] have been defined over the past few years. DVB by satellite is being widely deployed worldwide and is already evolving to a second generation (DVB-S [8]) to accommodate higher bandwidth efficiency, more elaborate service types and a better quality through new concepts such as hierarchical modulation. The general interference and attenuation environment for a satellite link in a broadcast system in shown in Fig 1. An earth station transmits on the uplink with a given power P 1. Other stations destined to the same transponder and interfering sources on the uplink may also be transmitting towards the same receiving antenna. At the transponder, the aggregate of all transmissions is filtered and pre-amplified. A frequency converter then shifts the uplink transmission to the appropriate band destined for the downlink. The traveling tube amplifier (TWTA) amplifies the signal with a gain β and the signal is radiated with a total power: Psat = Pg G. On the downlink, ST weather effects, such as rain and scintillations, may attenuate the signal, and co-channel interferences from adjacent satellites and ground stations can also degrade the broadcast signal. At the receiver, a low noise amplifier (LNB) shifts the incoming signal from 1 GHz band (Ku band) to about 1 to GHz. An tuner demodulates the signal and provides the (analog) I and Q components to a digital demodulator. This last stage provides the carrier recovery, symbol timing adjustment, equalization, Figure 1. Uplink and downlink channel path P is the maximum peak effective radiated power in max dbm. B is the back off amount in db of the total average 0 satellite power. β P is the percentage in db of the available 1 average power to be used for the carrier of interest (e.g. 10% corresponds to 10 db). G / ξ is the ratio of effective receiving antenna gain to received system noise temperature. κ is the boltzman constant, -198 dbm/k-hz; and L T is the total loss, including free space, antenna pointing loss, rainfall, and other to that terminal. The above C/N 0 ratio thus accounts for additive noise, as well as the radiated signal strength and the attenuation throughout the channel and the system. In typical manufacturers and service providers specs, separate ratios are provided for such factors as the added interferences from adjacent satellites, the inter-modulation products, the crosspolarization noise and others, as discussed in the sections below.

2 II. CHANNEL IMPAIRMENTS A. Free-Space Loss The loss in signal strength through free space is function of the frequency and the distance between the satellite and the receiver: L f = log10( f ) + 0log10( r). For geosynchronous satellites, orbiting at 35,784 km and a frequency of 1 GHz, the loss is: L f = 05. 5dB. B. Rain and Scintillation Rain has a dual effect on the C/N ratio: 1. It increases the antenna (and the system) temperature by as much as 00K relative to a clear day, resulting in degradation in the G / ξ ratio. The system temperature has 3 components: ξ σ ξ + ( 1 σ ) ξ + ξ, where : o o = f a f 0 ξ a : Antenna noise temperature due to external elements: rain, sun, and ground. σ : Loss in the antenna components (about 0.955) f o 0 ξ : Ambient temperature (about 80 K) o ξ R : Noise temperature of the LNB (e.g. 80 K) A typical scenario for the degradation in system temperature in heavy rain is show in TABLE I. below. TABLE I. EFFECT OF HEAVY RAIN OF SYSTEM TEMPERATURE Clear sky R Heavy rain C. of the Transponder Amplifier is due to the analog filters and amplifiers on the path from the earth station to the satellite and down to the (user) receiver. The dominant ones are due to the traveling tube amplifier (TWTA) and may be represented in terms of amplitude and phase effects, termed AM-AM and AM-PM between the input and the output. A general PAM signal at the input of a non-linear system x( t) = r( t) cos[ wt +θ] yields an output of the form y( t) = A[ r( t) ] cos{ wt +θ + φ [ r( t) ]}, where the two non-linear functions are expressed in terms of the input envelop r(t) through a set of curves, or in a parameterized form such as the commonly used Saleh model [14] : arr A( r) = 1+ b r r a r ; φ( r) = 