Signal generator and signal generation method with cyclic prefix generation

A signal generator for creating a measuring signal comprising a cyclic prefix comprises a baseband signal generator for generating a baseband measuring signal and a channel emulator for emulating a channel in the baseband measuring signal resulting in a channel modified signal. Moreover, it comprises a modulator for modulating the channel modified signal and a cyclic prefix generating unit for generating and inserting a cyclic prefix into the modulated signal. The cyclic prefix generating unit is set up for generating the cyclic prefix in such a manner that a channel modified cyclic prefix is emulated.

TECHNICAL FIELD

The invention relates to creating a measuring signal including a cyclic prefix, especially creating an OFDM signal.

BACKGROUND ART

In a classical signal generator, a baseband signal is generated and modulated. A cyclic prefix is then generated from a last part of the signal and added to the beginning of the signal. The resulting signal is passed through channel emulation means, which for example fade the signal and add noise. The resulting signal is the measuring signal, which is supplied to a device under test. Performing the signal generation by the above-described method though requires a great deal of processing power.

Recently, signal generators for multi-carrier signals have been proposed that include the application of static or fading channel models to the signal prior to signal modulation. Since the creation of a cyclic prefix though is a nonlinear operation, the resulting signal is different from a signal generated by the above-described classical signal generator. Since the channel affects the signal only before cyclic prefix insertion, the cyclic prefix, which is added to the beginning of each symbol, exactly corresponds to the end of the symbol. From a receiver perspective, this difference manifests in the autocorrelation properties of the received signal. The signal received from the signal generator will have ideal autocorrelation properties with distinct correlation peaks separated by symbol length plus cyclic prefix length. In comparison, a signal received from a classical signal generator will have degraded autocorrelation peaks that are shifted in time.

For example the US patent application US 2010/0118818 A1 shows a communication system using cyclic prefixes.

Accordingly, an object of the invention is to create a signal generator and a signal generation method capable of generating a measuring signal including a cyclic prefix which require a low computational power and at the same time achieve a measuring signal with auto correlation properties which are comparable to those of a measuring signal generated by a classical signal generator.

SUMMARY OF THE INVENTION

An inventive signal generator for creating a measuring signal comprising a cyclic prefix comprises a baseband signal generator for generating a baseband measuring signal and a channel emulator for emulating a channel in the baseband measuring signal resulting in a channel modified signal. Moreover, it comprises a modulator (e.g. in case of OFDM the modulator would usually be implemented as IFFT, Inverse Fast Fourier Transform) for modulating the channel modified signal and a cyclic prefix generating unit for generating and inserting a cyclic prefix into the modulated signal. The cyclic prefix generating unit is set up for generating the cyclic prefix emulating a channel modified cyclic prefix. It is therefore possible to retain the advantageous low-computational complexity of a signal generator employing the channel emulator before adding the cyclic prefix while, at the same time, achieving a measuring signal, which is comparable to the measuring signal of a classical signal generator, which employs the channel emulator as the last step.

Advantageously, the cyclic prefix generating unit is set up for generating the cyclic prefix emulating a signal-to-interference-ratio of a channel modified cyclic prefix. It is therefore possible to achieve auto correlation results, which are comparable to the results of a measuring signal generated by a classical signal generator.

Furthermore, it is advantageous that the channel used for emulating the signal to interference ratio is identical with the channel used for creating the channel modified baseband measuring signal. Especially ideal autocorrelation properties of the resulting measuring signal can thereby be achieved.

Advantageously, the cyclic prefix generating unit is set up for generating the cyclic prefix by generating a first signal corresponding to an end of a current symbol, generating a second signal corresponding to a beginning of a directly prior symbol, performing a windowing of the first signal and the second signal resulting in a windowed first signal and a windowed second signal, and adding the windowed first signal and the windowed second signal resulting in the cyclic prefix. The sequence of steps results in a very low computational complexity and, at the same time, in a measuring signal with the desired autocorrelation properties. By advantageously setting the window positions and width, it is possible to set the autocorrelation properties of the resulting measuring signal.

Even more advantageously, a third signal is generated corresponding to a part of a preceding symbol. Alternatively the third signal is generated by an independent signal source. For example, white Gaussian noise can be used. Also here, a windowing of the third signal is performed. This third signal is added to the first and second windowed signals. An even more accurate resemblance of a measuring signal generated by a classical signal generator can thereby be achieved. Therefore, the autocorrelation properties are even better matched to the autocorrelation properties of a measuring signal generated by a classical signal generator than using only a first and second signal.

