Introduction
Along with society’s progress and the technical development, the multimedia services grow unceasingly, people to network band width request also along with it growth. The communications network is turning toward IP, the wide band direction to develop. The communications network by the transmission network, the switching network and the access network three parts is composed. At present, our country transmission network already basically realized the digitization and the using optical fiber; The switching network has also realized the program control and the digitization; But the access network was still looks carefully the company through the twisted pair line and the bureau, can only achieve 56 kb/s the transmission speeds, cannot satisfy the people to the multimedia information urgent demands. Docking enters the net to make the large-scale transformation, promotes to FTTC (optical fiber to roadside) even FTTH (optical fiber to household), needs the soaring cost, in the short-term realizes with difficulty. The XDSL technology has realized in the telephone line the data high speed transmission, but the majority household telephone line are not many, limited has been possible to connect the surfer the computer number, moreover laid down in various rooms transmits the electric cable to be extremely inconvenient. Most economical effective moreover facilitates the foundation equipment is a power line, takes the transmission medium the power line, does not need to carry on the new line construction in the family interior, the cost is low. The power line as the communications channel, does not need to maintain or the maintenance quantity nearly is minimum, moreover may realize nimbly namely inserts namely uses. In addition, because does not need to pay the telephone bill, the month rent is cheap.
The power line high speed data transmission causes the power line to do has possibly become possibly for the correspondence medium. Lays down has power line’s place, transmits each kind of Internet’s data through the power circuit, may realize the data communication, including becomes the local area network or turns on the Internet. Transmits each kind of Internet data through the power source line, may advance Internet’s popularization greatly. This technology may also cause the home-use computer and the electric appliance union for the network which may communicate mutually, forms the new intellectualization electrical network, the user realizes domestic electric appliances’ monitoring and the management in any place through Internet; May realize the electric power to copy in directly the table and the electrical network automation the remote letter, the telemetering, the remote control, remote regulating each function, but does not need seperately to lay down the communications channel. Therefore, research electric power
The line correspondence is very essential.
1 OFDM basic principle
Orthogonal Frequency Division Multiplexing OFDM (Orthogonal Frequency Division Multiplexing) is one kind of orthogonal multi-carrier modulates the MCM way. In the traditional digital communication system, the symbol sequence modulation carries on the serial transmission in a carrier, each mark’s frequency may hold the channel completely the available bandwidth. OFDM is one kind of parallel data transmission system, uses in the frequency the equal-space N sub-carrier constitution. They modulate a group independent data message separately, after the modulation, N sub-carrier’s signal adding together also transmits. Therefore, each mark’s frequency spectrum only takes the channel complete band width a part. In the OFDM system, through the choice intercarrier distance, causes these sub-carrier maintains in the entire mark cycle the frequency spectrum the orthogonal characteristic, in each sub-carrier’s signal overlaps mutually in the frequency spectrum, but between the receiving end use carrier’s orthogonal characteristic, may not have the distortion to restore the transmitter data, thus raises system’s frequency spectrum use factor. Figure 1 has given the Orthogonal Frequency Division Multiplexing OFDM basic principle. Considered in one cycle transmits after symbol sequence (do, d1,…, dn-1) each mark di is undergoes the baseband modulation, letter in reply number di=ai jbi, serial symbol sequence gap for △t=l/fs, fs is system’s mark transmission speed. After the string and transforms, they modulate N sub-carrier separately (fo, f1,…, fn-1), these N sub-carrier frequency division multiplying entire channel bandwidth, between the neighboring child carrier’s frequency interval is 1/T, mark cycle T from △t increases N△t. Synthesis transmission signal D(t) may use its low pass duplicate envelope D(t) to indicate.
And ωi=-2π · △f · i,△f=1/T=1/N△t. In mark cycle [O, T], the transmission signal is D(t) =Re {D(t) exp (j2πfot)}, 0≤t≤T.
If as the sampling speed carries on the sampling take mark transmission speed fs to D(t), within one cycle, altogether has N sampling value. Makes t=m△t, sampling sequence D(m) may use the symbol sequence (do, d1,…, dn-1) separate to pay the inverse transformation expression. Namely
Therefore, the OFDM system’s modulation and the demodulation process equivalent pay the inverse transformation and the separate in the separate pay to transform processing. Its core technologies is the separate pays to transform, if uses the digital signal processing (DSP) technology and the FFT fast algorithm, does not need to tie the shape filter group, realizes is quite simple.
2 electric power line numbers pass on the equipment hardware constitution
Power line data transmitting equipment’s hardware diagram as shown in Figure 2.
2. 1 digital signal processing unit TMS320VC5402
Realizes MODEM with digital signal processing’s method to need the extremely high operational capability and the extremely high operating speed, before high speed DSP appears, the digital signal processing can only use the ordinary microprocessor. As a result of the speed limit, the MODEM maximum speed which realizes generally in 2400b/s. Since from the late-1970s, Intel Corporation has promoted first generation of DSP chip Intel 2920, in the recent 20 years emerge a large quantities of high speed DSP chip, thus cause words belt high speed DSP MCODEM possibly to realize into.
