Improved user experiences are possible with enhanced FM radio data system (RDS) reception

Transcription

1 Improved user experiences are possible with enhanced FM radio data system (RDS) reception Aravind Ganesan, Senior Systems Engineer, and Jaiganesh Balakrishnan, Senior Member of Technical Staff, Wireless Business Unit, Texas Instruments Introduction With the growing popularity of FM transmission in a number of developing countries, low-cost integrated FM receivers have become the norm in the mobile handset segment. Most users would be familiar with high-quality stereo audio FM transmission, which is the primary sub-channel in FM broadcast transmission. In addition to the audio channel, FM transmission includes Radio Data System (RDS), a low-bit-rate digital-information sub-channel. RDS is used to transmit auxiliary data like program information, traffic messages, and even region-specific advertisements. These features enhance the user experiences. The RDS sensitivity of FM receivers is critical to sustaining such enhanced user experiences under challenging mobile connections. In this article, we first look at the merits of exploiting FM RDS data in mobile devices which contain other functionalities, such as the Global Positioning System (GPS). We then explore details of RDS transmission along with traditional receiver processing. We also look at factors influencing RDS performance and discuss why it is important to improve the receiver performance. Finally we present the optimized receiver which enables improved user experiences. Why is RDS interesting? The ubiquitous nature of FM broadcast transmission, coupled with a low receiver complexity, has enabled the proliferation of FM receivers. It is therefore natural to use this medium to broadcast useful information over a wide area. Thus, RDS is a convenient medium for transmitting digital content across the airwaves. Some of the RDS features are tied to the content transmitted by radio stations. Radio text displaying song/program information, and Alternate Frequency (AF) indicator are a few examples. The RDS standard [Reference 1] provides ample flexibility to support additional features like song tagging, Traffic Messaging Channel (TMC), radio paging, Differential GPS (D-GPS) and more. A detailed and interesting history of the RDS standard can be found in the article RDS is now 25 the complete history [Reference 2]. User experiences can be improved in devices supporting multiple functionalities, like GPS positioning and Internet connectivity, by exploiting features supported by RDS. For

2 example, in a device with an FM receiver and a GPS receiver, data from the FM radio RDS-TMC can be used to improve the GPS-based navigation application. TMC data contains information about traffic incidents in the transmission range of the broadcasting station. A device with RDS and GPS reception capabilities can use the TMC data, matched with its current GPS position, to support a traffic-based dynamic re-routing. This feature determines the fastest route to a destination based on knowledge of current traffic conditions in the locality. Presence of road blocks due to accidents or weather conditions transmitted through the RDS-TMC are used to alter the route appropriately. Figure 1 illustrates a typical scenario where the dynamic routing facility can help the navigation application to combine information from the GPS and the FM-RDS modules to change the route on-the-fly based on exceptional circumstances indicated through the FM- RDS TMC channel. RDS TMC channel Accident! Original route Original route Quicker route Quicker route Figure 1: Example of dynamic routing feature Another example rests in the use of D-GPS assistance which compensates for errors introduced due to transmission delays in the ionosphere. For such features, improving the RDS performance automatically translates to an increase in the range over which the RDS signal can be decoded. Another interesting application that makes use of RDS channel is the song tagging feature, as found in some of today s portable music players. This feature allows a user, while listening to a song on an FM broadcast station, to tag that song on the listening device. While doing so, the listening device notes down a unique identification number, transmitted over the RDS ODA (Other Data Application) group, and identifies the song when the listening device connects to the Internet. This feature is useful in scenarios