Alternatively, in addition to first and second signals, further signals corresponding to parts of at least one preceeding symbol or from other signal sources can be used. For each of these signals, a windowing is performed resulting in windowed further signals. All of the resulting window signals are then added up resulting in the cyclic prefix. A very accurate resemblance of the autocorrelation properties of a measuring signal generated by a classical signal generator can thereby be achieved.

An inventive signal generation method serves the purpose of creating a measuring signal comprising a cyclic prefix. In a first step a baseband signal is generated. In a second step, a channel emulation is performed on the baseband measuring signal resulting in a channel modified signal. A modulation is performed on the signal resulting in a modulated signal. In a fourth step a cyclic prefix is generated and inserted into the modulated signal. The cyclic prefix is generated so that a channel modified cyclic prefix is emulated. While retaining a low computational complexity, advantageous autocorrelation properties of the resulting measuring signal can thereby be achieved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. However, the following embodiments of the present invention may be variously modified and the range of the present invention is not limited to the following embodiments.

First we demonstrate the setup and function of a cyclic prefix and of an exemplary signal generator alongFIGS. 1-3. With respect toFIGS. 4-7the function of an embodiment of the inventive signal generator is described in detail. Finally alongFIG. 8andFIG. 9, the function of an embodiment of the inventive signal generation method is described. Similar entities and reference numbers in different figures have been partially omitted.

InFIG. 1, a typical OFDM symbol9is shown. A cyclic prefix8is added at the beginning of the symbol9and is identical to a last part8aof the symbol9. The symbol9and the cyclic prefix8are referred to together as reference number10.

InFIG. 2, an exemplary signal generator1is shown. The signal generator1comprises a baseband signal generator2which is connected to a modulator3, which again is connected to cyclic prefix generation unit4, which again is connected to a channel emulator5. All of the units2-5are connected to control means6, which control the units2-5.

The control means6control the baseband signal generator2so that it generates a baseband signal. The baseband signal is passed on to the modulator3, which modulates the baseband signal. The resulting modulated signal is handed on to the cyclic prefix generating unit4, which generates a cyclic prefix from the last part8aof the symbol9and adds it to the beginning of the symbol9as cyclic prefix8. The resulting signal is then passed on to the channel emulator5, which emulates a transmission of the signal through a transmission channel. The resulting measuring signal is then handed on to a device under test7, which is not part of the signal generator1. A measurement is performed using the generated measuring signal.

The signal generator1is connected to the device under test7in a digital manner here. Alternatively, the signal generator can additionally comprise a digital-analog-converter connected to the channel emulator5and a radio frequency mixer connected to the digital-analog-converter. In this case, the device under test7is connected to the radio frequency mixer. The analog-digital-converter is then setup for converting the digital output signal of the channel emulator5into an analog signal. The radio frequency mixer is setup for mixing the analog signal to a radio frequency. The device under test then sees a radio frequency measuring signal.

InFIG. 3, a typical channel emulator5ofFIG. 2is shown in detail. The channel emulator5comprises a number of delay elements30,31,32, which are each connected in parallel to the cyclic prefix generating unit4ofFIG. 2. Therefore, each of the delay units30-32is provided with the not yet channel modified measuring signal. The delay units30-32each create a different delay time within the supplied signal. After passing the delay units30-32, the individually delayed signals are handed to multipliers33,34and35, which multiply the respective signal with additional signals h1(n′), h2(n′), hN(n′). The resulting signals are then handed over to an adder36, which adds up the individual multiplied signals and outputs a final sum signal as measuring signal y (n′). By the use of the delay units30-32, especially reflections in a real world-environment, can be emulated.

First Embodiment

InFIG. 4, an exemplary embodiment of the inventive signal generator11is shown. The signal generator11comprises a baseband signal generator12, which is connected to a channel emulator13, which again is connected to a modulator14, which is furthermore connected to a cyclic prefix generating unit15. All of these units are connected to control means16, which control them. A device under test17is connected to the cyclic prefix generating unit15.

The signal generator11is connected to the device under test17in a digital manner here. Alternatively, the signal generator can additionally comprise a digital-analog-converter connected to the cyclic prefix generating unit15and a radio frequency mixer connected to the digital-analog-converter. In this case, the device under test17is connected to the radio frequency mixer. The analog-digital-converter is then setup for converting the digital output signal of the cyclic prefix generating unit15into an analog signal. The radio frequency mixer is setup for mixing the analog signal to a radio frequency. The device under test then sees a radio frequency measuring signal.

The baseband signal generator12is configured for generating a baseband measuring signal18, which is handed on to the channel emulator13. The channel emulator13emulates a transmission channel over which the baseband measuring signal18is transmitted. A resulting channel modified signal19is handed on to the modulator, which modulates it resulting in a modulated signal20. The modulated signal20is handed on to the cyclic prefix generating unit15, which generates a cyclic prefix for each symbol of the modulated signal20and adds it to the beginning of the respective symbol. A resulting measuring signal21is supplied to a device under test17. A measurement may be performed using this measuring signal.

