WO2007040952A2

WO2007040952A2 - A transmitter, cellular communication system and method of transmitting therefor

Info

Publication number: WO2007040952A2
Application number: PCT/US2006/036029
Authority: WO
Inventors: Nick W. Whinnett
Original assignee: Motorola, Inc.
Priority date: 2005-10-03
Filing date: 2006-09-14
Publication date: 2007-04-12
Also published as: WO2007040952A3; GB2430844B; GB2430844A; GB0520099D0

Abstract

A transmitter comprises functionality (101) for receiving a sequence of input modulation symbols. An M-point discrete Fourier transform (105) is applied to the block of input modulation symbols resulting in a frequency domain symbol block. This block is fed to an N-point inverse discrete Fourier transform (105) (N>M) thereby generating a time domain transmit signal. In addition, the transmitter (200) comprises a phase rotation processor (201) which phase rotates the input modulation symbols in multi-symbol intervals. The phase rotations applied within each interval are constrained in accordance with a first phase rotation constraint requirement whereas the phase rotations between consecutive symbols belonging to different intervals are constrained in accordance with a different phase rotation constraint requirement. The invention may allow interference mitigation by reducing alignment between quadrature channels of the transmitter and interferers.

Description

A TRANSMITTER, CELLULAR COMMUNICATION SYSTEM AND METHOD

OF TRANSMITTING THEREFOR

Field of the invention

The invention relates to a transmitter, cellular communication system and method of transmitting therefor and in particular, but not exclusively, to a transmitter for a cellular communication system.

Background of the Invention

Cellular communication systems have become an increasingly important part of the communication infrastructure of many countries. Currently, second generation cellular communication systems, such as the Global System for Mobile communication (GSM) , is the most widespread technology for supporting mobile telephony and data communication. Furthermore, in recent years, third generation cellular communication systems, such as the Universal Mobile Telecommunication System (UMTS) , have been rolled out in many places to provide additional and enhanced communication services .

In order to continuously improve and enhance the communication services that can be provided, significant amounts of research and development are undertaken. For example, although third generation cellular communication systems are still in the process of the initial roll out, work is already undergoing in developing and standardising further enhancements. Specifically, the 3rd Generation Partnership Project (3GPP) , which is the standardisation body responsible for defining the third generation cellular communication systems (including UMTS) , is already considering new technologies for improved air interface communications . This work is 5 undertaken under the working title of E-UTRA (Evolved- UMTS Terrestrial Radio Access) .

A promising air interface technique proposed for E-UTRA is known as Discreet Fourier Transform-Spread Orthogonal 10 Frequency Division Multiplex (DFT-SOFDM) . In particular, DFT-SOFDM has been proposed for the uplink transmissions of E-UTRA.

FIG. 1 illustrates an example of a DFT-SOFDM transmitter 15 in accordance with prior art. The transmitter is arranged to receive a number of data bits in a serial-to-parallel converter 101 that converts the data into suitable groups . Each of the groups of data bits are then mapped into a modulation symbol by bit-to-constellation mappers 20 103. The modulation symbols have an order that corresponds to the number of data bits in each group.

The output of the bit-to-constellation mappers 103 consists in blocks of M modulation symbols. Each block of

25 M modulation symbols is fed to an M-point Discrete

Fourier Transform (DFT) 105 which specifically can be a Fast Fourier Transform (FFT) . The output of the DFT 105 consists in M frequency domain data values corresponding to the M input modulation symbols .

30

The M frequency domain data values are fed to an N-point Inverse Discrete Fourier Transform (IDFT) 107 which specifically can be an Inverse Fast Fourier Transform (IFFT) . N is larger than M and thus the M frequency domain data values are fed to a subset of M subcarriers out of the N subcarriers of the IDFT 107. The remaining N-M subcarriers are set to zero .

The output of the IDFT 107 corresponds to a time domain transmit signal which can be transmitted without modification. However, in the transmitter of FIG.l the time domain transmit signal is fed to a cyclic prefix processor 109 which adds a cyclic prefix as is well known from e.g. OFDM transmitters.

The overall effect of the DFT 105 and the IDFT 107 corresponds to an upsampling and frequency shift of the time domain signal made up of the input modulation symbols .

DFT-SOFDM has a number of advantages including reduced amplitude variations compared to basic OFDM; efficient implementation of transmitter and receiver processing by means of FFT/IFFT algorithms; high spectral efficiency due to lack of roll-off in the frequency response; and ability to position the M frequency subcarriers flexibly within the N available sub-carriers, which allows advanced techniques such as frequency domain scheduling to be employed.