3 where a person might not be able to identify the broadcasted song, but is interested in hearing it again or purchasing it later. In addition to RDS features that augment the performance of other IPs or enable use cases, there are compelling reasons to improve the stand-alone performance of RDS receivers. Take the example of RDS alternate frequency (AF) feature, which allows a receiver to automatically search for an alternate broadcast station transmitting the same program. This feature is useful in high-mobility scenarios where the signal condition of the tuned frequency deteriorates as the user moves away from the transmitter and there is a network of transmitters broadcasting the same program at different channel frequencies. This is similar to the handovers in cellular networks, except that the decision is made solely by the receiver. Since this feature needs the receiver to demodulate the RDS data correctly to extract the alternate frequency information, a receiver with poor RDS sensitivity performance will in turn have poor AF sensitivity performance. As discussed in the subsequent section, the RDS sensitivity requires higher input-power level than audio sensitivity. Thus, even though audio quality in the tuned channel might be good, the receiver might not be able to decode the RDS data and extract the relevant information required for the AF feature, due to a non-optimum RDS processing. A similar scenario exists for audio content-related features like song information display, program station name display and others. As the above examples prove, enhancing the RDS performance is essential to enabling improved support for RDS-based features and hence improved user experience. Details on RDS transmission The signal of interest in FM broadcasting is the Multiplexed (MPX) signal [Figure 2], which consists of four components: Mono (L+R), Pilot, Stereo (L-R) and RDS. These are frequency division multiplexed at DC, 19kHz, 38kHz and 57kHz. The pilot aids the recovery of the other components of the MPX signal. RDS data is differential BPSK modulated at a baud rate of khz. The pulse shape was designed such that it coexists with the now mostly defunct Autofahrer-Rundfunk-Informationssystem (ARI) transmission, which was used to transmit traffic information in Germany. Mono (L+R) Pilot Stereo (L-R) RDS Freq (KHZ) Figure 2: FM Multiplexed signal

4 The RDS transmitter s physical layer is shown in Figure 3. It consists of a simple frame buffer from which frames of 16 bits each (Block) are passed to the error control encoder. The code used is a (26,16) shortened cyclic code. This code was designed as a burst error code with capability to detect burst errors of lengths of five or less. From RDS Transport Layer Frame Buffer Un-coded Data Error control encoder Encoded bits KHz Differential BPSK Modulator Pulse shape filtering To MPX signal Combiner Figure 3: Simplified RDS transmitter To the encoded block, an offset is added to the 10 parity bits using a simple Exclusive-OR (XOR) operation. The offset addition aids the frame synchronization at the receiver. The modified coded bits are then passed to the modulator and the pulse shaping filter. This signal is then modulated to 57 khz and added to the other components of the MPX signal with different gain factors applied to the audio, pilot and RDS streams. These gains are referred to as the frequency deviation of the respective component. This composite signal is then frequency modulated and broadcast by the transmitter. More details on the RDS transmission standard can be found in the document RDS standard EN :1998 [Reference 1]. What is RDS performance dependent on? As mentioned in the previous section, different gains are applied to the three components of the FM MPX signal. Demodulated RDS data quality is dependent on: the received signal quality (strength), the loss due to non-idealities in the receiver and the frequency deviation of the RDS signal at the transmitter. In addition, FM demodulation involves differentiating the phase of the input signal. This causes the noise level at the output of the FM demodulator to have a squared dependency as a function of frequency. RDS signal situated around 57 khz will suffer the largest degradation due to this relationship. The bandwidth occupied by the FM signal is controlled by the sum of the frequency deviations. To ensure adjacent channels do not cause interference to each other, the total frequency deviation is restricted by the regulatory authorities like the FCC. Increasing the RDS share of frequency deviation while improving RDS performance comes at the cost of lowered audio performance. RDS frequency deviation is typically restricted to 2 khz, compared to 65 khz for the audio signal. Choices, like the one illustrated above, lead to RDS sensitivity being poorer than the audio sensitivity. RDS receiver processing Complementary actions are performed by the receiver on the received signal to recover the transmitted signal. The receiver performs frequency demodulation to recover the MPX signal from which different components can be extracted. Specifically for the RDS signal,

