Introduction
At present works in the midwave and the skip band AM broadcast quality is lower than the people to listen to the request by far, in 2004 formulated the digital audio frequency broadcast DRM technical standard, was one kind in the original intermediate wave frequency band, still took 9kHz (or l0kHz) the band width, might provide the non-disturbance the close frequency modulation three-dimensional acoustic mass broadcast technique. The DRM broadcast is after the AM broadcast, the FM broadcast third generation broadcasts the way, its appearance symbolizes the broadcast system by the simulation to the digital system transition. At present, domestic and foreign uses the DRM receiver based on the PC DRM software receiver, already was quite mostly mature, but its application scope receives certain limit. Therefore to promote the DRM system’s promotion, needs one kind of cost to be low, the reliability to be high, the volume small and carries the convenience the hardware DRM receiver.
The DRM system uses the OFDM modulation system, has many kinds of modes of transmission, is suitable in many kinds of channels and the band width transmission mode, may transmit the audio frequency class and the data stream. Figure 1 and Figure 2 has given the DRM launching system structure separately and receives the terminal schematic diagram.
Figure 1 DRM launching system schematic diagram
Figure 2 receives the terminal structure schematic diagram
DRM system key technologies
DRM key technologies including OFDM modulation demodulation, channel estimate and synchronization and so on.
OFDM modulation and demodulation technology
The OFDM system realizes as shown in Figure 3. The transmission data carries on the code mapping in the frequency range, transforms the time domain after the IFFT operation:
Figure 3 the OFDM modulation/demodulation realizes the diagram
(1)
In the formula Xn, k expressed the nth mark, on the kth sub-channel modulates signal T is in the sub-carrier mark cycle, during the sub-carrier’s frequency interval for δf=1/T, the entire mark cycle is T Tg, g(t) to transmit the filter profile. After IFFT, the frequency range signal modulated in each orthogonal sub-carrier, has completed the Orthogonal Frequency Division Multiplexing. Before each OFDM element, adds on the protection to be separated Tg=LT/N. The protection gap is bigger than the biggest latency to expand, such all latencies are smaller than the protection gap’s multi-diameter signal will not extend to next element period, thus effectively eliminated the code the crosstalk. The OFDM signal after adds the window box number to reduce outside the belt the signal power, after low-pass filtering modulates launches the channel to the main carrier frequency.
Receiving end’s treating processes and the transmitting end are opposite, the channel comes out the signal undergoes the main carrier frequency demodulation first, after the low-pass filtering A/D transformation and the string and transform, then carries on FFT to obtain a mark data. Carries on balanced to the obtained data, the correction channel distorts. Then carries on the decoding decision and the string transformation, restores the primitive dual data series. Table 1 has given DRM the system OFDM parameter.
The channel estimate is carries on the correlation detection, the demodulation and the balanced foundation. The DRM system in the transmitting end appropriate position insertion pilot frequency unit, the receiving end restores the pilot frequency position using the pilot frequency the channel information, then processing methods and so on use like interpolation, filter, transformation, obtain all times the channel information. The OFDM channel estimated that the algorithm uses the least squares to miss the (LS) algorithm and the linearity is smallest divides equally misses the (LMMSE) algorithm. And the LS algorithm definition is:
(2)
And the k distribution obedience pilot frequency distribution rule, H*k, LS represents in sub-carrier k the channel information which obtains after the LS estimate, Xk is transmits the value, Yk is passes through channel’s receive value, Nk is a noise, the condition variance is E (│Vk│2│Xk│) =2δ2n/ │Xk│2. The LS algorithm computation load are few, but erroneous Vk is also big. In order to reduce the error to affect, may use the LMMSE algorithm to carry on smoothly, it is based on estimated that channel’s autocorrelation function Rh and the channel noise variance δ2n obtains, the pilot frequency place channel estimated value is:
(3)
And h*LS and h*LMMSE are the pilot frequency place channel values which LS and the LMMSE estimate obtains. The channel correlation matrix is Rh=E(hhH). Xth, a diagonal matrix, in its diagonal line’s value for the corresponding pilot frequency’s in transmission value, superscript H represents altogether clutches the transposition. LMMSE must be friends with about 4dB compared to the LS performance, but computation load LMMSE must be more complex than LS. In the DRM system, uses in the channel estimate the pilot frequency definition is Ps, k=as, k ej2πν [s, k], usually as, k=21/2, regarding some boundary sub-carrier as.k=2.
Synchronized technology
The DRM system transmits the pattern continuously, with is different based on the 802.11a standard’s wireless local area network’s burst transmission system. The burst transmission pattern’s prefix code is very usually short, this request can use the limited prefix code to realize the fast synchronization. But the continual transmission mode in the formula system message is passes on continuously. Therefore, the receiver has more time to carry on the reliable desired signal examination, then carries on each kind of error estimate compensation. Synchronization including frequency synchronism, time synchronism (mark synchronization, fixed time synchronization) and sampling frequency synchronization. Figure 4 has given the DRM synchronization plan diagram.
Figure 4 DRM synchronization algorithm diagram
Figure 5 has given when uses the channel 2 (typical midwave), robust pattern B, SNR=30dB, to data channel MSC, in the synchronization and the channel estimated around receives signal star map simulation result. May see from the chart, after passing through the synchronization, channel estimate processing, star map distinct improvement.