However, although one of the advantages of DFT-SOFDM is that the amplitude variations may be reduced in comparison to a basic OFDM solution, it is still higher than that of many modulation techniques and results in the requirement for transmit power amplifiers to be significantly backed-off thereby resulting in reduced efficiency and transmit power and/or increased distortion.

A suitable measure for the amplitude variation and required power amplifier back-up is the Peak to Average Ratio (PAR) which is typically used to characterise the amplitude variation characteristic. A measure of the amplitude variation which tends to more closely reflect the required amplifier back-off is the Cubic Metric (CM) measure .

It has been proposed that π/2 BPSK modulation can be used for uplink communication from power limited User

Equipments in E-UTRA. π/2 BPSK refers to a BPSK signal which is rotated by 90 degrees between each modulation symbol. In the context of the DFT-SOFDM transmitter of FIG. 1, these π/2 BPSK symbols may form the input to the first DFT 105. The data rate achievable with π/2 BPSK is less than that of QPSK but the PAR/CM performance is very good resulting in a reduced power amplifier back-off.

However, the Inventor has realised that the use of π/2 BPSK introduces some disadvantages. In particular, the Inventor has realised that the use of BPSK may result in the possibility that different User Equipments may align in the IQ domain thereby degrading the interference averaging properties between the User Equipment.

For example, a first User Equipment can in a first symbol time transmit a first BPSK symbol which is received in the I-channel, the next symbol is received in the Q- channel, the following in the I-channel etc. Furthermore, an interfering User Equipment may also transmit a symbol which is received in the I-channel, followed by the Q- channel followed by the I-channel etc. Thus, if the phase of these two User Equipments is such that their respective I- and Q-channels align, the interference caused will also be aligned and will remain aligned for all transmitted symbols. This can substantially reduce the communication performance and can in particular increase the error rate significantly.

Hence, an improved transmitter system would be advantageous and in particular a system allowing increased flexibility, low amplitude variation, low power amplifier back-off, high efficiency, improved interference performance, increased communication quality, reduced error rate and/or improved performance would be advantageous .

Summary of the Invention

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to an aspect of the invention there is provided a transmitter comprising: means for receiving a sequence of input modulation symbols ; means for performing an M- point discrete Fourier transform on the sequence of input modulation symbols to generate a frequency domain symbol block; means for performing an N-point inverse discrete Fourier transform on the frequency domain block to generate a time domain transmit signal, N being an integer larger than M; and means for phase rotating the input modulation symbols in multi-symbol intervals wherein phase rotations within each interval are constrained in accordance with a first phase rotation constraint requirement and phase rotations between consecutive symbols belonging to different intervals are constrained in accordance with a different phase rotation constraint requirement.

The invention may allow improved performance of a transmitter. In particular, the invention may allow improved interference performance in a communication system. Specifically, the correlation and possible alignment between I- and Q-channel transmissions of BPSK signals may be substantially removed. The invention may provide improved error performance of communications . Furthermore, low complexity and easier implementation may be achieved. In addition, the invention may allow low amplitude variations and in particular low peak-to- average values .

The first phase rotation constraint or the different phase rotation constraint may correspond to the symbol phase not being constrained. The phase rotation of an input modulation symbol may be associated with a scaling of the input modulation symbol .

It will be appreciated that the frequency domain symbol block may be modified or processed before being applied to the means for performing an N-point inverse discrete Fourier transform (for example pulse shaping may be applied) .

According to an optional feature of the invention, the first phase rotation constraint comprises selecting a relative phase rotation between consecutive symbols from the group consisting of: a. π/2; and b. - π/2.

This may provide particularly advantageous performance and/or implementation. In particular, low amplitude variation and low error rates may be achieved. The system may be compatible with a π/2 BPSK modulation scheme.

According to an optional feature of the invention, the second phase rotation constraint comprises selecting a relative phase rotation between consecutive symbols from the group consisting of: a. 0; b. π/2. a . π; and b. - π/2.

This may provide particularly advantageous performance and/or implementation. In particular, low amplitude variation and low error rates may be achieved. In particular, the feature may allow for an efficient mitigation of the possibility of the alignment between I- and Q-channels of different transmitters while allowing simple I-and Q-based processing in each transmitter. According to an optional feature of the invention, the multi-symbol intervals comprise M input modulation symbols .

