SG187662A1 - Method and device for cancelling doppler shift induced inter carrier interference in an ofdm communication system by using signal pre-distortion - Google Patents

Abstract

A Transmitter for generating an OFDM signal is disclosed that comprises a Doppler pre-distortion unit with an input section for receiving a data vector S, an OFDM modulator, a Doppler Shift memory location for readably storing a pre-determined Doppler Shift value. The Doppler pre-distortion unit is adapted to generate a pre-distorted data vector S_PD from the data vector S. The components of the pre-distorted vector S_PD and of the data vector S represent data symbols and the pre-distorted data vector S_PD is a linear function of the data vector S, and the linear function is dependent on the Doppler shift value. The OFDM modulator is adapted to gener¬ ate an OFDM signal such that sub-carrier amplitudes of the ODFM signal are based on the components of the pre-distorted data vector S_PD.

Description

TITLE

METHOD AND DEVICE FOR CANCELLING DOPPLER SHIFT INDUCED INTER

CARRIER INTERFERENCE IN AN OFDM COMMUNICATION SYSTEM BY USING

SIGNAL PRE-DISTORTION.

OFDM (Orthogonal Frequency Division Multiplexing) is a spe- cific implementation of a multi carrier modulation method which is based on the use of orthogonal carrier signals for parallel data transmission. Different from other known multi carrier systems, the carrier distance can be diminished sig- nificantly through the orthogonality of the carrier func- tions. Hence, OFDM has established itself as a bandwidth ef- ficient method for data transmission. OFDM is used currently for DAB, DVB-T, WLAN and in fourth generation (4G) mobile phone protocols and it stands out especially in terms of ro- bustness against multi path propagation effects and efficient use of bandwidth.

US 2007/0030798 shows a Doppler frequency calculating appa- ratus that calculates a Doppler frequency, which is the mag- nitude of a time-dependent fluctuation of a characteristic of the transmission path through which an orthogonal frequency- division multiplexing (OFDM) signal is transmitted.

It is an object of the application to provide an improved method of data transmission for OFDM communication systems in the presence of a relative movement between a sending unit and a receiving unit.

This and other objects are solved by the subject matter of the application.

If a sending unit and a receiving unit of a wireless radio link move relative to each other with a velocity v and v << cp holds, the received signal experiences a frequency shift Af due to the Doppler effect according to

Af ==,

Co wherein cy stands for the velocity of light in vacuum and fg for the signal frequency. Under the assumption of the narrow- band approximation, which states that the signal bandwidth is considerably smaller than the mean frequency, the frequency shift Af is constant for the entire signal spectrum. It is positive, if the sending unit and the receiving unit approach each other and negative it they move away from each other.

If sending unit and receiving unit are stationary or move slowly with respect to each other (v*fy << cp), there is es- sentially no Doppler shift. Hence, the carrier functions of received signals are orthogonal and have essentially no over- lap in the frequency domain.

If, on the other hand, the sending unit and the receiving unit move relative to each other, the Doppler shifted carrier functions are no longer orthogonal and a superposition of signals is received on the various sub-carriers (also called carriers for brevity). This effect is known as inter carrier interference (ICI) and leads to a marked deterioration of the signal to noise ratio (SNR) on the various sub-carriers and causes an increased frequency of bit errors or even the in- terruption of the radio link.

Herein, the carrier functions are also referred to as "SI carrier functions". In OFDM, a transmitted pulse is obtained by modulating a sub-carrier, which is given by a rectangular pulse, with a sinusoid signal. Amplitude and phase of the si- nusoid function are used to represent a symbol to be trans- mitted. The sub-carrier and also the transmitted pulses cor- respond to sinc- or SI- functions in the frequency domain.

Modulation of a carrier signal may be carried out by modula- tion of an analog carrier signal or by digital signal pro- cessing.

With the subject matter of the application, it is possible to operate OFDM systems even if the maximum Doppler shift is larger than 2% of the sub-carrier distance. The application provides an increase in the transmission frequencies and the data rates and hence the carrier distance, which makes stand- ard OFDM effectively usable for communication systems with high velocities of the participants, such as for planes,

UAVs, cruise missiles and satellites.

The application discloses a transmitter for generating an

OFDM signal that comprises a Doppler pre-distortion unit with an input section for receiving a data vector S. The data vec- tor is generated from data of a data source using a symbol mapper. The data source may be provided, for example, by a camera, a microphone or another input device or also a com- puter readable memory in which multimedia files or other data is stored. The transmitter further comprises an OFDM modula- tor and a Doppler Shift memory location for readably storing a pre-determined Doppler Shift value. Herein, the Doppler shift value can be a characteristic value for the Doppler shift or alsc a higher-level data type such as a vector or matrix that can later be used to calculate the pre-distortion matrix. The Doppler Shift value can also be identical with the pre-distortion matrix. The Doppler pre-distortion unit is adapted to generate a pre-distorted data vector S PD from the data vector S, wherein the components of the pre-distorted vector S PD and of the data vector S represent data symbols.

As used herein, a data symbol does not only represent binary data but also refers to data that is sent together in one time slot and/or frequency slot, for example the data that is represented by a constellation point of a QAM modulator. The pre-distorted data vector S PD is a linear function of the data vector S and is dependent on the Doppler shift value.

The OFDM modulator is adapted to generate an OFDM signal such that sub-carrier amplitudes of the ODEM signal are based on the components of the pre-distorted data vector S PD.

The linear dependence of the pre-distorted data vector S PD as function of the data vector S applies to the Doppler cor- rection itself. For other corrections, further correction terms may be added, such as a constant value. This can result altogether in a non-linear function for the pre-distorted da- ta vector S PD.

The transmitter may further comprise a Doppler pilot signal generation unit, which is adapted to generate a Doppler pilot signal on a sub carrier. The pilot signal may be generated by setting only one input of an IDFT to a constant value and the other inputs to zero.

