CN103200145A - Orthogonal frequency division multiplexing (OFDM) transmitter implementing method based on DDS principle - Google Patents

Abstract

The invention discloses an orthogonal frequency division multiplexing (OFDM) transmitter implementing method based on the DDS principle. The OFDM transmitter implementing method based on the DDS principle is characterized in that a scheme through utilization of the DDS principle is used for replacing an inverse fast Fourier transform (IFFT) and low-pass shaping filtering portion of an existing OFDM transmitter. According to the OFDM transmitter implementing method based on the DDS principle, a frequency-domain-sequence-to-time-domain-sequence conversion module based on the DDS principle can realize conversion from a frequency-domain sequence X (k) to a time-domain sequence s (n), the sequence obtained through the conversion is the same as a sequence which is obtained through an IFFT + low-pass filtering scheme, and the complexity of a system is lowered; and not only is the IFFT and low-pass shaping filtering module which has to be used in a traditional OFDM transmitter eliminated, but also a multiplier does not need to be used, so that occupied hardware resources are few.

Description

A kind of implementation method of the OFDM transmitter based on the DDS principle

Technical field

The present invention relates to wireless communication field, particularly a kind of new OFDM transmitter design scheme.

Background technology

As far back as the sixties in 20th century, the multicarrier parallel transmission thought that channeling, signal spectrum cover mutually just is suggested.In the system of this multicarrier parallel transmission, channel is divided into N subchannel, and each channel can transmit data separately and can not disturb, and the signal spectrum of adjacent channel appearance 50% is overlapping.Compare with FDM before, the signal band of this technology can be overlapping, availability of frequency spectrum height, but implement the comparison difficulty; Namely need N orthogonal modulator and demodulator respectively at transmitting terminal and receiving terminal, if adopt filter to realize, be difficult to the orthogonality between the assurance filter.

Up to the seventies in last century, Weinsetin and Ebert are applied to discrete Fourier transform in the parallel transmission system, OFDM (OFDM has been proposed, Orthogonal Frequency Division Multiplexing) technology, utilize discrete Fourier transform (IFFT/FFT) to be realized the function of a plurality of modulation and demodulation devices, and do not adopt the scheme of a plurality of filters, thereby reduced the realization difficulty of OFDM.And along with adopting FFT to realize the proposition of OFDM scheme, and the development of semiconductor technology, Digital Signal Processing etc. in recent years, OFDM is widely applied in digital audio broadcasting, digital video broadcasting, WLAN (wireless local area network) and the mobile communication system, OFDM has been set at the standard of WLAN (wireless local area network) such as the IEEE802.11 standard.

OFDM transmitter schemes based on IFFT is generally to adopt at present, and almost be the OFDM implementation of unique employing, as shown in Figure 1, this OFDM transmitter is to utilize the IFFT module sequence contravariant of frequency domain to be changed into the sequence of time domain earlier, pass through the step such as filtering interpolation, D/A conversion, carrier modulation, power amplifier of several times again, finally by by antenna signal being sent.Yet, the structure more complicated of IFFT module itself, consumption of natural resource is also more, especially works as counting more for a long time of conversion, and its complexity and consumption of natural resource amount will be doubled and redoubled, and real-time requires also to have proposed stern challenge.

Summary of the invention

The objective of the invention is to propose a kind of OFDM transmitter implementation based on the DDS principle.

The structure of OFDM transmitter provided by the invention as shown in Figure 2, the nucleus module among Fig. 2 is the time domain frequency domain sequence transformation module based on the DDS principle, the specific implementation of this module specifically comprises as shown in Figure 3:

1) generating k way carrier wave sequence is , n=0,1,2 ..., MN-1, ω _kRepresent the digital angular frequency of k way carrier wave;

2) each way word subcarrier sequence and frequency domain signal X (k) are multiplied each other respectively, obtain the time domain sequences of each branch road, , n=0,1,2 ..., MN-1, wherein X (k) represents the frequency-region signal sequence, and N represents the number of OFDM subcarrier, and M represents the interpolation multiple;

3) with the time domain sequences s of N branch road _k(n) about the k addition, by calculating , n=0,1,2 ..., MN-1 directly obtains time domain sequences s (n), the time-domain digital sequence that D/A is given in s (n) representative.

