WO2008061595A1

WO2008061595A1 - Method for avoiding mirror crosstalk in an ofdm system

Info

Publication number: WO2008061595A1
Application number: PCT/EP2007/008938
Authority: WO
Inventors: Andreas Forck; Rüdiger Halfmann; Thomas Haustein; Volker Jungnickel; Clemens Von Helmolt
Original assignee: Nokia Siemens Networks Gmbh & Co. Kg; Fraunhofer Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2006-11-24
Filing date: 2007-10-16
Publication date: 2008-05-29
Also published as: EP1926269A1

Abstract

To avoid mirror crosstalk of subcarriers in an OFDM system, a method is proposed, wherein a source signal to be transmitted consisting of data symbols (N) is split before the IFFT to be applied into a number of disjoint - orthogonal in the frequency sense - carriers and then transmitted in a parallel manner by way of different frequencies to a demultiplexer in two sidebands (Bu, BL), with the mirror carrier corresponding to a carrier loaded with data symbols (N) not being loaded with data symbols.

Description

Method for avoiding mirror crosstalk in an OFDM system

The present invention relates to a method for avoiding mirror crosstalk in an OFDM system according to the preamble of claim 1.

In the text which follows technical English nomenclature and acronyms are used, such as for example

OFDM Orthogonal Frequency Division Multiplexing;

RF Radio Frequency; FFT Fast Fourier Transform Transform;

IFFT Inverse Fast Fourier Transform; etc.

The use of such technical terms and common acronyms prevents confusion. A list of the acronyms, abbreviations and terms used is given at the end of this document and forms an integral part of this document.

Modern communication systems use multi-carrier methods and multi-antenna systems. OFDM technologies allow simple channel equalization in the frequency range, particularly in frequency- selective channels. A cyclic prefix, the so-called guard interval, ensures orthogonality of the individual OFDM sub- carriers. Orthogonality here means disjoint carriers. The guard interval is dimensioned such that the temporal channel pulse response of an OFDM symbol has died out within the guard interval, so that so-called inter-symbol interference ISI from successive OFDM symbols is excluded. The length of the guard interval is thus defined by the physical channel and not by the waveform selection (e.g. OFDM or CDMA) . The term «cyclic» relates to the manner in which the guard interval is filled with signals. Since the FFT at the receiver requires a cyclic termination of the temporal receive signal for the correct reconstruction of the waveform generated by the IFFT at the transmitter, the time signal generated by the IFFT at the transmitter is continued cyclically by the length of the guard interval, in that the k time samples from the end of the signal are copied to the beginning. This allows a temporal displacement to be permitted within the guard interval at the receiver, without having impaired the cyclic characteristic of the received OFDM signal. In practice generally at least three times as many sub-carriers as channel taps (channel taps refer to the number of relevant time samples of the channel pulse response) are used, so that the effective channel per sub- carrier can be considered to be frequency-flat, in other words independent of frequency. As the number of sub-carriers increases, it is possible to reduce the capacity loss through the guard interval overhead while keeping the length of the guard interval constant. The guard interval is a type of unused redundancy, to achieve strict orthogonality of the sub-carriers and no ISI.

Commercial OFDM systems, e.g. WLAN according to IEEE 802.11a/g, operate with around 20 MHz bandwidth and approximately 80% of sub-carriers are used for data transmission. To increase data rates, either a number of antennas or more bandwidth can be used. With very large baseband signal bandwidths of for example 100 MHz or more it is difficult to achieve digital up and down- conversion, therefore direct up-conversion from the baseband and direct down-conversion to the baseband are good options for problem solving. When a data signal having a specific spectrum is mixed with an RF carrier frequency, cumulative and differential signals result in the frequency range, in other words after up-conversion a spectrum results, which is symmetrical about the carrier frequency.

When low-cost RF components are used, so-called signal imbalances result for the complex-valued elements of the baseband signal. These I/Q imperfections bring about a crosstalk of signals of the mirror frequencies in OFDM systems, see also [2] . In relation to FFT/IFFT we refer to the frequencies -f...+f, i.e. every sub-carrier has a «mirrored» equivalent on the other side of the zero carrier DC. The zeroth sub-carrier of the IFFT/FFT is always mapped precisely onto the mixed frequency of the local oscillator of the RF and represents the DC-Offst there. For this reason it can only receive a real-valued signal.

This applies both for direct up-conversion and direct down- conversion. This crosstalk is a function of the quality of the hardware components used and frequently determines the overall performance of the system in the case of low-cost solutions. In single-carrier systems I/Q imbalance produces crosstalk between the real and imaginary parts of the signal. This signal crosstalk between the real and imaginary parts can however be achieved simply by real-valued processing of the SISO or MIMO signals. This incurs an additional outlay by the factor 2, if matrix-vector multiplications are carried out in a real manner.