For computational efficiency, the Doppler pre-distortion unit may be adapted to derive the linear function of the data vec- tor S from one or more look-up tables which are selected ac- cording to the Doppler shift value.

In a further embodiment, the Doppler pre-distortion unit is adapted to store the Doppler shift value as a pre-distortion matrix and derive the linear function of the data vector S from the pre-distortion matrix. Herein, the pre-distortion matrix is calculated from a predetermined Doppler shift that the transmitter determines from an external input signal. 5

The Doppler pre-distortion unit may further be adapted to de- rive the pre-distorted vector S PD from a sum of the linear function and one or more non-linear correction terms in order to incorporate further corrections.

The transmitter may further comprise a Pilot Insertion unit for inserting the Doppler Pilot signal into an OFDM data frame. Moreover, the transmitter may comprises a multiplexer and an OFDM modulator, wherein the multiplexer is connected to the Doppler pre-distortion unit and the OFDM modulator is connected to the multiplexer, the OFDM modulator comprising an IDFT calculation unit which is adapted to generate an IDFT of the pre-distorted data vector S PD, and wherein the OFDM modulator is further adapted to generate an OFDM signal based on output of the IDFT calculation unit.

The application furthermore discloses a receiver for receiv- ing OFDM signals, which comprises an RF receiver wherein the radiofrequency RF can be an electro-magnetic wave of any fre- quency from a few kHz to several GHz, in principle even light. The receiver furthermore comprises a demodulator that is connected to the RF receiver, a de-multiplexer that is connected to the demodulator, and a Doppler estimation unit that is connected to the de-multiplexer. The Doppler estima- tion unit is adapted to generate an estimate of a Doppler shift based on amplitudes of a received OFDM signal at sub- carrier frequencies, the sub-carrier frequencies being fre- quencies of sub-carriers that are neighbours to a pilot sub-

carrier frequency. Advantageously, the neighbours are next neighbours because their amplitude is higher than that of other sub-carriers, which leads to an enhanced precision. It works in principle also with the n-th next neighbours, either alternatively or in addition to the next neighbours. For a good accuracy it is furthermore advantageous to use a pilot sub-carrier frequency that is at or at least close to, for example within +/- 2 sub-carrier distances, the centre of an

OFDM frequency band.

The application furthermore discloses an OFDM communication unit which comprising the aforementioned transmitter accord- ing and the aforementioned receiver. Moreover, the applica- tion discloses an OFDM communication system with a first com- munication unit, which comprises at least the aforementioned transmitter and a second communication unit that comprising at least the aforementioned receiver.

The application furthermore discloses a method for transmit- ting and processing a Doppler pilot signal comprising - generating a pilot signal with a predefined sub-carrier amplitude, wherein the pilot signal uses a pilot sub- carrier, the pilot sub-carrier being chosen from a plu- rality of OFDM sub-carriers; - sending the pilot signal (via a transmitter) of a first communication unit over a communication channel to a se- cond communication unit; - receiving the pilot signal and evaluating signal ampli- tudes on sub-carrier frequencies, the sub-carrier fre- quencies being frequencies of neighbours, especially of next neighbours, of the pilot sub carrier; - determining a Doppler frequency shift from the signal amplitudes.

For use with OFDM frames, the method may also comprise steps of inserting a Doppler pilot symbol into an OFDM frame to be transmitted and generating the Doppler pilot symbol according to the Doppler pilot symbol.

The method for estimating a Doppler frequency shift of a rel- ative movement between a first OFDM communication unit and a second OFDM communication unit may furthermore comprise steps of - deriving an estimated relative velocity of the second

OFDM communication unit (relative to first comm. unit); - deriving a magnitude of a Doppler shift from the esti- mated relative velocity; = transmitting the magnitude of the Doppler shift to the first OFDM communication unit over a communication chan- nel; - storing the magnitude of the Doppler shift into a com- puter readable memory of the first communication unit.

The step of deriving the estimated velocity may furthermore comprise the aforementioned steps of transmitting and pro- cessing a Doppler pilot signal. Alternatively, the step of deriving the estimated velocity may also comprise a deriva- tion of the estimated velocity from location signals, the lo- cation signals being received via an RF receiver of the se- cond communication unit.

The application furthermore discloses a method for transmit- ting pre-distorted OFDM signals over a communication channel, the method comprising - receiving input data from a data source; - deriving a data vector S$ from the input data;

- pre-distorting the data vector S to obtain a pre- distorted data vector S PD, wherein the step of pre- distorting comprises obtaining the pre-distorted data vector S PD as a linear function of the data vector wherein the linear function of the data vector S depends on a Doppler frequency shift; - deriving an OFDM signal from the pre-distorted data wvec- tor S PD, wherein sub-carrier amplitudes of the OFDM signal are based on the components of the pre-distorted data vector S PD; - transmitting the OFDM signal via an RF antenna.

Moreover, a method for transmitting pre-distorted OFDM sig- nals according is disclosed which further comprises deriving the Doppler frequency shift according to one of the claims 12 to 14 and using the derived Doppler frequency shift in the step of pre-distorting the data vector S.

The application furthermore discloses a computer readable program for executing one of the aforementioned methods and a computer readable memory with the computer readable program.

Moreover, the application discloses one or more signal pro- cessing units, the signal processing units each comprising a microprocessor, an instruction processor and a computer read- able memory, the one ore more computer readable memories com- prising the computer readable program. Alternatively, the one each comprise a microprocessor and a hard-wired circuit for executing one of the aforementioned methods. The computer programs, and signal processors may located be on a first and on a second communication unit in which case the word comput- er program/memory refers to both the first and the second communication unit.