The multi-symbol interval may correspond to size of the DFT used by the transmitter. This may facilitate implementation.

According to an optional feature of the invention, the multi-symbol intervals are aligned with data blocks of the M-point discrete Fourier transform.

The multi-symbol intervals may be aligned with the DFT block processing of the transmitter. This may facilitate implementation.

According to an optional feature of the invention, the input modulation symbols are Binary Phase Shift Keying (BPSK) modulation symbols.

The invention may provide improved performance and may in particular allow BPSK modulation symbols to be used while maintaining a high resistance to interference due to quadrature alignment.

According to an optional feature of the invention, phase rotations are constant amplitude phase rotations.

This may facilitate implementation and/or may provide advantageous performance. According to an optional feature of the invention, the transmitter is a Discrete Fourier Transform- Spread Orthogonal Frequency Domain Multiplex (DFT-SOFDM) transmitter .

The invention may in particular allow an improved DFT- SOFDM transmitter.

According to an optional feature of the invention, each interval comprises at least four input modulation symbols .

An improved performance, facilitated operation and/or facilitated implementation may be achieved by having relatively long intervals.

According to another aspect of the invention, there is provided a cellular communication system comprising a transmitter, the transmitter comprising: means for receiving a sequence of input modulation symbols; means for performing an M-point discrete Fourier transform on the sequence of input modulation symbols to generate a frequency domain symbol block; means for performing an N- point inverse discrete Fourier transform on the frequency domain block to generate a time domain transmit signal, N being an integer larger than M; and means for phase rotating the input modulation symbols in multi-symbol intervals wherein phase rotations within each interval are constrained in accordance with a first phase rotation constraint requirement and phase rotations between consecutive symbols belonging to different intervals are constrained in accordance with a different phase rotation constraint requirement.

According to an optional feature of the invention, the transmitter is an uplink transmitter.

The invention may allow particularly improved uplink performance in a cellular communication system.

According to an optional feature of the invention, the cellular communication system comprises a plurality of transmitters arranged to apply different phase rotations between consecutive symbols belonging to different intervals .

This may allow improved performance and may in particular allow a reduced quadrature alignment and thus reduced interference between the different transmitters .

According to another aspect of the invention, there is provided a method of transmitting comprising: receiving a sequence of input modulation symbols; performing an M- point discrete Fourier transform on the sequence of input modulation symbols to generate a frequency domain symbol block; performing an N-point inverse discrete Fourier transform on the frequency domain block to generate a time domain transmit signal, N being an integer larger than M; and phase rotating the input modulation symbols in multi-symbol intervals wherein phase rotations within each interval are constrained in accordance with a first phase rotation constraint requirement and phase rotations between consecutive symbols belonging to different intervals are constrained in accordance with, a different phase rotation constraint requirement.

These and other aspects , features and advantages of the invention will be apparent from and elucidated with reference to the embodiment (s ) described hereinafter.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 illustrates an example of a DFT-SOFDM transmitter in accordance with prior art; and

FIG. 2 illustrates a DFT-SOFDM transmitter 200 in accordance with some embodiments of the invention.

Detailed Description of Some Embodiments of the Invention

The following description focuses on embodiments of the invention applicable to a cellular communication system but it will be appreciated that the invention is not limited to this application but may be applied in many other communication systems .

FIG. 2 illustrates a DFT-SOFDM transmitter 200 in accordance with some embodiments of the invention. The transmitter 200 is specifically a transmitter of a remote terminal of a cellular communication system and is transmitting data to a base station of the cellular communication system using a suitable uplink air interface communication channel .

The transmitter 200 is a modified version of the prior art transmitter of FIG. 1 and comprises a serial-to- parallel a converter 101, an M-point DFT 105, an N-point IDFT 107 (wherein N is larger than M) and a cyclic prefix processor 109 as will be well known to the person skilled in the art and which have already been described with reference to FIG. 1.

In contrast to the transmitter of FIG. 1, the serial-to- parallel converter 101 of the transmitter 200 of FIG. 2 is directly coupled to the DFT 105. In the example, the transmitter 200 receives BPSK symbols which are phase rotated in a phase rotation processor 201. The phase rotated symbols are then fed to the serial-to-parallel converter 101 and become the input modulation symbols for the DFT 105.

In the transmitter 200 of FIG. 2, the phase rotation processor 201 comprises two cascaded phase rotators 203, 205. The two phase rotators 203, 205 are specifically complex multipliers which multiply an incoming complex BPSK symbol by a complex value having unity amplitude.