1+ φ bφ r The amount of non-linearity depends on the operating point, or input back off (IBO) relative to the point of saturation. The effect on a square constellation is significant and results in a distortion of the square shape into one that has more rounded edges. The effect on a constant modulus PSK constellation is not as significant, as far as the magnitude distortion, since the envelop of the signal is almost constant. This was one of the reasons for choosing concentric ring consteallations for DVB- S. However, the aggregate effect of non-linearity is not negligible either (Fig. shows the case for IBO = 1dB), particularly for the higher density constellation. The effect is much less pronounced for the QPSK and 8PSK cases used in the first generation DVB-S system. Antenna temp 47 K System temp K Degradation in G ξ 10*log(39/114) = 4.6 db. Rain produces an attenuation at the carrier level, thus contribute to the total loss L T (in Eq 1). In GHz, the attenuation coefficient variation is nearly linear. Analytically, the amplitude attenuation is log normal distributed [4], i.e. assumed slow and flat fading. Scintillations are distortions by small-scale irregularities of the refractive index in the path. The amplitude (attenuation) of scintillation is log normal distributed and over short durations (up to 1 minute), the variance is considered constant. As with rain, the density function is Gaussian. While this analytical model is mostly useful for simulation, for the sake of link budget computations, the actual attenuation is determined by a recipe-like guidelines, such as the one provided in IESS-308, Appendix J, [11]and is function of several factors, such as the geographical location, the angles and latitude of the antennas, the outage probability desired, etc.. Figure. Effect of non-linearity on 16 and 3APSK D. Co-Channel Interference Co-channel interference is an interfering QAM signal that overlaps partially or completely in the same band as the desired signal. This interference may be caused by a number of factors: 1. The side lobes of a ground station antenna.. A cross-polarized signal from the same satellite. 3. A signal from a ground microwave link 4. Adjacent satellites whose antenna side lobes may radiate across the spectrum of interest. 5. Inter-modulation products caused by non-linearity in the TWTA when or more carriers are used. Their amplitude is proportional to the operating power and is higher near saturation. For the case of carriers per amplifier, the level of intermod products is given by:

3 IM dbc = ( * TOPB + 1) db, where TOPB is the total output power back off from saturation. TABLE II. below, taken from the Optus satellite specifications [16], illustrates typical co-channel interference values. TABLE II. CO-CHANNEL CHANNEL INTERFERENCE Parameters Clear sky Faded Uplink Faded downlink Intermod noise C/Im 17 db 13 db 17 db Downlink x-pol C/I Downlink adj. satellite C/N E. Mutipaths In satellite broadcast systems, there is often though not always- a line of sight reception and thus the reflections due to signal bouncing off the neighboring obstacles (buildings, etc...) have much lower power compared to the main signal. However, in contexts where a cable distribution system is used, micro-reflections due to the cable and the connections are significant enough to cause noticeable signal degradation A Satellite Master Antenna System (SMAS) is defined in [7] as one which is intended for the distribution of television and sound signals to households located in one or more adjacent buildings. It is a means for sharing the same resources among several users for satellite and terrestrial reception combined micro-reflections model (shown in the figure below). In the model specified, micro-reflections between devices in consecutive floors, between head-end and the first device, and between tap-off and user outlet, are specified. In the aggregate model, the shortest echo is about 30 nsec and the longest about 40 nsec. The effect on higher order constellation is significant as may be seen in figure 3 below: 4 echoes at 38, 55, 85 and 15 nsec are shown for 8PSK and echoes at 55 