The subject matter is now explained in further detail with respect to the following figures in which

Fig. 1 shows sub-carrier amplitudes for a sent pilot sig- nal and for a received pilot signal in the presence of a Doppler shift,

Fig. 2 shows frequency-amplitude diagrams for sub-channel signals of a sent signal and of a received signal in the presence of a Doppler shift,

Fig. 3 shows frequency-amplitude diagrams for sub-channel signals of a sent signal and of a received signal in the presence of a Doppler shift using a pre- distorted signal,

Fig. 4 shows a communication system according to the ap- plication,

Fig. 5 shows quadrature coordinates derived from received signals for a Doppler shift of 0.01 sub carrier distances without the use of pre-distortion,

Fig. © shows quadrature coordinates derived from received signals for a Doppler shift of 0.01 sub carrier distances and using pre-distortion,

Fig. 7 shows quadrature coordinates derived from received signals for a Doppler shift of 0.05 sub carrier distances without the use of pre-distortion,

Fig. 8 shows quadrature coordinates derived from received signals for a Doppler shift of 0.05 sub carrier distances and using pre-distortion, and

Fig. 9 shows a simplified design of a communication unit according to the application.

According to the application, a sent signal is pre-distorted based on an estimated Doppler shift. In order to estimate the

Doppler shift, a frequency shift is determined with suffi- cient accuracy by using the transmission of a corresponding

OFDM pilot symbol. The OFDM pilot symbol uses only one sub- carrier in the centre of the signal bandwidth, whereas the other sub-carriers are set to zero. The upper diagram of Fig. 1 shows a corresponding sub-carrier occupation for a system with 64 sub-carriers. Only the sub-carrier 33 at the centre of the bandwidth is assigned to the pilot signal.

As a consequence of the Doppler shift Af and the loss of car- rier function orthogonality, the signal energy on the receiv- ing side is distributed over the neighbouring sub-carriers.

This is seen in the lower diagram of Fig. 1. At the sub car- rier frequencies, the amplitude of a received signal does not correspond to a single sub-carrier amplitude. According to the application, a Doppler shift compensation is based on the amplitude of received signals, so as to make the computations independent of phase rotations caused by the communication channel. When using SI-functions as orthogonal carriers, a proportion of an amplitude S of a left-sided neighbouring carrier (34) to an amplitude S of a right sided carrier (32) is given as

Shen neighbour _ r= [sil (Af + 1) — - sin(zAf (Af 4) — 1-4F (1), which for

Spapnsgnon SIA =D) | sin(zf)r(af +1) 1+ 41

Af # 1 can be solved for the Doppler shift Af to give

Af =F (2). +7

Advantageously, the maximum Doppler shift is smaller than the frequency distance between two neighbouring sub-carriers, Af

The relationship (1) provides a one-to-one mapping for the computation of the frequency shift from the ratio of the sig- nal amplitudes of neighbouring carriers which is easy to im- plement and permits an accurate determination of the frequen- cy shift also in the presence of additive white Gaussian noise (AWGN) or other noise.

The accuracy of the method increases with the number of used sub-carriers. For evaluating more than two sub-carriers while using the same pilot signal as above, the relation (1) can be generalized by considering the second nearest neighbours:

Ses, bled +2) 2-af which gives a —2.120 wherein m

S si(z(Af -2)) 2+4f 1+, is a sub-carrier number of a sub-carrier in the centre of the bandwidth. In general I — holds for n-th nearest 2 neighbours of the central sub-carrier. From this relation, the Doppler shift can be computed as a weighted sum of the ratios, Af = Swf © Fw, io , with > w =l. In one reali- zation, w; = 1/N carriers, wherein N carriers is the number of sub-carriers. In another realization, the weights w; decrease with increasing distance from the central sub-carrier m.

According to a first method of determining a Doppler shift Af for generating a pre-distortion matrix, a pilot signal is sent via a transmitter of a sending unit A to a receiver of a receiving unit B. The pilot signal uses a sub-carrier in the centre of the OFDM band and a predetermined sub-carrier am- plitude, as shown in the upper diagram of Fig. 1. The receiv- ing unit B receives a signal that is distributed over several sub-channels, as shown in the lower diagram of Fig. 1. From the ratio r of the amplitude Sictt neighvour ©f the left neigh- bouring sub-carrier to the amplitude S:ignt neignbour ©f the right neighbouring sub-carrier, the receiving unit B determines the

Doppler shift Af according to equation (2) and sends this in- formation back to the sending unit A. According to a second method, the pilot signal is generated by the receiving unit B after a communication with the sending unit B has been initi- ated, and the ratio r is evaluated by the sending unit A.

According to the application, a pilot signal for determina- tion of the Doppler shift is itself not pre-distorted but, rather, the Doppler shift is determined from the distortion of the pilot signal, which is known to the receiving unit or can be computed from values that are accessible to the re- ceiving unit.

Advantageously, the pilot symbol is transmitted regularly be- tween the communication units for determining an accurate frequency shift. In a specific embodiment, the pilot symbol is inserted into each OFDM frame to be transmitted. According to a method for transmitting OFDM message from a sending unit

A to a receiving unit B, the sending unit A sends a first message, for example a first OFDM frame, without pre- distortion. Hence, the first message may lead to a higher bit error rate than the following messages. The receiving unit B receives the first message and determines a Doppler shift from a pilot symbol that is included in the first message.

The receiving unit B sends a response message, which includes the determined Doppler shift. The response message from the receiving unit B may be pre-distorted using the determined

Doppler shift. The sending unit A receives the response mes- sage from the receiving unit B, stores the Doppler shift in a computer readable memory and uses the Doppler shift to pre- distort further messages according to the Doppler shift.

In a further modification, the pilot symbol may be included only in only some of the messages. For example, the pilot symbol may be sent after a pre-determined expiry time. In an- other embodiment, a new determination of the Doppler-shift is started when one of the communication units determines a sig- nificant change of its movement. To determine a velocity change, position determination via a GPS, phone cell loca- tions, acceleration sensors or other means may be used.