The TMS320 series performance-to-price ratio is high, the domestic existing development method is complete, since TI Corporation first generation of product TMS32010 has been published in the early-1980s, by new generation’s speed, is promoting TMS32020, TMS320C25, TMS320C30, TMS320C40 as well as fifth generation of product TMS320C54X one after another every two years.
Realizes the signal processing ability which according to the OFDM modem needs, this article chooses by TMS320VC5402 as the data pump completes FFT and so on each algorithm, uses its software, the hardware source fully, realizes has the high performance-to-price ratio OFDM high speed electric power line number to pass on the equipment.
TMS320C54X is TI Corporation the upscale 16 fixed point DSP series component which promotes in view of the communication application. This series component function is formidable, is flexible, compared with first several generation of DSP, has the following prominent merit:
◇ the speed is quicker (40~100 MIPS);
◇ the set of instructions is richer;
◇ more addressing system choice;
◇2 40 accumulator;
◇ hardware storehouse indicator;
◇ support block redundant and annular buffer management.
2. 2 high frequency signal processing unit
Mainly realizes to high frequency functions and so on signal enlargement, high frequency switch and line filter, and passes through the small processing union equipment to escort to the distribution line finally. Signal enlargement including sending terminal controllable gain amplification (forward power control), receive direction AGC low noise amplification part. And the high frequency switch completes the receiving and dispatching high frequency signal the transformation, realizes the diplex operation. Simultaneously causes receiving and dispatching to use in common line filter, like this may save the system cost.
2.3 RS 1232 connection units
The user data connection uses the RS 1232 standard serial ports. The serial port data interrupt uses the border triggering interrupt, the serial port interrupt routine completes the user data the transmission and the receive. Will receive the user data temporarily stores in the CPU transmission buffer, when full arises suddenly the package transmits to DSP carries on processing.
3 parameter designs
3.1 guard time choice
According to the OFDM signal design criterion, first chooses the suitable guard time, △=20μs, this can satisfy fully under the electrical power system environment, the OFDM information erasing multi-diameter latency expands goal.
3.2 mark cycle choice
T>200 μs, the corresponding child channel spacing, f<5kHz, must divide 5 sub-channels at least like this in the 25kHz band width. Another husband channel number cannot be too many, increases the child channel number, although may raise the frequency spectrum transmission efficiency, but the DSP component’s order of complexity will also increase, the cost rise, will meanwhile receive the channel time selective fading the serious influence. Therefore, considered that uses 7 sub-channels in the 25kHz band width.
3. 3 sub-channel number computation
Sub-channel spacing:
Each sub-channel’s mark cycle: T=250μs
Consideration guard time: △=20μs, then has Ts=T △=270μs
Each sub-channel actual mark rate:
Total bit rate: 3.71kbps×25 sub-channel ×2b/symbol=185.5kb/s
System’s frequency spectrum efficiency: β=185.5kbps/100kHz=1.855bps/Hz<2bps/Hz
May see, by now the system already had the high frequency spectrum efficiency. 25 group voice signal total speed with and transforms and on 4PSK mapping each sub-channel after the string the useful information mark rate compares, each sub-channel may also insert the redundant information to use in the synchronization, the carrier parameter, the frame protection and the user information and so on. What needs to point out:
①As a result of OFDM signal time-frequency orthogonal limiting condition, in this design, although used 25 sub-carrier parallel transmissions also only to be able to pass on 25 groups pronunciations. If must pass on 8 groups pronunciations, and transforms and the 16QAM mapping after the string, on each sub-channel the useful information’s mark rate is 1.855bps/Hz, most may also insert the redundant information is O.145bps/Hz, in the actual transmission this is the transmission quality which very difficult to guarantee, therefore this design is opposite in M-16QAM uses 4 sub-carrier transmission 6 groups voices not to be contradictory.
②In this design, has reserved many positions for the redundant information, its redundant information and useful information’s ratio is 0.59, is bigger than the iDEN system 0.44. This is considered the OFDM signal and the deviation is fixed time more sensitive regarding the carrier phase deviation, like this may insert the many reference signals by to realize carrier phase locking, the track and the position synchronization fast; On the other hand to guides the mark space the choice to be also more nimble, chooses guidance mark space L=10 in the design.
③The OFDM signal modulation demodulation’s core is the DFT/IDFT algorithm. At present, uses the DSP chip to complete DFT/IDFT generally, therefore has the necessity to the DSP performance which designs needs to carry on the estimate. According to the design requirements, must be able at least in 250μs to complete 32 plural number FFT operation. We knew that N plural number FFT altogether needs the 2Nlog2 N real number multiplication and the 3Nl0g2 N real number addition. The supposition real number multiplication and the real number addition are the monocycle instructions, take 32 plural number spots as the example, like this altogether needs 800 instruction cycles, namely 20μs, therefore uses TMS320VC5402 to be able to satisfy the design requirements (the TMS320VC5402 single instruction cycle is 10ns).
In summary, the OFDM number passes on plant parameter like table l to arrange in order.