and 85 nsec are shown for 16APSK, with the attenuation according to the echo path profile in [7]. quadratures (I and Q) as well as the magnitude imbalance (ε ) and phase imbalance ( ϑ ) [3]: I ' 1 + ε = Q' 0 ϑ cos 0 1 ε ϑ sin ϑ sin I ϑ Q cos For M-PSK modulations, the I/Q imbalance causes a warping of the (otherwise round) constellation. The effect is more pronounced for square constellations such as the 16- QAM, and is less significant for the QPSK case. For the multiring PSK constellations specified in DVB-S, the effect is quite pronounced since the distance between points on different rings is significantly altered (Fig. 4). Figure 4. Effects of I/Q imbalance on 16APSK and 3APSK Simulation also shows that the presence of the I/Q imbalance has a negative impact on the convergence capability of the equalizer when both multipaths and I/Q imbalance are present. The plots in Fig. 5 illustrate the case for 16APSK and 3APSK with a channel consisting of I/Q imbalance and multipaths ( echoes at 36 and 55 nsec with 15 and 0 db attenuation respectively). The figure demonstrates that proper equalization cannot be achieved. The cluster variance around each point is due to the residual error of the equalizer which does not converge to the expected small value. The case where explicit I/Q balancing is applied prior to equalization is shown in a later section. Figure 3. Effect of 4 and reflections on 8PSK and 16APSK F. Tuner I/Q imbalance An analog tuner is commonly used at the receiver front end to down-convert the signal and provide the baseband quadrature (I and Q) components. Due to imperfect analog multiplications and filtering, the quadratures are off-balance, in both phase and magnitude. The corrupted quadratures (I and Q ) may be written in terms of the un-corrupted (hypothetical) Figure 5. Improper equalization as a result of I/Q imbalance

4 G. Orbit Inclination & Doppler Shift The satellites used in broadcast services are geostationary but not perfectly equatorial: During the first 5 or so years, the satellite is maintained at equatorial orbit +/ deg. Afterwards, latitude correction ceases and the orbit inclination builds in a linear fashion at 0.8 deg / year up to 3 deg. The impact of an inclined orbit is two-folds: 1. The linear velocity between the satellite and the receiving earth station causes a Doppler shift in carrier frequency.. The total path length varies with time and causes a change in the total path delay. The linear velocity of the satellite may be determined analytically, given the inclination angle, the height of the satellite and the latitude of the user station. The case of a user at the equator is illustrated in Fig. 6. Given a height H, and an inclination angle I, it may be shown that the linear velocity (towards and away from the satellite) is given by: V = ( D H ) / 6 km / h, where H is the height of the satellite and D is the maximum distance to the satellite from the observer and is derived from the geometry of the figure to be : D = R{ 1 cos( I )} ( R + H ) + H, with R the radius of the earth. For a height of 35,784 km, a radius R of 6378 km and a 5 deg inclination, V = 4.76_ km/ h. In practice, satellite manufacturers and service providers supply a table of maximum frequency shift and maximum delay variances for all possible angles of inclination (for instance the figures for the VA and VI satellites in [1]). The specifications of a typical tuner phase noise are given in TABLE III. below. TABLE III. TUNER PHASE NOISE TYPICAL CASE Offset freq KHz Phase noise dbc/hz I. Frequency Error Frequency errors in the received baseband I and Q quadratures may be caused by a variety of sources along the path from the earth station to the user receiver: 1. The frequency conversion process in the satellite (from uplink to downlink frequency) which is roughly limited to +/- 5 khz.. The frequency converters/amplifier at the user site (LNB) that brings the signal from 1 to 1+ GHz. 3. The oscillators in the tuner at the user site. 4. The Doppler shift discussed earlier. III. SIMULATION RESULTS A DVB-S/S transmitter/receiver system using QPSK, 8PSK 16APSK, and 3APSK modulation according to