The first communication unit then uses the Doppler shift Af to pre-distort a signal to be sent. In an implementation of a pre-distortion method for an OFDM communication system using

IDFT/DFT (inverse discrete Fourier transform/discrete Fourier transform) according to the application, the SI-functions are replaced with the expression _sin(zll ~k +41) (3), wherein k and 1 are sub-carrier numbers sin] (1—k+ v)

N and N is the number of sub-carriers. The abovementioned rela- tion (1), which applies to the continuous Fourier transform, corresponds to the special case in which 1 - k = 1. Taking into account the approximation sir) = 300) for N>>x for win *

N the sinc function si(x), the abovementioned relation is also valid in the DFT case, provided the number N of sub-carriers is large enough.

According to the application, a frequency shift is determined with sufficient accuracy prior to a pre distortion of a sig- nal to be transmitted. In one embodiment, the frequency shift is determined by using a reception of the abovementioned pi- lot symbol. Further embodiments comprise the use of other system information to derive a relative movement such as the computation of a relative velocity from GPS data, especially from GPS data of the receiving unit or from other position data.

By determining a relative velocity and/or Doppler shift ac- cording one of the abovementioned frequency estimation meth- ods, the sending unit computes a pre-distorted signal.

The distortion of a signal due to the Doppler shift can be represented by an interference matrix 2 of the size [N x NJ, wherein N is the number of OFDM sub-carriers, which takes the form

Si Sin tt Siw =o So $a tt Sow

Sn Sno tt Su

Herein, the elements i; of the interference matrix determine the interference component of the 1-th sub-carrier on the k- th sub-carrier, or, in other words, the proportion of the signal from the sub-carrier 1, which appears on sub-carrier k on the receiving side. When using SI-carrier functions and for a time continuous system, the components of the interfer- ence matrix E and a given frequency shift Af is computed ac- cording to

Eel = si(z(k —1+ Af)) (4).

According to the application, the inverse E* of the distor- tion matrix E is computed and a data vector S to be sent is multiplied with the inverse matrix E*to obtain a pre- distorted data vector Sep, Sp, =2 +S. The inverse may be ob- tained by direct numerical calculation such as a variant of the Gaussian elimination algorithm or also by an iterative method, such as Jacobi iteration, conjugate gradient or oth- ers.

A modulating means of the receiving unit modulates the sub- carrier according to the pre-distorted data vector and trans- mits the signal over a communication channel. At the receiv- ing unit, a corresponding signal is received. The correspond- ing signal can be represented as a function of the pre- distorted signal, which represents the properties of the com- munication channel. The receiving unit demodulates the re- ceived signal. In an idealized case in which only the Doppler shift is present or in the case in which other signal distor- tions are at least sufficiently small, the channel can be represented with sufficient accuracy by the distortion matrix 2, and the reconstructed data vector R at the receiving side is equal to the original data vector Ss, R=E2-§, =S.

The sending unit and the receiving unit may comprise addi- tional units which are not shown in Fig. 4 for simplicity, such as scrambler/descrambler, an interleaver/deinterleaver, a channel coding/decoding unit, digital analog converters (DACs), analog digital converters (ADCs), low pass filters, oscillators for generating a carrier frequency and also fur-

ther error correction means, such as the channel coding indi- cated in Fig. 4.

A signal pre-distortion and recovery according to the appli- cation is now illustrated with reference to the Figures 2 and 3, wherein Fig. 3 shows the use of a pre-distortion according to the application. In the examples of Fig. 2 and 3, the OFDM system uses four sub-carriers and the data symbols are modu- lated by an amplitude modulation. The components of the data symbol S can also be used as the in-phase or qguadrate compo- nents of QAM modulations. In this case, the data symbol S represents only half of the information while the other half is transmitted via the respective other components of the QAM modulation. In the example shown in Figs. 2 and 3, the rela- tive movement is such that a Doppler shift of 0.4 sub-carrier distances results. The amplitudes of the sent and of the re- ceived signal at the sub channels are indicated by arrows.

By way of example it is assumed that the data symbol

S=[3 11 3] is to be transmitted, in which the components correspond to sub carrier amplitudes, which are numbered by the indices 0, 1, 2, 3. Amplitudes of the amplitude modula- tion corresponding to discrete values are indicated in Figs. 2 and 3 in arbitrary units on the Y-axis and by corresponding horizontal lines. Likewise, the sub-carrier frequencies are indicated in arbitrary units on the X-axis and by correspond- ing vertical lines. A signal is represented by the sum of the four frequency curves as shown in the lower diagrams of Figs. 2 and 3.

For comparison, the communication system without use of the pre-distortion is considered first. Under the assumption that there is substantially only the Doppler shift Af = 0.4, the interference between sub-carrier signals can be represented by an interference matrix E with the following components: 0.67 0.50 -0.19 0.12 = -022 0.76 0.50 -0.19 > 013-022 076 050 -0.09 0.13 -0.22 0.76

The resulting received signal is shown in the lower diagram of Fig. 2 in which the received signal, which is given by the sum of the individual signals from the four carriers, is in- dicated by a thick line. Accordingly, the following distorted data vector is derived from the demodulated signal: s=[2.93 005 243 1.91]

It can be seen in Fig. 2 and also in the Fig. 7 that a recon- struction of the original data vector becomes difficult to impossible for Doppler shifts of this size if the frequency shift Af is not determined: the positions at which one of the sub-carrier signals has the maximum amplitude and the other three sub-carrier signals are zero are shifted by the fre- quency shift Af which is not known to the receiving unit.

Hence, a receiving unit will still evaluate the sum signal at the sub-carrier frequencies at which the amplitude cannot be attributed uniquely to a sub-carrier signal.

According to the application, the sending unit generates a pre-distorted symbol by multiplying the symbol to be sent with the inverse of the interference matrix. Thereby, a sym- bol S,,=[1.39 2.89 -0.43 3.52] is obtained which is modulated and sent over the communication channel via a transmission antenna. The communication channel is assumed to be the same

Doppler channel as above, which is characterized by the in- terference matrix EZ. The upper diagram of Fig. 3 shows a fre- quency-amplitude diagram that represents the sent signal and the lower diagram shows a frequency-amplitude diagram that represents the received signal. As can be seen in the upper diagram, the amplitudes of the received signal coincide with amplitudes of the original signal at the positions of the four sub-carrier frequencies. The receiving unit uses the de- tected amplitudes at the sub-carrier frequencies to recover the original data vector S.