(before a) synchronization and channel estimate,
(after b) synchronization and channel estimate,
Figure 5 around synchronization and channel estimate MSC (64QAM) star map
Receiver terminal plan
Based on software radio DRM receiver
1) based on TMS320DM6446 receiver hardware platform
Based on software radio technology DRM receiver’s hardware platform as shown in Figure 6, considered has the volume to be small, the reliability is high, cost low and good timely request, uses TI Corporation in view of multimedia, low power loss hand-hold equipment application development dual processor nuclear chip TMS320DM6446 is the core hardware platform. Using the DSP chip formidable signal processing ability, completes the OFDM demodulation, the channel decoding and the demultiplexing duty, completes the audio frequency, the data decoding and system’s functional control and the management using ARM926.
Figure 6 based on software radio structure DRM receiver
This terminal hardware platform including the tuner, the control and DSP processes three modules. The tuner module mainly by the front end tuner, the mixer and the filter is composed, the function is provides the appropriate intermediate frequency signal for the system. The front-end of receiver with the sensitivity high, the dynamirange is elected big, the integration rate high component, here chooses the ST Corporation’s high performance vehicle to carry front end the radio tuner TDA7511, it including the mixing, the supersonic amplification, the automatic gain control, the AM/FM demodulation, the PLL phase-locked loop and the quality monitor and so on, has the integration rate high, to need the periphery component few and takes the circuit wafer area small merit, is front end the radio frequency core part. The control module is mainly composed of in DM6446 ARM9, matches realizes system’s control and the management by the peripheral circuit. The DSP processing module is this terminal nucleus module, is mainly composed of in DM6446 TMS320 C64x , is sent in after the modulus transformation’s data stream DSP, completes the DRM signal the demodulation, the channel decoding and the demultiplexing duty, finally gives the result output ARM9, obtains the tonic train signaling.
The tuner filter group is the signal which airborne receives divides for each wave band carries on the receive, with the aim of filtering the clutter, enhances the entire receiver’s signal-to-noise ratio, the enhancement sensitivity target. The filter choice elliptic function filter, it makes a sound in the pass band and the stop-band frequency presents and so on ripple characteristics, its main parameter for the filter exponent number is in 3, pass bands the fluctuation is in 0.25dB, the stop-band the weaken is 50dB, the impedance for 50Ω and the insertion loss is 6dB.
The TDA7511 interior has two mixers, it the signal mixing which receives the antenna arrives first 10.7MHz, then arrives at the signal two mixings 455kHz, then simulated signal after filter, enlargement sends in the exterior mixer. The exterior mixer selects PHILIPS Corporation’s SA612 chip, 455kHz simulated signal mixing to the 12kHz intermediate frequency. SA612 is the internal belt oscillator and the voltage reference double balanced mixer, has the low power loss, the integration rate high merit, its oscillator may dispose for the crystal or the harmonious operator schema, the operation is nimble. A/D sampling uses in the 12kHz simulated signal digitization, a/D chip sampling rate being supposed to be bigger than at least 24kHz (12kHz 2 times), here selects the AD9225 a/d conversion chip. AD9225 is the single power source power supply, 12 precisions, the 25MSPS high speed modulus switch, the signal-to-noise ratio is 71dB, the stray dynamirange is 85dB.
TMS320DM6446 has the binuclear Da Vinci construction product, has the high performance, the low power loss, the big storage capacity and the peripheral device connection nimble and so on merits, may increase the receiver hardware platform performance, the flexibility and the reliability. The external connection memory connects its EMIF connection, is composed of FLASH and SDRAM. FLASH selects a 512K×8b AM29LV040B chip, uses in depositing the application procedure, SDRAM selects 16MB the HY57V281620 chip. When system work, in FLASH procedure when work is duplicated DM6446 the internal storage space, and starts in the internal memory to move, but after exterior 16MB SDRAM mainly uses in saves processing the data. To with the computer exchange data, have established RS232 conveniently the connection /USB connection.
Figure 6 tuner part may also use Mirics Corporation MS1001 multi-frequency band migration broadcast tuner, realizes DRM, to simulate AM, FM and the DAB multi-service pattern migration broadcast function.
2) DRM receiver software overall flow
The receiver procedure is divided two parts, C64x DSP mainly completes the OFDM demodulation, the channel decoding and functions and so on Xie Fu, and gives the result output ARM926, then completes the audio frequency, the data decoding and system’s control and the management function by ARM926. Its procedure flow as shown in Figure 7.
Figure 7 DRM receiver software overall flow chart
Based on specific IC’s DRM receiver
Figure 8 has given based on the DRM specific IC TMS320 DRM350 multi-service pattern receiver plan.
Figure 8 based on TMS320 DRM350 DRM receiver
TMS320 DRM300/DRM350 is world first which TI Corporation promotes supports the DRM standard the special-purpose single chip. RS500 is RadioScape promotes based on TI DRM300 and the DRM350 chip module, supported DRM, to simulate AM, FM and the DAB standard, the RS500 module has realized Figure 6 the complete DRM receiver function. Take RS500 as the foundation, may fast, the low cost develop the reliable high multi-service pattern portable DRM receiver. Figure 8 is one kind of cost is low, the reliability to be high, the volume small and carries conveniently, may receive DRM, simulate AM and so on multi-service pattern broadcast signal the receiver plan, the exterior MCU monolithic integrated circuit, controls RS500 through I2C the disposition and the work.
Concluding remark
This article has mainly studied the DRM system design technology, the multianalysis DRM key technologies has realized the method. The digital broadcast substitutes for the simulation broadcast is the inevitable trend of development gradually, but in transition period the two is coexistent. In view of the present DRM receiver mostly was based on the PC DRM software receiver, the application scope receives certain limit, the article proposed two kind of costs were low, the reliability to be high, the volume small and carried the convenience the multi-service pattern hardware DRM receiver plan. Uses this kind of digit, to simulate the AM mix receiver, may promote the DRM system’s promotion, realizes broadcasts from the current simulation to the digital broadcast smooth transition.
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