CA2508463A1 - Method to compensate for rain fade in a digital video broadcast satellite system - Google Patents

Description

METHOD TO COMPENSATE FOR RAIN FADE
IN A DIGITAL VIDEO BROADCAST SATELLITE SYSTEM
FIELD OF THE INVENTION
The present invention relates to satellite communications and to earth stations (terminals) for geostationary satellites using "bent pipe" (analog repeater) transponders.
More particularly, the invention relates to large networks of receive only stations and one transmit station. This is usually the case for broadcast of television signals over the satellite medium.
BACKGROUND OF THE INVENTION
This invention is a method to solve a long standing problem of broadcasting digital video signals by satellite. Older methods of broadcasting video signals by satellite used frequency modulation which where analog in nature and are not covered by this invention.
The problem occurs mostly on broadcast frequencies in Ku and Ka band.
The propagation characteristics of those bands are such that moisture present on the propagation path absorbs the signal and thus causes impairments in the reception. The impairments can cause the signal to noise ratio of the signal to go below the receiver threshold, thus causing complete signal loss. This loss results in loss of video content for the duration of the fade, which causes viewer dissatisfaction. The effect of signal attenuation is three fold:
1. A first effect is to reduce signal strength while the noise caused by the receivers low noise amplifier remains constant.

2. A second effect is to increase slightly the noise floor of the station because the moisture is warmer than the background space. This effect is small for the low cost, small sized parabolic dish.

3. Depolarization of the received signal: part of the signals energy appears in the orthogonal polarization. Experimental results reported indicate this effect to be negligible compared to the first item.
Several methods exist to control the uplink part of the transmission to ensure that the broadcast signal from the satellite is essentially immune to fade.
Downlink fades are a different problem. Because the plurality of receivers are dispersed in wide areas that can experience fade events that are specific to the location, conventional means to control uplink power do not apply.
It is well known from communication theory the relationship between the signal to noise expressed as the ratio Eb/No, the bit rate and the forward error coding (fec) rate. It is assumed that the transmit power out of the satellite transponder to the multiple receivers is constant. The only 2 other variables to improve the received signal to noise are the bit rate and the fec rate. A bit rate reduction of a factor of 2 results in a signal to noise improvement of 3 dB.
The receiver threshold is usually defined as the worse bit error rate that can be tolerated for a given Eb/No. The bit error rate is a function of the Eb/No, modulation and type of forward error coding. Thus a threshold improvement of 3 dB is obtained by reducing the bit rate by a factor of 2. An decrease in fec rate from %Z ( 1 coding bit for each data bit) to 1/3 (2 coding bits for each data bit) will produce an improvement in bit error rate performance and thus reduced threshold that depends on the particular scheme used.
There are many combinations of bit rate and fec rates that can be used to improve the receiver threshold.

The new scheme proposed in this invention multiplexes into a single signal digital streams of at least 2 different bit rates. These alternate bit rates can also use varying coding rates. Several multiplexing scheme can be devised and some are given in a following example. In all cases the different choices of combination bit rates and coding rates are known in advance by the receiver. The receiver decodes the different streams and selects the one that has the best signal to noise.
During clear sky reception conditions, the receiver would select to display to the viewer the digital stream of the highest bit rate that corresponds to the best audio and video quality. During fade conditions, the receiver would select to display one of the lower bit rate streams that correspond to a lower audio and video quality.
As an example, assume a high quality broadcast signal that has a channel bit rate of 10 Mb/s and a high compression lower quality broadcast signal of 1 Mb/s.
Assume that both have the same forward error coding method and the same modulation: qpsk.
Assume further that the link budget (sizing of antenna size, power from the satellite, noise contribution from low noise amplifier) is designed (under clear sky conditions) to provide negligible bit error rate for the 10 Mb/s broadcast signal at an Eb/No of 5 dB
(see Figure 8 for the definition of this parameter) . The bit error rate performance in presence of additive noise is given by articles published in learned journals. These results are given for a specific modulation method and forward error coding scheme.
When a received signal is attenuated by 10 dB by a fade event (received power is 1/10 the clear sky value), the signal power is decreased by 10 dB while the noise density remains approximately constant. From Figure 8, the Eb/No is decreased by 10 dB
for the Mb/s signal. This worsens the bit error rate and renders the high quality transmission unusable because of the high number of bit errors. Note however that in Figure 8, the lower bit rate link has a bit period that is 10 times longer than the high bit rate. The result is that the Eb/No for the low bit rate is the same SdB under the fade condition. In this condition, the bit error rate is negligible for this stream. The appropriate decompression scheme is selected by selecting the best output stream and a reduced quality broadcast is provided to the viewer instead of no reception at all.
While the signal strength is reduced by one tenth, those normally skilled in designing wireless demodulators can design signal processing schemes that can handle this requirement.
The previous example is independent of the exact method used to multiplex the two streams. Those normally skilled in the design of modems will select the method most appropriate.
Furthermore, the method can be used to provide preview of broadcast material using the lower resolution stream for the receivers that do not have access to the better quality material. When the receiver does not have the authorization from the service provider to view the broadcast material, the receiver can operate in low resolution mode to provide a preview of the broadcast.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the invention, reference will be made to the accompanying drawings illustrating embodiments thereof, in which:
Figure 1 is an overall block diagram of a satellite video broadcast systems;
Figure 2 is a block diagram of an improved transmitter scheme Figure 3 is a block diagram of variable transmission streams;