In OFDM systems signal distortions due to I/Q imbalances cannot be compensated for separately on each sub-carrier due to the mirror carrier crosstalk. It is possible to proceed with mirror carrier crosstalk in the same manner as with real-valued signal processing SP. Here the signals always have to be processed in a common manner on a carrier -f and its mirror carrier +f. This would also increase the SP outlay by the factor 2, however the MIMO matrices to be processed also grow by the factor 2 in every dimension and frequently used algorithms for calculating weighting matrices for MIMO equalization such as matrix inversion or QR decomposition have an outlay of ~ N3, so the computation outlay for double the edge length of the matrix is increased eightfold. Taking into account the fact that only half as many matrices have to be processed, this gives an additional outlay by the factor 4, which is not negligible in the case of broadband real-time processing (baseband signals - A -

with 100MHz signal bandwidth) and high numbers of antennas (more than 4 antennas on each side) .

The impact of the above-mentioned I/Q imperfections is shown in [I].

It is possible to dispense with mirror carrier suppression in signal processing (SP) as a function of the modulation used, if the phase and amplitude imbalance is small enough: a few degrees for phase and a few percent for amplitude. This is unrealistic for low-cost components.

The problem of mirror carrier crosstalk has been resolved as follows until now: i) In the case of small bandwidths of 20 MHz and therefore also low clock speeds in digital signal processing mirror carrier crosstalk can be avoided with digital up- conversion (DUC) and digital down-conversion (DDC) . ii) In the case of bigger bandwidths (e.g. 100 MHz and more) it is difficult, because of the limited clock speed in the FPGA and with the ADCs and DACs, therefore either expensive pre-compensated hardware components are used or additional signal processing (SP) has to be carried out in the baseband after appropriate calibration.

US 2005/0207333 Al [3] discloses a method and an apparatus for an enhanced OFDM-system, whereby subcarriers of the OFDM-system are multiplexed between at least a first and a second transmit antenna. The subcarriers are separated between the at least two transmit antennas, so that a subcarrier and a mirror subcarrier are not transmitted on the same transmit antenna and that adjacent subcarriers on the same antenna are at least two subcarrier frequency bandwiths apart. Due to imperfect hardware the mirror cross talk is a fact as explained above. This causes a distortion of trainigs sequencies as well as the data sent on a mirror frequency. As a consequence by transmitting a signal from an antenna 1 subcarriers which are not intended to be used contain already a signal. This signal causes a distortion of a signal sent from an antenna 2.

The object of the present invention is to avoid performance degradation due to mirror carrier crosstalk, without carrying out complex signal processing or using expensive hardware.

For a method of the type mentioned in the introduction this object is achieved by the features specified in claim 1.

The inventive solution, according to which

«the mirror carrier corresponding to a carrier loaded with data symbols (1, N) is not loaded with data symbols», avoids the performance degrading effect of mirror carrier crosstalk in that the respective mirror carrier has no signal and crosstalk does not occur. One advantage of this solution compared with a half-sideband solution is a very similar spectrum to that of full loading. Generally this means that any loading pattern is permissible on a sideband, as long as the other sideband is not «loaded».

The specified method is in principle suitable for all OFDM- based transmission methods. The WLAN market with low-cost solutions would be a major target area here as would modern cellular mobile radio standards, for example 3GPP-LTE, WiMAX etc., which are based on OFDM transmission. This allows access for the transmission band as a whole or frequency-symmetrical parts thereof to be operated in TDMA access. The inventive method, in other words the omission of mirror carriers during transmission, is particularly suitable for systems having no extreme requirements in respect of spectral efficiency, or to integrate low-cost, low-end devices simply into the existing network.

In one specific application a terminal can report by means of an agreed signaling bit, whether or not it wishes to be supported without a mirror carrier. This allows even so-called "low cost devices" to be deployed for broadband (i.e. bandwidth > 20MHz) or high-modulation (<16QAM) OFDM transmission.

This allows throughput to be increased, in that used bandwidth can be exchanged for higher modulation. Because certain frequency ranges may have a lower effective SNR in the frequency-selective channel, it is possible to dispense specifically with these ranges during transmission, possibly using the frequency-mirrored range with higher modulation and double output. The spectral efficiency achieved within the same bandwidth therefore does not necessarily have to be reduced by the omission of mirror carriers.