In contrast to an alternative method that uses a reverse shift by Af of the sent signal by the frequency shift, a method according to the application is technically easier to realize as it can be done by simply adjusting the signal am- plitude at the sending unit and it does not require a rescal- ing of pulse lengths or a tuning of oscillator frequencies.

In a further embodiment, a system according to the applica- tion comprises a digital (time discrete) signal processing system. The sender comprises an IDFT unit for executing an inverse discrete Fourier transform and the receiving unit comprises a DFT unit for executing a discrete Fourier trans- form. In a QAM realization of OFDM, the real part and imagi- nary parts of the IDFT input values and the DFT output values represent in-phase and quadrature amplitudes. As is known for

OFDM, the IDET is used to transform the QAM components for the N sub-carriers into a time dependent complex valued sig- nal, the real and imaginary part are converted into analog signals and up-converted into two 90° phase shifted signals.

Herein, complex values are interpreted in the usual way by identifying phase and amplitude in the complex plane as phase and amplitude of a signal or of a digital representation of the signal. In a known way, the process is reversed at the receiver.

The DFT is represented with complex valued exponential func- tions exp(x + jy), wherein exp(x + jy) = exp(x) *(cos(y) + J sin(y)) and j = v-1. Correspondingly, the interference matrix also comprises complex valued exponential functions. More specifically, the elements of the interference matrix are given by the sinc function or by the function (3) as before, but in addition they are multiplied by a complex exponential function. This is explained below in further detail.

By way of example, a data vector to be sent is given by the ’ vector s=s, Sy ee Sea] - The DFT is represented by the ma- trix 0:0 01 ., _O0(N-1) oN ew eT _j2a0 _j2aid _ja HD e N e N vee e N .

F= and the IDET is represent- _(-1)0 LL v- ’ (N-1N-1) eT eT A ed by the Hermitian transpose F* (complex conjugate transpose) of the matrix F. A signal t to be transmitted is represented by the relation t=F -S , wherein the N components of t de- fine modulation parameters for the N sub-carriers. For the

DFT, the matrix F is multiplied by a normalization factor of 1/N. The normalization factor 1/N can also be attributed to the IDFT or a normalization factor of 1/VN can be attributed to both the DFT and the IDFT.

Under the assumption that the communication channel causes a phase rotation 6 and a Doppler frequency shift of Af to a signal, wherein the Doppler shift is represented in units of the sub-carrier distance, the communication channel can be represented by a diagonal channel matrix H with the following elements (0a —j2n| ——+6

SU

(laf —j2m| —+6 al 0 SUE, ’ ’ ’ . (N-1)AF —j2m| ——+8 0 0 a ol oy (O-Af oo (1AfF Lo ((N-D)Af —j2r| ——+6 —j2m| —+6 —j2m| —=+8 do] 5 ) e 5 ) ee 5

The diagonal channel matrix H can be represented as a product of a diagonal matrix Hp and a diagonal matrix He which repre- sent the Doppler shift and the phase shift, respectively, and which have the following elements: (0a —j2m —-

SL

(laf —jem| =

H, = 0 e ) ‘ee 0 ’ ’ ’ , (N-1)ar —jom| LE 0 0 eee N (oar (LAF ((N-1)aF —j2r —— —j2r| — —j27| ——— du ¥) e 5) cee 5 2 0 a. 0 0 id a. 0

H,=| |. . . Co |=e1. 0 0 ce em

The received signal r is given by the relation r=H,-t-e/””=H, F -S-e*” and the received symbol is given by

1 1 sane

R=—F-r=S-—F-H, -F ¢/7. Hence, the interference matrix E

N N

— 1 . is given by the product ==5 FH, -F.

The components &,,, von = are computed according to 1 & jangle) sin(z(l — k + Af )) sali Jase)

Su=m2e =—¢ (5) = (7 ’ Nsin| (I —k + Af)

N

With the definition of m as carrier distance 1-k, the inter- ference can be brought into the symmetric form: o & te Sva == & Eo tt Eva , wherein $a $n te c 1 3 pete) sin(a(m+ AF) sory fo) “ = —_— e = . " N & (7 ’ Nsin| (m+ Af)

N

The receiving unit computes the pre-distorted data vector us- ing the interference matrix according to the relation

S,,=2"S . Under the abovementioned assumption that the com- munication channel causes only a Doppler distortion and a phase shift and provided that the relative velocity has not changed significantly since the last determination, the re- constructed data vector R is equal to the sent data vector S times a phase factor exp (j 2n8) that is due to a phase rota- tion 2u8,

R=S, H=E"-S-F-H-F =8-¢""".

The phase rotation is corrected for using a known method, for example based on pilot sub-carriers. The term "pilot sub- carrier" refers to the sub-carrier that is used for sending a pilot signal to determine the phase shift. The pilot sub- carrier may also be used for sending a pilot signal to deter- mine the Doppler shift according to the invention.

Fig. 4 shows a flow diagram of an OFDM communication unit 10 according to the application. The communication unit 10 com- prises a transmitter 11 and a receiver 12 for sending and re- ceiving messages via a communication channel 13 to another communication unit, which is not shown in Fig. 4. The other communication unit may be of the type shown in Fig. 4 or also of the simpler type shown in Fig. 9, for example.

In the transmitter 11, a data source 14 is connected to an input of a channel coding and interleaving unit 15. An output of the channel coding and interleaving unit 15 is connected to an input of a symbol mapper 16. An output of the symbol mapper 16 is connected to an input of a channel estimation and pilot signal insertion unit 17 and an output of the chan- nel estimation and pilot signal estimation unit 17 is con- nected to a Doppler pre-distortion unit 18. An output of the

Doppler pre-distortion unit 18 is connected to an input of a multiplexer 19. A further input of the multiplexer 19 is con- nected to a Doppler pilot signal generation unit 20. An out- put of the multiplexer 19 is connected to an input of an OFDM modulator 21 and an output of the OFDM modulator 21 is con- nected to an input of an RF transmitter 22, which comprises a

RF transmitter antenna.

