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An Improved Digital Automatic Gain Control Method for DAB Receivers Hongsheng Zhang, Guoyu Wang, Mingying Lu International Journal of Digital Content Technology and its Applications. Volume 5, Number 5, May 2011
An Improved Digital Automatic Gain Control Method for DAB Receivers *1,2
*1
Hongsheng Zhang, 1,2Guoyu Wang, 2Mingying Lu Department of Microelectronic Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China, [email protected] ; 2 Key Lab of Microelectronic Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, P. R. China doi:10.4156/jdcta.vol5.issue5.27
Abstract Digital Audio Broadcasting (DAB) is designed for reliable reception up to vehicle speed of 120 km/h. The high vehicle speed reception requires fast Automatic Gain Control (AGC) to compensate the Rayleigh fading effects in wireless environment. But fast AGC will also incorrectly amplify DAB's null symbol and the following phase reference symbol to cause synchronization problem. Traditional DAB receivers use an 'agc_hold' signal to hold the AGC voltage to avoid the incorrect amplification during the null symbol. But this method requires one extra pin on both the RF tuner and the baseband decoder chips of the DAB receiver. Also it requires good time alignment between the 'agc_hold' signal and the actual null symbol, which brings the complexity to the receiver design. In this paper we proposed an improved AGC method which does not require the 'agc_hold' signal, while still keeps the null symbol quite clear. The new AGC method can be digitally implemented and integrated into the baseband decoder. Comparing to the traditional AGC method, the proposed method reduces the pin numbers of both the RF tuner and the baseband decoder chips. Also it reduces the communications between the RF tuner and the baseband decoder, thus simplifies the receiver design.
Keywords: Digital Audio Broadcasting, Automatic Gain Control, Rayleigh Fading 1. Introduction DAB is a digital audio broadcast system developed by Eureka 147 project. The target is to provide high audio quality for mobile reception. In 2005, South Korea proposed the Digital Multimedia Broadcasting (DMB) technology based on DAB. Besides the high quality audio services, DMB also offers a wide range of innovative services, such as mobile TV, traffic and safety information, data information and many other applications. DMB uses the same channel coding method as DAB. So DAB and DMB are the same if only the channel decoding part is concerned [1][2][3]. The field strength of the DAB signal is dynamically changing due to the multipath propagation caused by reflection and scattering. Typically these effects can be described by the Rayleigh channel model [4]. In the Rayleigh channel model, the radio signal can vary greatly at approximately every half wavelength [5][6]. If the receive moves in such environment, the receiving signal strength will change time by time. The faster the receiver moves, the faster the signal strength varies. Such effect is also called time selective fading [5][7]. Both DAB and DMB are designed for mobile reception. The target is to obtain reliable reception up to vehicle speeds of 120 km/h [1][4][7]. The high vehicle speed requires the DAB receiver to be very robust against the time selective fading. There are two technologies applied in DAB: the time interleaving technology and the fast Automatic Gain Control (AGC) design. The time interleaving technology is standardized by DAB [1], and has been proven to be an effective method against the time selective fading, especially for the vehicle speed faster than 50 km/h [8]. Below that speed, the benefit of the timing interleaving will be decreased, and AGC becomes the main factor to eliminate the time selective fading. In this situation the AGC must be fast enough to keep the receiving signal stable. Nowadays many DAB receivers, especially the portable and hand-hold DAB receivers, have applied the power saving technology to save the power consumption [9][10][11]. The power saving technology means that the DAB receiver will periodically turn off the RF tuner as well as the baseband decoder during the unwanted DAB symbols. This also requires the AGC to be very fast so that the receiving signal can quickly turn back to the normal level after the RF tuner is awaked.