X of the present invention (k) represents the frequency-region signal sequence, the time-domain digital sequence that D/A is given in s (n) representative, and N represents the number of OFDM subcarrier, and M represents the interpolation multiple.Such as in OFDM transmitter in the past, M=8 just means sequence X (k) is carried out obtaining sequence x (n) after the IFFT conversion, n=0, and 1,2 ..., N-1 carries out x (n) 8 times filtering interpolation again, obtains s (n), n=0, and 1,2 ..., MN-1.

Each multiplication unit represents I among Fig. 2, the Q two-way is multiplied by cosine Serial No. and sinusoidal Serial No. respectively; Each X (k) is plural number, such as for 16QAM, X (k) equals ± 1 ± j, ± 1 ± 3j, ± 3 ± j or ± among 3 ± 3j one.s _k(n) be actually X (k) and

Multiply each other, get the result of real part again.If X (k)=I (k)+jQ (k) calculates s _k(n) expression formula is as follows:

$s_k\left(n\right)=\mathrm{Re}\left[X\left(k\right){\mathrm{Ae}}^{{\mathrm{j\ω}}_{k}n}\right]$

＝Re{[I(k)+jQ(k)][Acosω _kn+jAsinω _kn]}

＝I(k)·Acosω _kn-Q(k)·Asinω _kn （1）

N represents subcarrier number, ω among Fig. 2 ₀, ω ₁, ω ₂..., ω _k..., ω _N-1Be digital relative angle frequency, and ω _k=k ω ₁Obviously, ω _kf _sBe the corresponding simulation angular frequency of sine and cosine sequence, f _sSample frequency for D/A among Fig. 1.Each X (k) remained unchanged in the time period of an OFDM symbol; And the pace of change of n is very fast, and is equally high with the sample frequency of D/A.If the duration of an OFDM symbol is T ₁, T ₁Also be the cycle of number one subcarrier simultaneously, the subcarrier period T ₁With digital relative angle frequencies omega ₁Satisfy relational expression: $2\mathrm{\&pi;}{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="HDA00003007418300013.png,HDA00003007418300021.png"\; state="\{\{state\}\}">f</figure-callout>}}_1=\frac{2\mathrm{\&pi;}}{{\mathrm{<figure-callout\; id="1"\; label="T"\; filenames="HDA00003007418300013.png,HDA00003007418300021.png"\; state="\{\{state\}\}">T</figure-callout>}}_1}={\mathrm{\&omega;}}_1{f}_s.$

Prima facie from Fig. 2, aspect memory, need 2N based on the look-up table principle just, cosine register group, each register group need be deposited NM quantized data; Aspect computing, need the adder of 2N multiplier and 2 N inputs.These register groups, it is more to seem the stock number that takies, actually not so.Reason is, in fact this 2N register group only need use one or half just much of that; And if use several memory set more, 2N multiplier also can all omit.

Principle according to Direct Digital frequency synthesis (DDS) is just understood above-mentioned simplification process easily.Each register group need be deposited NM quantized data, and N subcarrier arranged, and the X of each subcarrier (k) has N time domain point x (n) afterwards through the IFFT conversion, and each time domain point will be through M interpolation doubly, and this MN point will be at T ₁Send into D/A in time.Use one or half register group just can realize that all digital subcarriers multiply each other, when realizing Q (1) Asin ω ₁During n, with the taking out to multiply each other with Q (1) and get final product one by one of the data in the register group; When realizing Q (2) Asin ω ₂N=Q (2) Asin (2 ω ₁N) time, the data in the register group are multiplied each other with Q (2) every a taking-up again to get final product; When realizing Q (k) Asin ω _kDuring n, only need be with the data in the register group every k-1 taking-up, multiplying each other with Q (k) gets final product again.