In the receiver 12 of the communication unit 10, an output of an RF receiver 23 is connected to an input of an OFDM demodu-

lator 24. An output of the OFDM demodulator is connected to an input of a de-multiplexer 25. An output of the de- multiplexer 25 is connected to a channel correction unit 26.

A further output of the de-multiplexer 25 is connected to a

Doppler estimation unit 27. An output of the channel correc- tion unit 26 is connected to an input of a symbol demapper 28. An output of the symbol demapper 28 is connected to an input of a channel decoding and deinterleaving unit 29. An output of the channel decoding and deinterleaving unit 29 is connected to a data sink 30. The receiver 12 may comprise further components such as a phase locked loop and an adap- tive equalizer.

In this embodiment, the Doppler estimation unit is connected the Doppler pre-distortion unit. However, the Doppler estima- tion unit may also be connected to the multiplexer for send- ing the value of the estimated Doppler frequency back to a second communication unit. The communication unit may also be realized without a Doppler pre-distortion unit 18 and without the Doppler pilot signal generation unit 20. This is shown in

Fig. 9. The pre-distortion is then provided by a second com- munication unit, for example a base station, to which the communication unit 10 is connected via the channel 13.

A computation of the pre-distortion matrix E * requires the computation of an inverse of the [N x N] matrix E and in gen- eral is computationally expensive. Therefore, in a further modification, pre-computed pre-distortion matrices or ele- ments thereof are computed for various frequency shifts and are stored in look-up tables of the sending unit. Pre- distortion matrices for intermediate values of frequency shifts may be determined by interpolation of the pre-computed pre-distorticon matrices.

According to the application, the receiving unit only needs to comprise an additional functionality for estimating the

Doppler shift or the relative velocity and does not need to include further additional functionality. Thus, the adapta- tion of existing receiving units and the use of inexpensive receiving units is facilitated. The use of pilot signals makes efficient use of the bandwidth as compared to known correction methods, such as for example the cancelling out of interferences by transmission of inverse signals on neigh- bouring sub-carrier frequencies.

The following Figs. 5 to 8 show data obtained from an OFDM- system with 64 sub-carriers and using a 16-QAM (quadrature amplitude) modulation of data symbols. The abovementioned pre-distortion method is based on an adaptation of the sub- carrier amplitudes. Hence, it also applies to a QAM modula- tion or other modulation methods using amplitude as well as phase values. The data symbols are represented preferentially by discrete values but may also comprise continuous values, as in continuous QAM.

In Figs. 5 to 8, the horizontal axis represents an "in-phase" coordinate and the vertical axis represents a "quadrature" coordinate which can be regarded as amplitudes of a sine and a cosine oscillation that are superposed to obtain a signal of an RF transmitter. Especially when many sub-carriers are used it is advantageous, however, to use QAM coordinates, that are generated by the symbol mapper 16, to derive an in- put signal to an IDFT calculation unit within an OFDM modula- tor 21, as in the embodiment of Fig. 4.

The circles in Figs. 5 to 8 represent received signals corre- sponding to a sent data symbol. The grid lines form a square grid around the discrete QAM amplitude and phase values. If the amplitude and phase pairs of received data symbols lie within the corresponding squares, they can be attributed uniquely to a QAM amplitude. Otherwise, bit errors will re- sult.

It can be seen from Fig. 7 that in an OFDM system without pre-distortion or other frequency corrections, already at a

Doppler shift of +/-5 % of the sub carrier distance, some of the circles lie outside the squares. Therefore, bit errors are to be expected. Especially for larger Doppler shifts, the bit errors eventually become so large that they cannot be compensated for by standard error correction methods. By con- trast, the pre-distortion method according to the application allows an exact reconstruction of the sent symbol, as can be seen in Figs. 6 and 8. Even for frequency shifts as high as half the sub-carrier distance, the sent symbol will be re- ceived correctly.

Fig. 9 shows a simpler embodiment of a communication unit 10° that does not comprise the Doppler pilot signal generation unit 20 and the Doppler pre-distortion unit 18 shown in Fig. 4. In this case, the Doppler pilot signal generation unit 20 and the Doppler pre-distortion unit 18 are provided by anoth- er communication unit, for example the communication unit 10 shown in Fig.4. Advantageously, the other communication unit is dedicated for downlink traffic, such as a radio base sta- tion.

Fig. 10 shows a communication system comprising a first com- munication unit 10 and a second communication unit 10' ac-

cording to the application. OFDM frames belonging to OFDM messages from communication unit 10 to communication unit 10° are indicated by rectangles 37 and OFDM frames belonging to

OFDM messages from communication unit 10' to communication unit 10 are indicated by rectangles 38. The communication units 10, 10' comprise signal processing units 35, 35' re- spectively. The signal processing unit 35, 35' comprise com- ponents for processing and evaluating the OFDM frames, as shown in Fig. 4 or Fig. 9. In a modification, the same anten- na is used as RF transmitter 22 and as RF receiver 23. If different antennas are used for transmitting and receiving, the communication units 10, 10' may be adapted not to trans- mit during reception of a Doppler pilot signal.

In an alternative embodiment, just a one way traffic of OFDM data packets 37 from the communication unit 10 to the commu- nication unit 10' is provided and the Doppler shift estimate is transmitted back from the communication unit 10' to the communication unit 10 by other means, for example by a fur- ther modulation technique such as AM, QAM etc. and/or by a further communication channel.