It will be appreciated that in other embodiments, the complex values may have a non-unity amplitude and may in addition to rotating the phase of the symbol also scale the amplitude. This may be beneficial in some embodiments , for example to reduce the amplitude variations of the time domain transmit signal.

The two phase rotators 203, 205 of the phase rotation 5 processor 201 are arranged to rotate the phase of the incoming BPSK symbols in accordance with different phase rotation constraints.

In the specific example, the first phase rotator 203 is 10 arranged to operate with the constraint that for every consecutive symbol an additional phase rotation of π/2 or -π/2 relative to the phase rotation of the previous symbol is achieved. Thus, the first phase rotator 203 results in the alignment of the BPSK symbol with the 15 quadrature changes between the I- and Q- channel for alternating symbols.

As a specific example, the first phase rotator 203 can multiply the incoming BPSK signals by a sequence of 20 complex values corresponding to:

..., j , 1 , j , 1 , j , 1 , j , 1 , j , 1 , j , 1 , j , 1 , j , 1, ...

In this example, the output of the first phase rotator 25 203 thus corresponds to a π/2 BPSK modulated signal.

The second phase rotator 205 is arranged to multiply the π/2 BPSK by a sequence of phase rotation symbols which only changes every K' th symbol , where K is an integer 30 that is larger than two and preferably significantly larger . Thus, the second phase rotator 205 applies a phase rotation which is constant within the πvulti-symbol intervals of the π/2 BPSK sequence but which changes between these intervals. Thus, the operation of the second phase rotator 205 can be considered to correspond to the application of a phase rotation scrambling code with a rate of 1/K of the BPSK symbol rate.

For example, the second phase rotator 205 can multiply the incoming π/2 BPSK signals by a sequence of complex values corresponding to :

..., j,j,j,j,j, j,j,j,j,j, j,j,l, 1,1, 1,1, 1,1, 1,1, 1,1, 1,-1,-1, -

1,-1,-1,-1,-1,-1,-1,-1,-1,-1,...

etc corresponding to a scrambling code of (...j,l,-l, ...) and K=12.

Furthermore, the phase rotations introduced by the second phase rotator 205 may use other phase rotation values than used by the first phase rotator 203. For example, the scrambling code of the second phase rotator 205 can specifically use all four of the symbols in the set of (l,j,-l,-j) .

It will be appreciated that by using the phase rotations corresponding to the real and imaginary axes , a substantially facilitated operation can be achieved and that in particular, the complex multiplications can be reduced to simple data move operations and sign inversions (e.g. swapping the real and imaginary values and potentially inverting the sign) . Alternatively, the scrambling code of the second phase rotator 205 can specifically use all four of the symbols in the set of (1+j , 1-j , -1+j , -1-j ) .

It will be appreciated that the overall effect of the first phase rotator 203 and the second phase rotator 205 is that the symbols fed to the serial-to-parallel converter 101 are divided into intervals of K symbols in which the phase rotations are constrained by the π/2 BPSK modulation (i.e. by an alternating ± π/2 phase rotation.

Furthermore, for every K' th symbol, i.e. for the interval boundaries, a phase rotation is introduced which is not constrained by the π/2 BPSK modulation. Indeed, for the example of FIG. 2, the phase rotation of every K' th symbol can be any phase rotation from the group of 0 ; π/2, π, - π/2.

Thus, within each interval of K symbols, the transmitted symbols are aligned with the I-and Q-channel in accordance with π/2 BPSK modulation. This, alignment is changed every K' th symbol in accordance with a given phase rotation scrambling code. Thus, the interference caused by an alignment of the I-and Q-channel for different transmitters can be substantially reduced resulting in improved performance and reduced error rates .

The 1/K. rate phase rotation scrambling code can be selected to be different for different user equipments of the cellular communication system. This will cause the phase rotations to be different for different user equipment as they will only be the same if the current phase rotation scrambling code symbols are the same. Thus, when the signals from different transmitters are received at the base station, the different scrambling codes will result in the phase alignment being limited to short intervals typically of only a few Ks of symbols or less .

In the transmitter 200, the intervals of symbols are selected to align with the blocks of the DFT 105. Thus, K is set equal to M and the start of each interval is set to coincide with the boundaries of the blocks which are fed to the DFT 105. Thus, in this example, the π/2 BPSK phase rotation is applied to the transitions between the M input modulation symbols in each block processed by the DFT 105 whereas the additional phase rotation is applied between the different blocks. This may allow efficient performance and may facilitate implementation.