-4-Figure 4 is a block diagram illustrating two streams being multiplexed;
Figure 5 is a block diagram providing an example of a multiplexing scheme where three streams are multiplexed into a single carrier;
Figure 6 is a block diagram providing a time domain view of Figure 5;
Figure 7 is a block diagram of the receiver; and Figure 8 illustrated the definition of E,,//No in a data transmission system Referring to Figure l, for an overall block diagram of a satellite video broadcast system, a transmitter emits a signal to the satellite. This signal is often controlled by an uplink power control scheme that compensates for fades in the transmitter to satellite path. The output of the satellite transponder has a constant output power on the path from satellite to the receivers, irrespective of fades in the uplink path when such a scheme is used.
The receivers can however be in a clear sky reception area or individually affected by fades that are specific to their geographic location. The uncompressed broadcast material supplied by the producer of the material is fed to at least 2 compressors shown in Figure 3 that use the same compression scheme but with different compression ratios.
These compressors perform both the voice and image compression. Furthermore, they multiplex the compressed voice and image into a single data stream. Each compressor has a different output bit rate corresponding to the different broadcast quality required. To compensate for the delay difference in the variable ratio compression, a delay equalizer in Figure 3 is introduced to simplify the task of changing dynamically between compressors at the receiver. Examples of compression scheme are mpeg 2 and mpeg 4.

-5-Referring to Figure 2, several streams of data corresponding to the different broadcast quality are multiplexed together into a single signal split between the in phase and quadrature channel of a conventional IQ modulator.
Several schemes are possible to perform this multiplexing. One example is provided in Figure 4 for the case where only 2 streams are multiplexed. In such a scheme, often called unbalanced qpsk, the 2 unequal bit rate streams are simply filtered with a filter to ensure inter symbol interference (isi) free reception with a Nyquist type filter.
After conversion to an analog format and harmonics removed, the signal is fed to a conventional modulator.
A well known alternate format for IQ modulator that converts to analog after the modulation is equivalent to the implementation in the diagram Figures 4 and 5.
Figure 6 shows an I channel time domain representation the scheme in Figure 5.
In that case, two bursts of different data rates are shown. In the event the signal to noise is too low for the highest data rate burst to synchronize and decode correctly, the following lower rate burst can receive the data. The preamble is used for initial synchronization of the Garner phase, symbol timing and signal amplitude. Figure 5 shows a preamble insertion block that formats the incoming 2 streams into separate data bursts.
For synchronization purposes, each burst is preceded by a preamble. A guard time is included between each burst to prevent interference between successive bursts.
Numerous variations of multiplexing schemes on the transmitter are possible and are not shown.
A block diagram of the receiver is shown in Figure 7. After processing by the receive electronics and the IQ demodulator the signal is converted to a digital form and

-6-filtered with the appropriate matched filter. Each stream has a signal to noise estimator.
The stream that has the best signal to noise is selected for decompression.
It will be understood that the above described embodiment is for purposes of illustration only and that changes and modifications may be made thereto without departing from the spirit and scope of the invention.

Claims