The embodiments can also be described with the following lists of elements being organized into items. The respective combinations of features which are disclosed in the item list are regarded as independent subject matter, respective- ly, that can also be combined with other features of the ap- plication. 1. Transmitter for generating an OFDM signal comprising a Doppler pre-distortion unit with an input section for receiving a data vector S, an OFDM modulator, a Doppler Shift memory location for readably stor- ing a pre-determined Doppler Shift value, wherein the Doppler pre-distortion unit is adapted to generate a pre-distorted data vector S PD from the data vector S, wherein the components of the pre-distorted vector S PD and of the data vector S represent data symbols, wherein the pre-distorted data vector S PD is a linear function of the data vector S, wherein the linear function is dependent on the Doppler shift value, and wherein the OFDM modulator is adapted to generate an

OFDM signal such that sub-carrier amplitudes of the ODFEM signal are based on the components of the pre-distorted data vector S PD. 2. Transmitter according to item 1 further comprising a Doppler pilot signal generation unit which is adapted to generate a Doppler pilot signal on a sub carrier. 3. Transmitter according to item 1 or 2, wherein the Doppler pre-distortion unit is adapted to derive the linear function of the data vector S from one or more look-up tables which are selected according to the Dop- pler shift value. 4, Transmitter according to one of the previous items, wherein the Doppler pre-distortion unit is adapted to store the

Doppler shift value as a pre-distortion matrix and de- rive the linear function of the data vector S from the pre-distortion matrix. 5. Transmitter according to one of the previous items, wherein the Doppler pre-distortion unit is adapted to derive the pre-distorted vector S PD from a sum of the linear func- tion and one or more non-linear correction terms. 6. Transmitter according to one of the previous items fur- ther comprising a Pilot Insertion unit for inserting the Doppler Pilot signal into an OFDM data frame. 7. Transmitter according to one of the previous items fur- ther comprising a multiplexer and an OFDM modulator, wherein the multiplexer is connected to the Doppler pre- distortion unit and the OFDM modulator is connected to the multiplexer, the OFDM modulator comprising an IDFT calculation unit which is adapted to generate an IDFT of the pre-distorted data vector S PD, and wherein the OFDM modulator is further adapted to gener- ate an OFDM signal based on output of the IDFT calcula- tion unit. 8. Receiver for receiving OFDM signals comprising an RF receiver, a demodulator that is connected to the RF receiver, a demultiplexer that is connected to the demodula- tor, and a Doppler estimation unit that is connected to the demultiplexer, wherein the Doppler estimation unit is adapted to gener- ate an estimate of a Doppler shift based on amplitudes of a received OFDM signal at sub-carrier frequencies, the sub-carrier frequencies being frequencies of sub- carriers that are neighbours to a pilot sub-carrier fre- quency. 9. Receiver according to item 8, wherein the pilot sub-carrier frequency is at the centre of an

OFDM frequency band. 10. OFDM communication unit comprising a transmitter according to one of items 1 to 7 and a receiver according to one of items 8 to 9. 11. OFDM communication system comprising a first communication unit which comprises at least a transmitter according to one of the item 1 to 7 and a second communication unit comprising at least a receiver according to one of the items 8 to 9.

12. Method for transmitting and processing a Doppler pilot signal comprising

- generating a pilot signal with a predefined sub- carrier amplitude, wherein the pilot signal uses a pilot sub-carrier, the pilot sub-carrier being cho- sen from a plurality of OFDM sub-carriers,

- sending the pilot signal of a first communication unit over a communication channel to a second com- munication unit,

- receiving the pilot signal and evaluating signal amplitudes on sub-carrier frequencies, the sub- carrier frequencies being frequencies of neighbours of the pilot sub carrier, and

- determining a Doppler frequency shift from the sig-

nal amplitudes.

13. Method for transmitting and processing a Doppler pilot signal according to item 12,

the generation of the pilot signal further comprising

- inserting a Doppler pilot symbol into an OFDM frame to be transmitted, and

- generating the Doppler pilot symbol according to the Doppler pilot symbol.

14. Method for estimating a Doppler frequency shift of a relative movement between a first OFDM communication unit and a second OFDM communication unit, comprising - deriving an estimated relative velocity of the se-

cond OFDM communication unit,

- deriving a magnitude of a Doppler shift from the estimated relative velocity,

- transmitting the magnitude of the Doppler shift to the first OFDM communication unit over a communica- tion channel, and - storing the magnitude of the Doppler shift into a computer readable memory of the first communication unit. 15. Method for estimating a Doppler frequency shift accord- ing to item 14, wherein the step of deriving the estimated velocity comprises the steps of transmitting and processing a Doppler pilot signal according to one of the items 12 to 13. 16. Method for estimating a Doppler frequency shift accord-

ing to item 14, wherein the step of deriving the estimated velocity comprises deriving the estimated velocity from location signals, the location signals being received via an RF receiver of the second communication unit.

17. Method for transmitting pre-distorted OFDM signals over a communication channel, the method comprising - receiving input data from a data source, - deriving a data vector S from the input data,

- pre-distorting the data vector S to obtain a pre- distorted data vector S PD, wherein the step of pre-distorting comprises obtaining the pre- distorted data vector S PD as a linear function of the data vector wherein the linear function of the data vector S depends on a Doppler frequency shift,

- deriving an OFDM signal from the pre-distorted data vector S PD, wherein sub-carrier amplitudes of the

OFDM signal are based on the components of the pre- distorted data vector S PD, and - transmitting the OFDM signal via an RF antenna. 18. Method for transmitting pre-distorted OFDM signals ac- cording to item 17 further comprising - deriving the Doppler frequency shift according to one of the items 12 to 14 and - using the derived Doppler frequency shift in the step of pre-distorting the data vector S. 19. Computer readable program for executing a method accord- ing to one of the items 12 to 18. 20. Computer readable memory with a computer readable pro- gram according to item 19. 21. One or more signal processing units, the signal pro- cessing units each comprising - a microprocessor, - an instruction processor, and - a computer readable memory, the one or more comput- er readable memories comprising a computer readable program according to item 19.

22. One or more signal processing units, the signal pro- cessing units each comprising - a microprocessor and - a hard-wired circuit for executing a method accord- ing to one of the items 12 to 18.