4 software constitutions
Above had determined the OFDM number passes on equipment’s main parameter and the algorithm, below showed realizes the software design and the flow with TMS320VC5402, as shown in Figure 3.
4. 1 modulation part software design
This procedure before the subroutine transfers, must transmit the data already by the data input memory, and transmitted the data area’s first address and the length as the intake conditions for the subroutine. When program execution, first transmits the memory clear, then disposes AD9708 the sampling speed, afterward permits the serial port transmission interrupt production, causes the interrupt service automatic to read in the transmission memory’s content in turn, sends in AD9708 to transform the simulated signal. Afterward the procedure reads a data from the data-carrier storage, after the code, parallel will put in the IFFT work area the relevant position, the insertion pilot frequency mark and will not need to snack to allay hunger zero. Afterward carries on the IFFT, IFFT algorithm to use the commonly used time domain to pull out algorithm DIT, the butterfly-shaped operation needs WN may look up the N=512 character the fixed point trigonometry tables to obtain. Because the TMS320VC5402 numerical calculus is 16 word length fixed-point arithmetic way, therefore IFFT uses the group method of fixed points, both increased the operation precision and to guarantee the operating speed. Then adds the window after the IFFT transformation’s result expansion, before and expands this signal’s, the part after on frame signal expands part adding together, adds the window to need the window box number to be possible the table look-up to obtain. The window box number deposits in the window box mathematical table, will be beforehand and will transform finally using the C language floating point calculation for the fixed-point number deposits in the table.
After actual, to adds window completion of work most takes 75 sampling periods (75×125 μs) the time from the read serial data, but transmits a signal to need the 512 32=544 sampling period (544×125 μs). This indicated that C5402 the operating speed enough meets the needs.
Worked as a signal end of transmission, this signal which the procedure will process immediately sends in transmits the memory to continue to transmit, and through intake conditions judgment data whether end of transmission.
4. 2 demodulation part software design
The flow which realizes with TMS320VC5402 is divided the synchronization to catch and to demodulate two stages. When synchronized capture stage execution, first receives the memory clear, disposes AD9057 the sampling speed, then opens the serial port receive interrupt, causes the receive interrupt service receive and stores automatically in turn from the AD9057 sampled data receives the memory.
Obtains a new sampling point every time, the procedure uses DFT the recursion algorithm to demodulate 25 group pilot frequency marks first, and is balanced to the pilot frequency. Afterward carries on the dot product separately with reference pilot symbolic vector 600h j600h, here and replaces the dot product with the pilot frequency symbolic vector’s real part and the imaginary component, then reflected that the correlation function the rule, simplifies the operation. After obtaining 25 groups pilot frequencies and the reference pilot correlative values, preserves temporarily, and separately various pilot frequencies correlative value comparison which preserves with the preceding sampling point (cancellation), (each group pilot frequency occupies 1bit) with a byte preservation comparison result’s plus sign. In processes the preceding sampling point in the process, also uses a byte to preserve it before its same spot pilot frequency correlative value comparison plus sign. Carries on the simple logic operation to these two bytes, then judges various pilot frequencies whether to present the peak value in the preceding sampling point place. If in 25 groups pilot frequencies has 20 above pilot frequencies simultaneously to present the peak value, then thought that this sampling point beforehand N=512 sampling point namely for a signal which catches, the procedure enters the demodulation stage; Otherwise waited for that receives the new sampling point to continue to carry on the synchronized capture.
The demodulation stage first to the frame signal which catches carries on the solid signal the FFT transformation, still used the group method of fixed points, afterward carries on balanced. Then figures out using the pilot frequency local samples clock’s detention Tau, should avoid in the computation presenting the division as far as possible, after may takes the constant denominator from the bottom, figure out ahead of time, takes the multiplication the coefficient. In order to guarantee after that the two-dimensional AGC precision, in computation Tau precisely to O.1μs. Then the basis Tau adjustment sampling clock, after the procedure will adjust the quantity notice serial port transmission interrupt service, continues to carry out two-dimensional AGC, but by the interrupt service in each time the interrupt response time issue order, each time may the adjustment next sampling time ahead of time (or backwardness) 1μs.
Two-dimensional AGC divides two steps to carry on. First acts according to Tau the modulation vector carries on the phase correction after balanced, here needs to use 512 character trigonometry tables which the FFT transformation uses, with an indicator direction trigonometry tables’ meter, figures out how many pathway channel according to Tau and the trigonometry tables angle gap only then to need to move the indicator under a standard, may determine each sub-channel’s correction succinctly through this table look-up’s method. After phase correction, then carries on the scope adjustment using the pilot frequency.
Then passes through the decision, and/the string transformation and the decoding then demodulate this data. Then uses the LMS algorithm to the balancer weight to carry on the adjustment. The procedure through carries on the simple peak-to-peak value threshold analysis to this part of signals, very easy to judge whether to have received the signal. If has, then continues to receive; Otherwise conclusion returns.
Conclusion
This article introduced the OFDM technology’s basic principle, narrated passed on communication facility’s software and hardware design based on the OFDM technology electric power line number, has given this design concrete parameter.