Because cosine is the phase shift of sine only Radian wants to finish I (k) Acos ω _kThe calculating of n only needs the data from sinusoidal register group to separate

Individual some value just.Quantized data in the register group can reduce to MN/2, this is because MN data have been deposited a complete cycle of sinusoidal signal, and the positive and negative half period of sinusoidal signal is antisymmetric, have only a negative sign every, so can only deposit the data of MN/2 positive half period, and just can obtain the data of negative half-cycle through simply getting " bearing ".

Because the value of Q (k) and I (k) is normally very limited, can be these numbers and just, the cosine factor multiplied result stores away in advance, thereby avoids using multiplier.Such as for 16QAM, the value of Q (k) and I (k) in an OFDM symbol be ± 1 or ± in 3 one, want to calculate the 3Acos ω in (1) formula _kN or-3Acos ω _kN can be earlier with 3 * Asin (ω ₁N) sequence stores away, again according to 1 * Asin (ω ₁N) method is the same, takes out addition every k-1 point.

Need N digital sinusoidal signal generator among Fig. 3 altogether, but because the data in their employed look-up tables can share, therefore can only use a common storage group to store the sinusoidal sequence data in the system, the system that makes so only need take less storage resources.When needs produce the sinusoidal subcarrier of different frequency, only need from memory, to get final product with different interval peeks.Since the value of the real part of frequency domain signal X (k) and imaginary part few (such as for 16QAM, real part and imaginary part all be ± 1 or ± some in 3), this has just determined when calculating X (k) and sinusoidal sequence

, n=0,1,2 ..., when MN-1 multiplies each other, do not need to carry out multiplication and calculate, and multiplication result is stored in the different memory set, and then from different memory set, choose good sinusoidal sequence.Such as when X (k)=1, directly from

, n=0,1,2 ..., peek in the memory set of MN-1, when X (k)=3, from another memory set 3

, n=0,1,2 ..., peek among the MN-1, so just removed multiplication calculating from, system configuration is simplified, and the resource occupation amount also can reduce.

According to the introduction of above principle, this patent is further simplified, as shown in Figure 4.The role of X among Fig. 4 (k) serves as switch, when n clk arrives (n=0,1,2 ..., MN-1), each branch road all can be selected data in the interval with k-1 according to the value of X (k) from the sinusoidal sequence memory of certain, and the data that choose directly are exactly s _k(n), n=0,1,2 ..., MN-1.Such n each branch road of the moment can be selected data, selects N data altogether, and this N data addition just obtains s (n) value of this moment, along with the arrival one by one of clk, and s (n), n=0,1,2 ..., each value of MN-1 can both be calculated.The number r of sinusoidal sequence memory determines by modulation system, such as for 16QAM since the value of real part and imaginary part have only ± 1 and ± 3, take absolute value and just have only 2 kinds of situations, so r=2; For 64QAM, the value of real part and imaginary part has ± 1, ± 3, ± 5 and ± 7, take absolute value and have only 4 kinds of situations, so r=4.

Core of the present invention is to adopt the scheme of DDS principle to come IFFT in the alternate figures 1 and the part of low pass molding filtration.The sequence that the sequence that this scheme conversion is come out obtains with IFFT+ low-pass filtering scheme is consistent, and system complexity significantly reduces, and shared hardware resource is also low.

Advantage of the present invention is as follows:

1. saved the IFFT part.The IFFT Module Design of especially counting a lot is difficult problems, such as 10000 subcarriers are arranged in the example, needs IFFT and the FFT of 10000 of designs, and this all is very difficult.Moreover count and be not the more bad design of FFT of 2 integral multiple.

2. saved the low pass molding filtration.The natural design bandwidth with OFDM of sequence s (n) that new departure generates does not need special digital filter to carry out low-pass filtering.

3. relatively save hardware resource.Even do not need multiplier in the new scheme, removed IFFT and molding filtration simultaneously from; Only need to use the adder of a N input, 1 output, add simple logic and judge and to realize that this just greatly reduces system complexity, saved ample resources simultaneously.

4. can realize the ofdm system of norator carrier wave number easily.For the OFDM transmitter system of norator carrier wave number, as long as design according to Fig. 2, not being subjected to counting of IFFT must be the restriction of 2 integral multiple.