Claims (22)

Patent claims

1. Transmitter for generating an OFDM signal comprising a Doppler pre-distortion unit with an input section for receiving a data vector S, an OFDM modulator, a Doppler Shift memory location for readably stor- ing a pre-determined Doppler Shift value, wherein the Doppler pre-distortion unit is adapted to generate a pre-distorted data vector S PD from the data vector S, wherein the components of the pre-distorted vector S PD and of the data vector S represent data symbols, wherein the pre-distorted data vector S PD is a linear function of the data vector S, wherein the linear function is dependent on the Doppler shift value, and wherein the OFDM modulator is adapted to generate an OFDM signal such that sub-carrier amplitudes of the ODFEM signal are based on the components of the pre-distorted data vector S PD.

2. Transmitter according to claim 1 further comprising a Doppler pilot signal generation unit which is adapted to generate a Doppler pilot signal on a sub carrier.

3. Transmitter according to claim 1, wherein the Doppler pre-distortion unit is adapted to derive the linear function of the data vector S from one or more look-up tables which are selected according to the Dop- pler shift value.

4. Transmitter according to claim 1, wherein the Doppler pre-distortion unit is adapted to store the Doppler shift value as a pre-distortion matrix and de- rive the linear function of the data vector S from the pre-distortion matrix.

5. Transmitter according to claim 1, wherein the Doppler pre-distortion unit is adapted to derive the pre-distorted vector S PD from a sum of the linear func- tion and one or more non-linear correction terms.

6. Transmitter according to claim 1 further comprising a Pilot Insertion unit for inserting the Doppler Pilot signal into an OFDM data frame.

7. Transmitter according to claim 1 comprising a multiplexer and an OFDM modulator, wherein the multiplexer is connected to the Doppler pre- distortion unit and the OFDM modulator is connected to the multiplexer, the OFDM modulator comprising an IDFT calculation unit which is adapted to generate an IDFT of the pre-distorted data vector S PD, and wherein the OFDM modulator is further adapted to gener- ate an OFDM signal based on output of the IDFT calcula- tion unit.

8. Receiver for receiving OFDM signals comprising an RF receiver, a demodulator that is connected to the RF receiver, a demultiplexer that is connected to the demodula- tor, and a Doppler estimation unit that is connected to the demultiplexer, wherein the Doppler estimation unit is adapted to gener- ate an estimate of a Doppler shift based on amplitudes of a received OFDM signal at sub-carrier frequencies, the sub-carrier frequencies being frequencies of sub- carriers that are neighbours to a pilot sub-carrier fre- quency.

9. Receiver according to claim 8, wherein the pilot sub-carrier frequency is at the centre of an OFDM frequency band.

10. OFDM communication unit comprising a transmitter according to claim 1 and a receiver according to one claim 8.

11. OFDM communication system comprising a first communication unit which comprises at least a transmitter according to one of the claim 1 to 7 and a second communication unit comprising at least a receiver according to claim 8§.

12. Method for transmitting and processing a Doppler pilot signal comprising - generating a pilot signal with a predefined sub- carrier amplitude, wherein the pilot signal uses a pilot sub-carrier, the pilot sub-carrier being cho- sen from a plurality of OFDM sub-carriers, - sending the pilot signal of a first communication unit over a communication channel to a second com- munication unit,

- receiving the pilot signal and evaluating signal amplitudes on sub-carrier frequencies, the sub- carrier frequencies being frequencies of neighbours of the pilot sub carrier, and - determining a Doppler frequency shift from the sig- nal amplitudes.

13. Method for transmitting and processing a Doppler pilot signal according to claim 12, the generation of the pilot signal further comprising - inserting a Doppler pilot symbol into an OFDM frame to be transmitted, and - generating the Doppler pilot symbol according to the Doppler pilot symbol.

14. Method for estimating a Doppler frequency shift of a relative movement between a first OFDM communication unit and a second OFDM communication unit, comprising - deriving an estimated relative velocity of the se- cond OFDM communication unit, - deriving a magnitude of a Doppler shift from the estimated relative velocity, - transmitting the magnitude of the Doppler shift to the first OFDM communication unit over a communica- tion channel, and - storing the magnitude of the Doppler shift into a computer readable memory of the first communication unit.

15. Method for estimating a Doppler frequency shift accord- ing to claim 14, wherein the step of deriving the estimated velocity comprises the steps of transmitting and processing a Doppler pilot signal according to claim 12.

16. Method for estimating a Doppler frequency shift accord- ing to claim 14, wherein the step of deriving the estimated velocity comprises deriving the estimated velocity from location signals, the location signals being received via an RF receiver of the second communication unit.

17. Method for transmitting pre-distorted OFDM signals over a communication channel, the method comprising - receiving input data from a data source, - deriving a data vector S from the input data, - pre-distorting the data vector S to obtain a pre- distorted data vector S PD, wherein the step of pre-distorting comprises obtaining the pre- distorted data vector S PD as a linear function of the data vector wherein the linear function of the data vector S depends on a Doppler frequency shift, - deriving an OFDM signal from the pre-distorted data vector S PD, wherein sub-carrier amplitudes of the OFDM signal are based on the components of the pre- distorted data vector S PD, and - transmitting the OFDM signal via an RF antenna.

18. Method for transmitting pre-distorted OFDM signals ac- cording to claim 17 further comprising - deriving the Doppler frequency shift according to claim 12 and - using the derived Doppler frequency shift in the step of pre-distorting the data vector S.

19. Computer readable program for executing a method accord- ing to claim 12.

20. Computer readable memory with a computer readable pro- gram according to claim 19.

21. One or more signal processing units, the signal pro- cessing units each comprising - a microprocessor, - an instruction processor, and - a computer readable memory, the one ore more com- puter readable memories comprising a computer read- able program according to claim 19.

22. One or more signal processing units, the signal pro- cessing units each comprising - a microprocessor and - a hard-wired circuit for executing a method accord- ing to claim 12.