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An Improved Digital Automatic Gain Control Method for DAB Receivers Hongsheng Zhang, Guoyu Wang, Mingying Lu International Journal of Digital Content Technology and its Applications. Volume 5, Number 5, May 2011
Due to the reasons above, the AGC circuit for DAB receivers, especially the portable receivers, is designed to have a fast settle time [8]-[12]. Receivers which have long AGC settle time usually work well at the receiving condition of stationary (where the receiving signal strength does not change with time) or driving (where the time interleaving will eliminate most of the time selective fading), but are easy to lost signal when users are walking or jogging (where the benefit of timing interleave is greatly decreased), especially in the urban areas. The first symbol of a DAB transmission frame is a special symbol which is called null symbol. During the null symbol, there are only a limited number of Orthogonal Frequency Division Multiplexing (OFDM) carriers to convey the Transmitter Identification Information (TII). This means the power of the null symbol is nearly zero, as shown in Figure 1 (a). The null symbol lasts for 1.3 ms for DAB transmission mode I, which is the mostly used transmission mode for terrestrial DAB. The null symbol is used for coarse time synchronization. The DAB receiver detects the amplitude level of the receiving signal to decide the start of the null symbol and a DAB transmission frame. Following the null symbol, there is another special symbol which is called phase reference symbol. The phase reference symbol carries a set of predefined training information which are used for fine time/frequency synchronization. The null symbol and the phase reference symbol together are called DAB synchronization channel because they are essential for the synchronization between the receiver and the transmitter. If either of the symbols is disturbed, the synchronization will be lost, or take longer time to setup.
Figure 1. The affection of the improper AGC in a DAB receiver. (a) shows the original DAB signal. (b) shows the big peaks generated at the beginning of the phase reference symbol due to fast AGC. The fast AGC circuit may have a settle time less than 1 ms, i.e. less than the length of the null symbol. So during the null symbol, AGC will consider the input signal very weak and thus generate a big feedback voltage to try to amplify the signal to bigger level. The big feedback voltage will last after the null symbol for at least one or two AGC detection windows. This big feedback voltage will hence amplify the phase reference symbol and generate big peaks after the null symbol, as shown in Figure 1 (b). The big peaks will cause the ADC saturation and the synchronization failure of the DAB receiver. So the AGC of a DAB receiver must be designed to not only have a short settle time, but also avoid incorrectly amplifying the null symbol and the following phase reference symbol.
2. Traditional AGC Method for DAB Receivers The DAB test standard requires the input signal level ranges from -81 dBm to -10 dBm [4]. Recent DAB RF tuners have extended this range to from nearly -100 dBm to 0 dBm [11]- [15]. Due to the big dynamic range of the input signal level, the AGC system of a DAB receiver usually comprises 2 stages: the Radio Frequency (RF) AGC stage and the Intermediate Frequency (IF) AGC stage. The RF AGC is
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An Improved Digital Automatic Gain Control Method for DAB Receivers Hongsheng Zhang, Guoyu Wang, Mingying Lu International Journal of Digital Content Technology and its Applications. Volume 5, Number 5, May 2011
integrated in the RF tuner and performs the first stage of the signal adjustment. The IF AGC comprises the IF VGA, the power detector (PD), and the AGC voltage generator. The IF VGA is inside the RF tuner chip. The PD and the AGC voltage generator can be either inside the RF tuner or inside the baseband decoder. If the two parts are inside the RF tuner, the AGC is totally controlled by the RF tuner itself and is completely an analog circuit. If the two parts are integrated in the baseband decoder, the feedback voltage of the IF AGC will be generated by the digital circuit of the baseband decoder. Such an AGC system is usually called digital AGC system. In this paper we focus on the improvement of the digital AGC system.