The invention avoids the drawback according to the method disclosed in [3] , by for at least one antenna a mirrorfree transmission and if at least two antennas carry out a transmission, there is no use of frequencies, which have been used by the at least one antenna.

Advantageous refinements of the invention are specified in further claims. The basic principles and an exemplary embodiment of the invention are described in more detail below with reference to the drawing, in which:

Figure 1 shows the principle of an OFDM transmission link; Figure 2A shows a first example of the carrier arrangement in the upper/lower sideband Figure 2B shows a second example of the carrier arrangement in the upper/lower sideband;

Figure 2C shows a third example of the carrier arrangement in the upper/lower sideband;

Figure 2D shows a fourth example of the carrier arrangement in the upper/lower sideband; Figure 2E shows a fifth example of the carrier arrangement in the upper/lower sideband; Figure 2F shows a sixth example of the carrier arrangement in the upper/lower sideband.

Figure 1 shows the principle of an OFDM transmission link: a word of length N having data symbols 1 is split in an S2P converter 10 into a parallel stream of data symbols 1. These data symbols, which are now parallel data symbols 1, are allocated in a carrier mapper 11 to the individual carriers located around the zero carrier DC. After mapping to suitable sub-carriers and the insertion of zeros on all the other carriers, an IFFT 12 effects a transformation to the time range and the orthogonality of the individual OFDM sub-carriers is ensured in the cyclic prefix insertion 13 with the so-called guard interval.

The different embodiments of the present invention are achieved by carrier assignment 11, in which the coded data symbols are allocated to specific carriers according to the inventive method.

To this end reference is made to the illustrations in the frequency range according to Figures 2A to 2F. The upper sideband Bu and lower sideband BL are disposed around the zero carrier DC (only shown thus in Figure 2A for reasons of simplicity) . The carriers are numbered +2, +4, etc. for even numbers in the upper side band BL and -1, -3, etc. in the lower sideband BL from the zero carrier out. According to Figure 2A, which represents a first embodiment of the present invention, to avoid mirror crosstalk in the upper sideband By carriers with an even number and in the lower sideband BL carriers with an odd number are provided with data symbols.

A particular embodiment of the present invention results according to Figure 2B with a one-sideband modulation, where all the carriers are loaded with data symbols in just one sideband.

A development of the assignment is shown in Figure 2C, where a specific range in a sideband is not used, with carriers with consecutive numbering being loaded with data symbols in the correspondingly mirrored range in the other sideband instead.

The above-mentioned assignment of carriers to the sidebands BU and BL is particularly suitable for the use of high modulation formats, e.g. 64 qam or higher.

Figures 2D to 2F show a carrier assignment to the sidebands, which are particularly advantageous for multi-user access, because different users can use different quality classes of the RF interface; because all the carriers can be used for expensive hardware or with more complex signal processing. It should be anticipated that mirror carrier crosstalk should be avoided for so-called low-cost hardware. The advantages of the different embodiments of the present invention are summarized once again below:

1. Mirror carrier crosstalk during FFT/IFFT processing is excluded, since the mirror carrier pendant is not modulated in each instance.

2. Very high modulation formats can be used, for example 64/256 QAM, which would not be possible in conventional single-carrier methods due to the I/Q imbalance. When OFDM with perfect mirror carrier suppression is used, there are no restrictions, since I/Q imbalances do not impact on the individual carriers .

3. When the entire bandwidth is used the entire frequency- diversity of the frequency-selective channel can be exploited. This is particularly relevant in respect of very wide frequency bands, e.g. UWB.

4. Channel-adaptive transmission with high modulation formats can in principle be supported with the single restriction that the mirror carriers of very good frequency resources must remain unused. In the case of an SINR with one-sided degradation the full throughput is maintained as in the system without mirror carriers, since the degraded resources would not be used anyway due to adaptive resource management (including adaptive modulation and coding (AMC)) . 5. When only every second sub-carrier is used, a higher tolerance results in respect of Doppler shifts (relevant at high speeds) .

6. A UE can signal by means of an agreed bit, whether it is able to transmit or receive the full OFDM spectrum with corresponding quality or whether it is to be supported with the mirror carrier-free transmission version.

7. The principle can be extended without restriction to MIMO.

8. The output per carrier can be doubled with mirror carrier- free transmission, whilst maintaining the transmit output emitted as a whole and as a result may possibly be used directly for a higher modulation order. 9. The method can be operated in combination with full-band utilization, if certain frequency-symmetrical bands are used for mirror carrier-free transmission.