the specifications in [][6][8][9] is simulated using Matlab. The symbol rate is set to 30 Msymb/s and a sampling rate of 3-4 samples per symbol is used. A baseband channel model is assumed and various combinations of impairments are added, using the baseband channel model shown in Fig. 7. Equator I R H D H : height of satellite = 35,784 km R : radius of earth = 6378 km D : distance to satellite from an observer on the equator I : inclination angle of satellite orbit (I+jQ) symbols TWTA Phase Noise Tx Nyq Tx Intermod products Inband QAM interference (adj satellites) Adjacent Channel Interf Observer on the equator TWTA distortion Weather attenuat In this position, the distance from the observer to the satellite is D. After a ¼ earth revolution, the distance is H since the satellite is right above the observer. ( D H ) Thus, the motion speed to user: V = km / h 6 { 1 cos( I) } ( R + H ) D = R + H M = ( R + H )sin( I); N = ( R + H )cos( I) R H Equator R D M I N Figure 6. Orbit Inclination Effects AWGN Antenna Temperature noise LNB Phase Noise Micro reflections Tuner Phase Noise Freq Err Rx Nyq I / Q Imbalance Figure 7. Basedband Channel Model for Impairments Rx Timing jitter (I+jQ) symbols H. Phase Noise Phase noise arises from various sources: 1. The frequency conversion process in the satellite including the TWTA phase noise.. The frequency converters and amplifier (LNB) at the user site that shifts the signal from 1 to 1 GHz. 3. The tuner at the user site which down-converts the IF signal to baseband. An ideal receiver consisting of a matched Nyquist filter and a down sampler is used. The cluster of the received soft symbols is calculated for each impairment case. The results for an 8PSK constellation are shown in TABLE IV. below.

5 TABLE IV. IMPAIRMENTS EFFECTS ON CLUSTER FOR 8PSK Impairments Parameters 8PSK Cluster AWGN only Eb/No = 4 db 9.5 db AWGN, Phase noise khz offset 9.5 Echoes at 8.16 {55, 85} nsec 4 Echoes at 7.5 {36, 55, 85, 155} nsec IBO = 7 db 8.4 IBO = 5 db 7.96 IBO = 0 db 6.7 IQ Imbalance Amplitude = 0.15 Phase = deg IQ Imbalance Amplitude = 0.1 Phase = deg IQ Imbalance Amplitude = 0. Phase = 3 deg 8.16,, I/Q Imbalance 4 Echoes, IBO = 5 db, Amp= 0.15; Phase = 5 TABLE V. compares the cluster for 8PSK, 16APSK, and 3APSK constellations for given impairment scenarios. In each case, the same combination of impairments is used for all. The following set is used: 1. I/Q imbalance, withε = 0.15; ϑ = 5 deg.. echoes at 55 and 85 nsec as defined in [7]. 3. Additive white Gaussian noise at E b E0 = 6 db, thus C N = 10.7, 1.1, 1.9 db for 8PSK, 16APSK and 3APSK respectively. 4. Non-linearity with a back off point of 5dB. 5. Frequency offset with f = 10 khz. TABLE V. 5.9 CLUSTER FOR QPSK AND 8-PSK Impairments Eb / No 8PSK Cluster 16APSK Cluster 3APSK Cluster AWGN 6 db 11.1 db 1.3 db db 6 db db db db IQ Imbalance 6 db I/Q Imbalance 6 db Non-linearity Freq Offset I/Q Imbalance Freq Offset 6 db IV. IMPLICATIONS FOR RECEIVER DESIGN From the results above, the following may be inferred regarding the signal processing functionality required at the receiver: Equalization: An equalizer is required to counter the effect of the echoes in the antenna cable distribution. Given the echo profile, it is estimated that a T-space equalizer of about 4 taps will be sufficient to compensate for the echoes specified in [6]. Frequency and phase offset tracking: Frequency and phase offsets are large enough to cause significant errors. In addition, the higher density constellation are susceptible to small phase offsets. Therefore, a multistage correction scheme is required: a first stage providing a coarse frequency correction at the front end of the receiver, followed by a fine phase correction that is decision-directed and/or uses the pilot training symbols for an accurate phase estimate. Non-linearity: The multi-ring constellations used in DVB-S are less susceptible to non-linearity than square constellation (e.g. 16QAM), and thus an explicit remedy such as non-linear equalization may not be necessary. However, the effect of non-linearity is manifested as a phase error (Fig. ) and needs to be factored into the design