Figure 2. Block diagram of the traditional AGC system for DAB receivers. In order to get a fast AGC settle time, the detection window of PD must be short enough. However if the detection window is shorter than the length of the DAB null symbol, PD will consider the input signal very weak during the null symbol and thus generate a big feedback voltage. The big voltage will then cause the big peaks shown in Figure 1(b). Such peaks will affect the phase reference symbol and cause the DAB receiver hard to get into synchronization state. Users will feel that the receiver is hard to search the DAB programs even in a very well covered region. In order to avoid the big peaks, most RF tuners provide a special pin, which is usually called ‘agc_hold’, to hold the AGC feedback voltages during the null symbol [10]. The ‘agc_hold’ signal is provided by the baseband decoder. Once the null symbol is detected, the ‘agc_hold’ signal is validated so that PD is paused and the control voltage on IF VGA is kept constant to avoid the incorrect amplification during the null symbol. After the null symbol, the ‘agc_hold’ signal is invalidated and the AGC returns to normal state. So the traditional AGC method needs a null symbol detection block to get the position of the null symbol.
Figure 3. The affection of agc_hold in the traditional AGC method. At the time of Tb , the required AGC gain is Gb, while the actual gain is Ga due to agc_hold. Because Ga > Gb, peaks will be generated at Tb.
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An Improved Digital Automatic Gain Control Method for DAB Receivers Hongsheng Zhang, Guoyu Wang, Mingying Lu International Journal of Digital Content Technology and its Applications. Volume 5, Number 5, May 2011
The traditional AGC method requires good time alignment between the null symbol and the ‘agc_hold’ signal. If ‘agc_hold’ signal ends earlier, the unprotected part of the null symbol will still be incorrectly amplified to cause the peaks at the phase reference symbol. If ‘agc_hold’ signal ends too later, peaks may still be generated because the AGC value which held during the null symbol may becomes too bigger than required at the start of the phase reference symbol, as shown in Figure 3. So the ‘agc_hold’ signal is usually controlled and adjusted by the synchronization block of the channel decoder. Also the traditional method requires one extra ‘agc_hold’ pin on both RF tuner and the baseband decoder. This increases the cost of the chips and the complexity of the receiver design.
3. Proposed AGC Method for DAB Receivers In this section we proposal an improved AGC method which does not need the ‘agc_hold’ pin on either the RF tuner or the baseband decoder. The AGC feedback voltage is automatically held on when the null symbol is detected. Also the proposed AGC method does not need to communicate with the channel decoding part of the baseband decoder, thus simplify the design of the DAB receiver.
3.1. Description of the Proposed AGC Method The null symbol always starts from a falling edge of the signal level. The falling edge of the signal level will then bring an increase in the AGC feedback voltage, if the AGC response time is fast enough. So we can detect the null symbol by just observing the AGC feedback gain. If the ratio between the new AGC gain and the previous AGC gain is bigger than a threshold, then the current time tfall_edge can be marked as the start of the null symbol. In this way the null detector in the traditional method can be removed. The max length of the null symbol is 1.3 ms. During the 1.3 ms after the null symbol is detected, i.e. from tfall_edge to (tfall_edge + 1.3 ms), the power detector PD is paused, and the AGC feedback gain is held to a value smaller than the gain calculated at tfall_edge. The held value is purposely set smaller in order to reduce the affections stated in Figure 3. In this way, we only need to pause PD in the baseband decoder, not the VGA of the RF tuner, so the ‘agc_hold’ pin on both RF tuner and baseband decoder can be removed. The block diagram of the proposed AGC is shown in Figure 4.
Figure 4. Block diagram of the proposed AGC system for DAB receivers.
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An Improved Digital Automatic Gain Control Method for DAB Receivers Hongsheng Zhang, Guoyu Wang, Mingying Lu International Journal of Digital Content Technology and its Applications. Volume 5, Number 5, May 2011
Figure 5. Control flow chart of the proposed AGC method. After (tfall_edge + 1.3 ms), PD is activated and the AGC returns to normal operation mode. Several actions are done to avoid the suddenly change of the AGC gain. First, the new AGC feedback gain G(n) is calculated and a counter cnt is set to 1. cnt will be increased by 1 after each AGC detection. Then G(n) is compared with the last AGC gain G(n-1) (also the AGC gain held during the null symbol). If G(n)/ G(n-1)>1.125, then force G(n) to G(n-1)*(1+1/64) to avoid a big increase of G(n). If G(n)/ G(n1)