The ascending and descending parts of autocorrelation functions of a multipath signal are depicted inFIG. 5andFIG. 6. InFIG. 5the ascending part is depicted, while inFIG. 6the descending part is depicted. In these figures it is assumed that we are dealing with a multipath fading channel with 3 paths or equivalently NTap=3 and τ1=0. The figures show the signal and its delayed versions that would be received from a device under test if the signal is generated according toFIG. 2. Actually, the device under test would see the sum of the three signals depicted per figure. The once hatched signal parts40,42,51,54,61,64,80,83,90,93contribute positively to the autocorrelation. The double hatched parts50,53,60,63,70,72,81,84,91,94give no positive contribution to the autocorrelation. According to this interpretation, two (in general NTap−1, wherein NTapcorresponds to a number of taps in the used channel model) signal-to-interference-ratio regions (where the term interference denotes the signal parts that give no positive contribution to the autocorrelation, i.e.50,53,60,63) with finite signal-to-interference-ratio SIR(2)(1), SIR(2)(2) can be defined with respect to the ascending part of the autocorrelation function, compareFIG. 5:

Also two signal-to-interference-ratio regions (again the term interference denotes the signal parts that give no positive contribution to the autocorrelation, i.e.70,72,81,84,91,94) with finite signal-to-interference-ratio SIR(1)(1), SIR(1)(2) can be defined with respect to the descending part of the autocorrelation function, compareFIG. 6:

Here σtap2denotes the variance of htap(n′), where htap(n′) is defined through the tapped delay line channel model depicted inFIG. 3, tap is a index with 1<=tap<=NTapand n′ is a sample index with reference to Ts. (Ts=1/fsand fsis the sample rate of the signal x(n′)). Note that the channel is in general but not necessarily time variant.

Generally for NTaptaps we have

InFIG. 7, a detail of the cyclic prefix generation unit15ofFIG. 4is shown in a block diagram. In this figure and in the following text n is a sample index with reference to Ts(Ts=1/fsand fsis the sample rate of the signal20at the output of the modulator) and 0<=n<NCP+NDelta, NCPdenotes the number of CP samples and NDeltais defined as:

NDelta=max⁢{0,⌈max1<=tap<=NTap⁢{τtap}Ts⌉-NCP}(4)
where τtapdenotes the delay of the respective tap in the channel model. InFIG. 7, NDeltais 0.

The bandwidth of the lowpass filter107should preferably be chosen in a way that the bandwidth of x3(n) corresponds to the bandwidth of x2(n) (note that filtered AWGN is only one of numerous possibilities to generate x3(n) and the usage of x3(n) is by itself optional).

X1(n), x2(n) and x3(n) are first, second and third signals used for generating the cyclic prefix.

Functions f1(1)(n), f2(1)(n), f1(2)(n) and f2(2)(n) could for example be chosen such that the signal-to-interference-ratio with respect to the generated signal is adjusted according to equations (3):

SIR⁡(n)=(f2(1)⁡(n))2(f1(1)⁡(n))2
and the variance of Interference(n) is given as
var(Interference(n))=((f1(1)(n))2+(f2(1)(n))2)*P
demanding that
var(Interference(n))=var(x3(n))=var(x2(n))=Pand
SIR(n)=SIR(1)(n), where

SIR(1)⁡(n):={SIR(1)⁡(i)τi≤nTs<τi+10τNTap≤nTs<(NCP+NDelta)⁢Ts
is equivalent to

which leads to the following linear system of equations with the unknowns (f1(1)(n))2and (f2(1)(n))2:
(f1(1)(n))2+(f2(1)(n))2=1
(f2(1)(n))2−SIR(1)(n)*(f1(1)(n))2=0
the solution of this system is easily found as

(f1(1)⁡(n))2=11+SIR(1)⁡(n)(f2(1)⁡(n))2=SIR(1)⁡(n)1+SIR(1)⁡(n)
which directly leads to equations (5) and (6).

Equations (7) and (8) can be derived accordingly except that SIR(2)(n) is defined as

Another reasonable choice of the functions f1(1)(n), f2(1)(n), f1(2)(n) and f2(2)(n) could be to choose them in a way such that the SIR with respect to the autocorrelation function of the generated signal is adjusted according to equations (3):

The derivation of equations (9) to (12) follows the same steps as the derivation of equations (5) to (8) except that

SIR⁡(n)=(f2(1)⁡(n))2(f1(1)⁡(n))2
is replaced by

The first signal, second signal and third signal are equivalent to x1(n), x2(n) and x3(n) respectively.