5 which occupies a +/- 2.4kHz band around 57 khz, a conversion to baseband followed by pulse matched filtering produces the optimal signal for demodulation. Since the original data was differential BPSK modulated, a simple way to recover the data is to perform differential demodulation which does not require a phase recovery module. The demodulated bits are then processed by a frame-synchronization unit which is required to determine the block of 26 bits that need to be passed to the error control decoder along with removing the effect of offset word before the block is decoded. The decoder output consists of blocks of 16 bits along with status bits indicating the presence of errors, block ID and other relevant information, which are passed to higher layers. A simplified block diagram of RDS processing is shown in Figure 4. From FM demodulator output Demodulated bits KHz Conversion to Baseband Pulse matched filtering Demodulator Frame Sync Error Decoder Frame Buffer To Receiver RDS data processor Figure 4: Simplified RDS receiver block diagram Optimizing the RDS receiver performance The key modules that determine the performance of an RDS receiver are the RDS demodulator, frame synchronizer, and the error decoder. Typically, each of these modules is optimized independently to improve the RDS performance. However, additional performance gain can be obtained by jointly designing and optimizing these three modules. In a joint design, the nature of the signal processing performed in the preceding module is taken in to account in the design of subsequent modules. For instance, the noise statistics at the output of the demodulator can be exploited in the design of the error decoder. Additionally performance can be improved by feeding back useful information from one module to another. As an example, the frame synchronization module typically works with the raw (received) data block and provides the initial synchronization estimate in addition to monitoring the status of synchronization. However, exploiting information from the error control decoder can potentially improve the synchronization maintenance capability. Figure 5 shows a simplified block diagram of the optimized RDS receiver.

6 From FM demodulator output Conversion to Baseband Pulse matched filtering Demodulator Jointly optimized modules. Frame Sync Error Decoder Frame Buffer To Receiver RDS data processor Figure 5: RDS receiver block diagram showing the optimized modules The Block Error Rate (BLER) performance of the jointly optimized RDS demodulator/decoder is compared with that of an exemplary conventional design in Figure 6, illustrating a significant performance improvement over the conventional design. With a jointly optimized design, our receiver achieves an RDS Sensitivity of -99 dbm, for an RDS deviation of 2 khz. This is at least 4 db better than the nearest competitor part [Reference 3]. Assuming free space propagation, an improvement of 4 db in RDS sensitivity translates to approximately a 150% increase in the reception coverage area over which RDS data can be successfully decoded, thus enabling the enhanced use cases discussed previously. Additionally, the gap between the audio and RDS sensitivity performances is reduced by improving the performance of RDS sub-system. Figure 6: Performance of optimized RDS receiver Conclusion While audio performance is important in conventional FM broadcast receivers, user experiences can be clearly enhanced by supporting better RDS performance. Current use cases of FM RDS span a multitude of domains, from position location and routing to enabling a variety of features based on audio content like song tagging and more. While the allocation of more frequency deviation for RDS signal can improve the performance, it comes at the cost of loss in audio performance, so it is important to improve the receiver performance.

7 Related products from Texas Instruments Incorporated (TI) TI s current generation FM receivers support enhanced RDS receivers that achieve best in class performance and enhance user experiences. For more information, please visit References 1. RDS standard EN : RDS is now 25 the complete history 3. Silicon Labs, Si 4706: High performance FM RDS/RBDS data receiver, Datasheet About the authors Aravind Ganesan is a senior systems engineer for wireless connectivity solutions within the Texas Instruments Incorporated (TI) wireless business unit. In this role, he is responsible for developing algorithms for communication systems in the wireless connectivity segment for mobile devices at TI. Ganesan joined the TI family in 2004 as a Digital Signal Processing engineer at TI Bangalore. In this role, he worked on implementing PHY layer enhancements for the next generation DRP-based GSM-GPRS chipset. Ganesan received his Bachelor of Technology degree in Electrical Engineering from the Indian Institute of Technology in In addition, he received a Master of Science degree in Electrical and Communications Engineering from Texas A&M University in Jaiganesh Balakrishnan is a senior systems engineer for wireless connectivity solutions within the Texas Instruments Incorporated (TI) wireless business unit in Bangalore. In this role, he focuses on the design of highly integrated, power efficient and high performance connectivity transceiver cores for the mobile segment at TI. In recognition of great technical achievements and impact at TI, Balakrishnan was elected as Senior Member Technical Staff (SMTS), a group composed of TI's top six percent of technical achievers company wide. In 2002, Balakrishnan joined TI s Digital Signal Processing Solutions R&D Center in Dallas where he co-developed the multi-band orthogonal frequency-division multiplexing ultra wideband communication system and worked on the design of digital video broadcast receivers. In 2005, he moved to TI Bangalore where he was placed in his current role. Balakrishnan received a Bachelor of Technology degree from the Department of Electrical Engineering at the Indian Institute of Technology in In addition, he received a Master of Science degree and his Ph.D. from the School of Electrical and Computer Engineering at Cornell University in 1999 and 2002.