5. can improve system frequency.Can remove multiplier from new departure, and multiplier reduces the bottleneck of system works frequency often, therefore can improve system frequency.

● description of drawings

Fig. 1 is for adopting the OFDM transmitter architecture schematic diagram of IFFT and molding filtration;

Fig. 2 is the OFDM transmitter architecture schematic diagram that the present invention is based on the DDS principle;

The time domain frequency domain sequence transformation module that Fig. 3 the present invention is based on the DDS principle is used for realizing from frequency domain sequence X (k) to the schematic diagram that transforms the time domain sequence s (n);

Fig. 4 the present invention is based on the structural representation behind the time domain frequency domain sequence transformation module reduction of DDS principle, this module is only used less memory cell, and do not need to calculate multiplication, only the adder by the single output of input more than just can realize from frequency domain sequence X (k) to the conversion the time domain sequence s (n).

● embodiment

Use Examples set the present invention below, but these examples should not be interpreted as limitation of the present invention.

The temporary transient problem of not considering to protect interval cp supposes that the sub-carrier frequencies of ofdm system of the present invention is spaced apart 1000KHz, and the subcarrier number is 10000, that is:

, N=10000 supposes difference multiple M=10 again.Suppose that the sequence after binary source is through the 16QAM modulation is X (k), k=0,1,2 ..., 9999, then system only needs 2 sines or cosine memory set, and one has sequence

, another has

N=0 wherein, 1,2, ..., 49999, need to store 100000 quantized datas so altogether, if each quantized data is 16bit, system needs the 200k byte altogether so, and saved multiplication computational process, this IFFT module than 10000 implements much simple, has also saved resource greatly.

Being easy to the shared bandwidth of calculation system is: W=Δ f * N=1000 * 10000=10 ⁷Hz=10MHz.Because D/A converter is at T ₁Need to change M * N=10 * 10000=100000=10 in time ⁵Individual point.Calculate thus the operating frequency of D/A converter:

$f_s=\frac{M\&times;\mathrm{<figure-callout\; id="1"\; label="N\; T"\; filenames="HDA00003007418300013.png,HDA00003007418300021.png"\; state="\{\{state\}\}">N</figure-callout>}}{T_{}}=M\&times;N\&times;\mathrm{\&Delta;f}=10\&times;10000\&times;1000={10}^8=100\mathrm{MHz}---\left(2\right)$

Analyze for the frequency spectrum to new departure, at first list the expression formula of this scheme output sequence s (n), and the sequence number of hypothesis OFDM symbol is p that p is integer, and change to+∞ from-∞.Because the duration T at each OFDM symbol ₁In, n increase M * N time describes its mathematical procedure and need use rectangular function R _MN(n).R _MNWhen (n) being illustrated in 0≤n≤MN-1, value be 1, n during for other values value equal 0; Obvious R _MN(n-pMN) expression R _MN(n) along the axial right translation pMN of a n point.Like this, the sequence s (n) that gives D/A can be write as following expression formula (3):

$s\left(n\right)=A\&CenterDot;\underset{p=-\&infin;}{\overset{+\&infin;}{\mathrm{\&Sigma;}}}\mathrm{Re}\left\{\right[\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{X}_p\left(k\right)\exp \left(j{\mathrm{\&omega;}}_{k}n\right)\left]R_{\mathrm{MN}}(n-\mathrm{pMN})\right\}---\left(3\right)$

Obviously the summation order about p and k can exchange in the following formula, and namely formula (3) is equivalent to:

$s\left(n\right)=A\&CenterDot;\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}\mathrm{Re}\left\{\right[\underset{p=-\&infin;}{\overset{+\&infin;}{\mathrm{\&Sigma;}}}{X}_p\left(k\right)R_{\mathrm{MN}}(n-\mathrm{pMN})\left]\exp \left(j{\mathrm{\&omega;}}_{k}n\right)\right\}---\left(4\right)$