Then the inventive CP is given as:
CP(n)=w1(n)x1(n)+w2(n)x2(n)+w3(n)x3(n)  (16)

The processing steps corresponding to the means depicted inFIG. 7have to be performed for each symbol.

Note that equations (14) and (15) show that the values of f1(1)(n) and f2(1)(n) on the interval τNTap≦nTs<(NCP+NDelta)Tswhich are formally derived in equations (5), (6), (9) and (10) as 1, 0, √{square root over (2)} and j respectively, could also be set to zero (or any other value) for easier implementation, since on this interval w2(n) and w3(n) are nulled from f1(2)(n) anyway.

Note that in this embodiment the functions fx(y)(n) are real functions. In general they could also be complex. In this embodiment our inventive cyclic prefix generation method is applied for a Single Input Single Output system with one transmit and one receive antenna (NTx=NRx=1, wherein NTxis a number of transmit antennas and NRxis a number of receiver antennas). It can of course also be applied for Multiple Input Multiple Output systems with NTx>1 and/or NRx>1. In this case our inventive method has to be applied to each of the NRxsymbols to be generated per symbol plus cyclic prefix time.

More precisely, the cyclic prefix generation unit15comprises a multiplier109, which is connected to an adder110. Moreover, a filter107is connected to a further multiplier108, which again is connected to the adder110. The adder110is furthermore connected to a multiplier112, which again is connected to an adder113. A further multiplier114is also connected to the adder113.

The last part105of the current symbol104is used as a first signal x1(n). This signal is supplied to the multiplier114and multiplied with a signal f2(2)(n). The resulting signal is supplied to the adder113.

A signal part101of a prior symbol102, which starts directly after the cyclic prefix of the previous symbol102and has the length of the cyclic prefix NCPis used as a second signal x2(n) and supplied to the multiplier109. It is multiplied by the signal f2(1)(n). The generation of this signal is described earlier.

The resulting signal is passed on to the adder110. A noise signal106is supplied to the filter107, which performs a filtering in such a manner that the noise signal has the same bandwidth as the intended measuring signal. The noise signal106can advantageously be a wide noise signal. Alternatively, it can also be derived from prior symbols.

The resulting signal x3(n) is supplied to the multiplier108and multiplied thereby with the signal f1(1)(n). The resulting signal is also supplied to the adder110and added to the signal resulting from the multiplier109. The resulting signal111is furthermore multiplied by the multiplier112with the signal f1(2)(n). The resulting signal is supplied to the adder113.

The adder113adds the previously described signal and the signal resulting from the multiplier114resulting in a channel modified cyclic prefix signal115. This signal is used as the cyclic prefix103of the current symbol104.

Second Embodiment

InFIG. 8, an embodiment of the inventive signal generation method is shown. In a first step120, a baseband signal is generated. In a second step121, a channel emulation is performed on the baseband signal. In a third step122, the resulting signal is modulated. In a fourth step123, a cyclic prefix is generated as explained with regard toFIG. 5-7. In a fifth step124, the generated cyclic prefix is inserted. The resulting signal is the final measuring signal.

InFIG. 9, a detail of the embodiment of the inventive signal generating method shown inFIG. 8is shown. InFIG. 9, the fourth step123is expanded. In a first step130, a first signal corresponding to the end of the current symbol is taken. The signal corresponds to the signal part105ofFIG. 7. In a second step131, a second signal corresponding to a beginning of a prior symbol is taken. This second signal corresponds to the signal part101ofFIG. 7. In a third step132, which is an optional step, a third signal corresponding to a part of a preceeding symbol or corresponding to alternate signal sources is taken. In a fourth step133, window functions are applied to all of the above-taken signals. In a final fifth step134, the windowed signals are added in order to emulate the channel modified cyclic prefix.

In addition to the optional third step132, further signals also corresponding to parts of preceeding symbols or alternate signal sources can be added in order to achieve a measuring signal even better matched to the autocorrelation properties of a measuring signal generated by a classical signal generator.

The invention is not limited to the examples and especially not to the OFDM transmission scheme. The invention discussed above can be applied to any signals comprising a cyclic prefix. The characteristics of the exemplary embodiments can be used in any combination.

The embodiments of the present invention can be implemented by hardware, firmware, software, or any combination thereof. Various embodiments of the present invention may be implemented by one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, or the like.

Various embodiments of the present invention may also be implemented in the form of software modules, processes, functions, or the like which perform the features or operations described above. Software code can be stored in a memory unit so that it can be executed by a processor. The memory unit may be located inside or outside the processor and can communicate data with the processor through a variety of known means.