of such blocks as the phase lock loop. I-Q imbalance: Simulation showed that the convergence of the equalizer is negatively impacted by the I/Q imbalance and yields a worsened constellation at the output of the receiver. Thus, an explicit compensation scheme possibly at the front end of the receiver is needed to remove the effect of the I/Q imbalance. Figure 5 shows comparative results when a scheme such as the one in [13] is used for the case of 3 APSK. Input at the receiver with I-Q imbalance and Multi-paths Output after equalization without I-Q balancing Output after I-Q balancing and equalization Figure 5. Equalizer output with and without I/Q balancing

6 A typical DVB-S receiver block diagram is shown in figure 6 below. Figure 6. DVB-S receiver functional block diagram V. CONCLUSION This paper provided a qualitative and quantitative description of the physical layer impairments in a DVB-S system and their effects on the received cluster. The analysis was illustrated with comparative results for 8PSK, 16APSK, and 3APSK constellations for a number of impairment combinations. The results show that these highdensity constellations are very susceptible to the commonly encountered impairments on the satellite link and that specific countermeasures are needed particularly for channel multipaths, I/Q imbalance, frequency offset. Simulation also show that the presence of the I/Q imbalance has a negative effect on the convergence capability of a typical linear equalizer and requires an I/Q balancing scheme at the front end of the receiver. The effect of non-linearity, while not significant in the first generation DVB-S system, may become pronounced enough for the case of the multi-ring constellations of DVB-S (depending on the operating IBO level), requiring either proper non-linear equalization at the receiver or a pre-distortion scheme at the transmitter. REFERENCES [1] A/53. ATSC standard: digital television standard, revision B. Advanced Television Systems Committee (ATSC). Aug 001. [] A/80. Modulation and coding requirements for digital TV applications over satellite. ATSC standard July [3] M. Buchholz, A. Schuchert, R. Hasholzner, Effect of tuner I-Q imbalance on multicarrier-modulation systems, Proc of the 000 conf on Devices, Circuits, and Systems, March 000. Vol. 1, pp. T65/1 T65/6. [4] J.P. Choi & V.W. Chan Predicting and adapting satellite channels with weather-induced impairments. IEEE trans. on aerospace and electronic systems, Vol. 38, issue 3, July 00, pp [5] EN Digital video broadcasting. Framing structure, channel modulation for digital terrestrial television. ETSI V [6] EN Digital video broadcasting (DVB). Framing structure, channel coding and modulation for 11/1 GHz satellite services. ETSI. V [7] EN Digital video broadcasting (DVB) satellite master antenna television (SMATV) distribution systems. ETSI. V [8] EN Digital video broadcasting: second generation framing structure, coding and modulation systems for broadcasting, interactive services, news gathering and other broadband satellite applications. ETSI. DVBS-74r [9] ITU-R BO Common functional requirements for the reception of digital multi-programme television emissions by satellite operating in the 11/1 GHz frequency range. 001 [10] ITU-T J.83. Digital multi-programme systems for television, sound and data services for cable distribution [11] Intelsat Earth Station Standards (IESS) 308. Rev 10, Feb 000. Performance characteristics for intermediate data rate digital carriers using convolution encoding / viterbi encoding and qpsk modulation. [1] Intelsat Earth Station Standards (IESS) 411. Rev 4, Oct 00. Requirements for earth stations accessing Intelsat VA & VI Satellites having higher than nominal orbital inclination. [13] E. Nemer & A. Said. An iterative feedback algorithm for correcting the I/Q imbalance in DVB-S receivers. IASTED Conf on Communication Systems and Networks [14] Optus Satellite Network Designer s Guide. [15] A.A.M. Saleh, Frequency-independent and frequency-dependent nonlinear model of TWT amplifiers, IEEE trans. Commun. vol. 9, pp , Nov [16] J. Spilker. Digital Communications by Satellite. Prentice Hall