X _pWhen (k) representative sends p OFDM symbol, k the frequency-region signal that subcarrier is entrained, X _p(k) equal ± 1 ± j, ± 1 ± 3j, ± 3 ± j or ± among 3 ± 3j one.Formula (3) is earlier to the k summation, and to the p summation, expression is earlier added together the output result of all multipliers in the single OFDM symbol, considers to constitute final sequence s (n) by many OFDM symbols again again; To OFDM numbering p summation, to branch number k summation, the subcarrier sequence s of each branch road is calculated in expression earlier to formula (4) more earlier _k(n), n ∈ (∞ ,+∞), again the sequence addition of this N branch road, obtain total sequence s (n), second summation symbol here

It is the adder in the representative graph (2-4).Following relationship is arranged between total sequence s (n) of the sequence of each branch road and last output:

s(n)＝s ₀(n)+s ₁(n)+s ₂(n)+...+s _k(n)+...+s _N-1(n),n＝0,1,2,...,MN-1 （5）

Obviously, the scheme of formula (3) and these two kinds of sequence of calculation s (n) of formula (4) all is feasible.

According to the understanding of formula (4), study the sequence s of k bar subcarrier separately _k(n), n ∈ (∞ ,+∞), list its expression formula, economize the symbol Re that takes by force real part for simplicity and obtain:

$s_k\left(n\right)=\underset{p=-\&infin;}{\overset{+\&infin;}{\mathrm{\&Sigma;}}}[X_p\left(k\right)R_{\mathrm{MN}}(n-\mathrm{pMN})\&CenterDot;\mathrm{Aexp}\left(j{\mathrm{\&omega;}}_{k}n\right)]$ （6）

$=\left[\underset{p=-\&infin;}{\overset{+\&infin;}{\mathrm{\&Sigma;}}}{X}_p\left(k\right)R_{\mathrm{MN}}(n-\mathrm{pMN})\right]\&CenterDot;\mathrm{Aexp}\left(j{\mathrm{\&omega;}}_{k}n\right)$

The following formula explanation, s _k(n) multiplied each other by two sequences and obtain, establish

, d _k(n) obviously represent the baseband signal of k branch road, this baseband signal is every just to be changed once through the MN point, and therefore, its shared bandwidth is narrow.

ω in the formula (6) _kThe digital relative angle frequency of representing the k road, and ω _k=k ω ₁According to the knowledge of Digital Signal Processing, d _k(n) Aexp (j ω _kN) sequence is f via frequency _s=10 ⁸The D/A converter of Hz and upper cut off frequency are between 10 ⁷Hz and 10 ⁸Low pass filter between the Hz, the simulation angular frequency of resulting sinusoidal signal (no matter being I road or Q road) is Ω _k=ω _kf _s=k ω ₁f _sAnd the frequency of these sinusoidal signals equals the frequency of the analog submodule carrier wave expected just, obtains the last composite signal desired analog signal of OFDM modulating system just according to formula (5) again.

Above-described embodiment is not for limiting the present invention, and any those skilled in the art without departing from the spirit and scope of the present invention, can do various changes and retouching, so protection scope of the present invention is looked the claim scope and defined.

Claims (3)

1. the implementation method of an OFDM transmitter is characterized in that, the time domain frequency domain sequence transformation of this OFDM transmitter is based on the DDS principle, and concrete steps are:

1) generating k way carrier wave sequence is

N=0,1,2 ..., MN-1, ω _kRepresent the digital angular frequency of k way carrier wave, N represents the number of OFDM subcarrier, and M represents the interpolation multiple;

2) each way word subcarrier sequence and frequency domain signal X (k) are multiplied each other respectively, obtain the time domain sequences of each branch road,

N=0,1,2 ..., MN-1, wherein X (k) represents the frequency-region signal sequence;

3) with the time domain sequences s of N branch road _k(n) about the k addition, by calculating

, n=0,1,2 ..., MN-1 directly obtains time domain sequences s (n), the time-domain digital sequence that D/A is given in s (n) representative.

2. the method for claim 1 is characterized in that, adopts the digital sine signal generator to generate each way carrier wave.

3. the method for claim 1 is characterized in that, uses several common storage storage sinusoidal sequence data, when needs produce the sinusoidal subcarrier of different frequency, peeks with different intervals from described common storage.

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