To implement the present method, a terminal or device must be able to signal such signal mapping requirements to a base station or vice versa, since even RF components which are not OFDM-capable can then deal with OFDM signals, without the performance on the sub-carriers used suffering. The bit enabling such signaling should preferably be agreed by means of a standard. This also ensures simple verifiability of the use of the present method, since all standard-compatible devices/terminals not only have to be able to transmit this bit but also have to be able to evaluate this information correctly in the receive direction.

List of reference characters used; glossary

1 Data symbols

10 Serial to parallel converter, S2P converter

11 Carrier mapper; carrier assignment

12 Inverse Fast Fourier Transformation

13 Cyclic prefix insertion (guard interval)

B_L Lower sideband

Bu Upper sideband

DC Zero carrier, carrier frequency

PB Part of a sideband; fixed frequency range of a sideband; part of an upper or lower corresponding sideband, "partial Band"

N Number of data symbols

List of acronyms used

CDMA Carrier Division Multiple Access

DDC Digital Down-Conversion

DUC Digital Up-Conversion

FFT Fast Fourier Transform I/Q In-phase component & quadrature component (real & imaginary part)

IEEE the Institute of Electrical and Electronics Engineers, Inc.

IFFT Inverse Fast Fourier Transform ISI Inter-symbol interference

MIMO Multiple Input Multiple Output

SC-FDMA Single Carrier Frequency Division Multiple Access

SISO Single Input Multiple Output

OFDM Orthogonal Frequency Division Multiplexing QR Decomposition of a matrix H into a unitary matrix Q and a upper right triangular matrix R.

RF Radio Frequency

SINR Signal to Interference plus Noise Ratio; SP Signal Processing

Bibliography http: //www. vodafone-chair . com

[1] Dirty RF: A New Paradigm Gerhard Fettweis et al.

Technische Universitat Dresden, Vodafone Chair Mobile Communications Systems; D-01062 Dresden. Published under http://www.vodafone-chair.com

[2] Performance Degradation de to I/Q Imbalance in Multi- Carrier Direct Conversion Receivers:

A Theoretical Analysis

Marcus Windisch, Gerhard Fettweis

Dresden University of Technology; Vodafone Chair Mobile

Communications Systems; D-01062 Dresden. Published under http://www.vodafone-chair.com

[3] US 2005/0207333 Al

«Method and apparatus for an enhanced OFDM SystemRon Roststein et al . »

Correspondance Address: Motorla, Inc Schaumburg, IL 60196.

Claims

1. A method for avoiding mirror crosstalk in an OFDM system, wherein a source signal to be transmitted consisting of data symbols (1, N) is split before the Inverse Fast Fourier Transform (IFFT) to be applied into a number of disjoint - orthogonal in the frequency sense - carriers (11) and then transmitted in a parallel manner by way of different frequencies, characterized in that the mirror carrier corresponding to a carrier loaded with data symbols (1, N) is not loaded with data symbols and the transmission takes place over at least one antenna.

2. The method as claimed in claim 1, characterized in that transmission takes place by way of frequencies in two sidebands (Bu, BL), with carriers with even carrier numbers (+2, +4, +6, .. ) being used in one sideband (Bu, BL) and carriers with odd carrier numbers (-1, -3, -5, .. ) being used in the other sideband (Bu, B_L) .

3. The method as claimed in claim 2, characterized in that carrier numbers in both sidebands (Bu, BL) are consecutive (-1, -3, -5, ..; +2, +4, +6, ..) in each instance.

4. The method as claimed in claim 1, characterized in that transmission only takes place by way of frequencies in one sideband (Bu, B_L) with carriers (+1, +2, +3, ..).

5. The method as claimed in claim 4, characterized in that carriers with consecutive carrier numbers (+1, +2, +3, ..) are used.

6. The method as claimed in claim 1, characterized in that transmission takes place by way of frequencies in two sidebands (Bu, BL) , with carriers with consecutive carrier numbers (+1, +2, +3, ..) being used in a fixed frequency range (pB) of one sideband (By, BL) .

7. The method as claimed in claim 1, characterized in that transmission takes place by way of frequencies only in a fixed frequency range (pB) of one sideband (Bu, BL) and the corresponding frequency range (pB) of the other sideband (Bu, B_L ⁾.

8. The method as claimed in claim 1, characterized in that transmission takes place by way of frequencies only in a fixed frequency range (pB) of one sideband (Bu, BL) .

9. The method as claimed in claim 8, characterized in that all the carriers in the fixed frequency range (pB) of the one sideband (Bu, BL) are loaded with data symbols.

10. The method as claimed in one of claims 1 to 9, characterized in that a device using this method signals in at least one bit to a partner device that it is able to receive or transmit in a mirror carrier-free manner.