The described approach may be particularly advantageous for the application of DFT-SOFDM type modulation schemes in cellular communication systems, such as e.g. proposed for E-UTRA.

For E-UTRA, the uplink intra-cell interference should be largely eliminated for the DFT-SOFDM type modulation schemes since a user will be assigned time and frequency resources which are non-overlapping with other users in the cell . However inter-cell interference will not be suppressed and must therefore be taken into consideration. However, as inter-cell interference is likely to be lower than intra-cell interference, the impact of the alignment of the quadrature channels of a wanted transmitter and an interferer is likely to be reduced. Accordingly, although it is desirable to avoid prolonged periods where transmitters can be aligned in the IQ domain, it is not typically necessarily to apply a QPSK-like phase transition frequently such as every or every other symbol time .

Rather, in the example of the transmitter 200, the intervals are relatively long with the second phase rotator 205 only occasionally adding an extra phase offset to combat any quadrature alignment. Specifically, K can be set to four or higher to provide particularly advantageous performance. Indeed in the example of the transmitter 200 of FIG. 2, the majority (e.g. 15 out of 16) of phase transitions between modulation symbols are π/2 BPSK phase transitions with only an occasional QPSK- like phase rotation (e.g. 1 out of 16) . This scheme is- - referred to as M-HPSK, where in this example M=I6.

By reducing the number of additional phase transitions caused by the second phase rotator 205, a reduced amplitude variation (and in particular reduced PAR at a given statistical probability (e.g. amplitude not exceeded 99.9% of the time), and CM) can be achieved since a higher proportion of phase transitions between symbols are constrained to +/- π/2 (+/- 90 degrees) .

Simulations have been performed which compares the PAR and CM for π/2 BPSK and M-HPSK for a DFT-SOFDM transmitter. The results are indicated in the following table:

The simulations indicate that 16-HPSK offers similar PAR/CM performance to π/2 BPSK but with added inter-cell interference protection.

It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization. The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors . The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors .

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps .

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims does not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order.

Claims

1. A transmitter comprising: means for receiving a sequence of input modulation symbols; means for performing an M-point discrete Fourier transform on the sequence of input modulation symbols to generate a frequency domain symbol block; means for performing an N-point inverse discrete Fourier transform on the frequency domain block to generate a time domain transmit signal, N being an integer larger than M; and means for phase rotating the input modulation symbols in multi-symbol intervals wherein phase rotations within each interval are constrained in accordance with a first phase rotation constraint requirement and phase rotations between consecutive symbols belonging to different intervals are constrained in accordance with a different phase rotation constraint requirement.

2. The transmitter of claim 1 wherein the first phase rotation constraint comprises selecting a relative phase rotation between consecutive symbols from the group consisting of: a. π/2; and b. - π/2.

3. The transmitter of claim 1 or 2 wherein the second phase rotation constraint comprises selecting a relative phase rotation between consecutive symbols from the group consisting of: a . 0 ; b . π/2 . a . π ; and b . - π/2 . 5

4. The transmitter of any previous claim wherein the multi-symbol intervals comprise M input modulation symbols .

10 5. The transmitter of any previous claim wherein the multi-symbol intervals are aligned with data blocks of the M-point discrete Fourier transform.

6. The transmitter of any previous claim wherein the 15 input modulation symbols are Binary Phase Shift Keying

(BPSK) modulation symbols.

7. The transmitter of any previous claim wherein phase rotations are constant amplitude phase rotations.

20

8. The transmitter of any previous claim wherein the transmitter is a Discrete Fourier Transform- Spread Orthogonal Frequency Domain Multiplex (DFT-SOFDM) transmitter .

25

9. The transmitter of any previous claim wherein each multi-symbol interval comprises at least four input modulation symbols .

30 10. A method of transmitting comprising: receiving a sequence of input modulation symbols; performing an M-point discrete Fourier transform on the sequence of input modulation symbols to generate a frequency domain symbol block; performing an N-point inverse discrete Fourier transform on the frequency domain block to generate a time domain transmit signal, N being an integer larger than M; and phase rotating the input modulation symbols in multi-symbol intervals wherein phase rotations within each interval are constrained in accordance with a first phase rotation constraint requirement and phase rotations between consecutive symbols belonging to different intervals are constrained in accordance with a different phase rotation constraint requirement.