This application claims the benefit of priority of Korean Patent application No. 10-2011-0064163 filed on Jun. 30, 2011, which is incorporated by reference.
1. Field of the Invention
The present invention relates to wireless communication and, more particularly, to a method and apparatus for demodulating a downlink signal in a wireless communication system.
2. Related Art
Effective transmission and reception schemes for broadband wireless communication systems and methods for utilizing the schemes have been proposed in order to maximize the efficiency of limited radio resources. One of systems taken into consideration in the next-generation wireless communication system is an orthogonal frequency division multiplexing (OFDM) system capable of attenuating an inter-symbol interference (ISI) effect with low complexity. In the OFDM system, data symbols received in series are transformed into N parallel data symbols, carried on respective subcarriers, and then transmitted. The subcarriers maintain orthogonality in the frequency dimension. The orthogonal channels experience independent frequency selective fading. Accordingly, ISI can be minimized because complexity in a receiving stage is reduced and spacing between the transmitted symbols is lengthened.
Cooperative communication may be performed in a wireless communication system. The cooperative communication method is a method in which several neighboring nodes detect signals propagated through radio channels and improve the performance of a wireless communication system by using the detected signals to the maximum extent. Nodes, such as a repeater, a small-sized femto base station, and a mobile station, can perform cooperative communication, such as simple cooperative relaying (SCR) performing only a relay function or mutually cooperative relaying (MCR) performing transmission and relay functions between the nodes.
Meanwhile, inter-symbol interference (ISI) may occur in an OFDM system. ISI is one of distortion phenomena of a signal in which one symbol and a symbol subsequent to the one symbol interfere with each other. When ISI is generated, one symbol may act as noise for a subsequent symbol. In general, ISI may be generated by multi-path propagation or a non-linear frequency response unique to a channel. ISI generates an error in the decision device of a receiving stage. Accordingly, various efforts are being made to reduce the influence of ISI.
In a cooperative communication system, a mobile station may receive signals from a plurality of nodes. The signals received from the plurality of nodes may have different delays. The intensity of a reception signal having longer propagation delay may be stronger because some of the signals may be received through a relay station (RS). Accordingly, an ISI phenomenon according to multi-path propagation may occur.
There is a need for an efficient method of reducing ISI in a cooperative communication system.
The present invention provides a method and apparatus for demodulating a downlink signal in a wireless communication system. In particular, the present invention provides a method of selecting a downlink demodulation section so that optimal performance can be achieved according to a cooperative communication mode when a mobile station receives signals having various reception signal intensities and various propagation delays in an OFDM system in which cooperative communication is performed.
In an aspect, a method of demodulating, by a user equipment, a downlink signal in a wireless communication system is provided. The method includes receiving a first orthogonal frequency division multiplexing (OFDM) signal from a first node serving the user equipment, receiving a second OFDM signal from a second node different from the first node, and demodulating the first OFDM signal and the second OFDM signal in a fast Fourier transform (FFT) section, wherein the first OFDM signal and the second OFDM signal are either normal mode signals each having a first cyclic prefix (CP) or cooperative mode signals each having a second CP, and the second CP has a longer length than the first CP.
If the first OFDM signal and the second OFDM signal are the normal mode signals, the FFT section may be set based on a demodulation timing of the first OFDM signal.
If the first OFDM signal and the second OFDM signal are the cooperative mode signals, the FFT section may be set based on a demodulation timing of the first OFDM signal.
The method may further include whether the second OFDM signal is received anterior to the first OFDM signal.
The first OFDM signal may be received via a relay station.
If the second OFDM signal is received anterior to the first OFDM signal and the first OFDM signal and the second OFDM signal are the cooperative mode signals, the FFT section may be set based on a demodulation timing of the second OFDM signal.
If the first OFDM signal may be received anterior to the second OFDM signal or the first OFDM signal and the second OFDM signal are the normal mode signals, the FFT section is set based on a demodulation timing of the first OFDM signal.
In another aspect, a user equipment in a wireless communication system is provided. The user equipment includes a radio frequency (RF) unit configured to transmit or receive a radio signal, and a processor connected to the RF unit, wherein the processor is configured to receive a first orthogonal frequency division multiplexing (OFDM) signal from a first node serving the user equipment, receive a second OFDM signal from a second node different from the first node, and demodulate the first OFDM signal and the second OFDM signal in a fast Fourier transform (FFT) section, wherein the first OFDM signal and the second OFDM signal are either normal mode signals each having a first cyclic prefix (CP) or cooperative mode signals each having a second CP, and the second CP has a longer length than the first CP.
Hereinafter, some embodiments of the present invention are described in detail with reference to the accompanying drawings in order for those skilled in the art to be able to readily implement the invention. However, the present invention may be modified in various different forms and are not limited to the following embodiments. In order to clarify a description of the present invention, parts not related to the description are omitted, and the same reference numbers are used throughout the drawings to refer to the same or like parts. Furthermore, a description of parts which may be easily understood by those skilled in the art is omitted.
In the entire specification and claims, when it is said that any element “includes (or comprises)” any element, it means the corresponding element does not exclude other elements other than the corresponding element and may further include other elements which fall within the scope of the technical spirit of the present invention.
The following technologies may be used in a variety of multiple access schemes, such as code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), and single carrier frequency division multiple access (SC-FDMA). CDMA may be implemented using radio technology, such as universal terrestrial radio access (UTRA) or CDMA2000. TDMA may be implemented using radio technology, such as global system for mobile communications (GSM)/general packet radio service (GPRS)/enhanced data Rates for GSM evolution (EDGE). OFDMA may be implemented using radio technology, such as institute of electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, or evolved UTRA (E-UTRA). IEEE 802.16m is an evolution of IEEE 802.16e, and it provides backward compatibility with systems based on IEEE 802.16e. UTRA is part of a universal mobile telecommunications system (UMTS). 3rd generation partnership project (3GPP) long term evolution (LTE) is part of an evolved UMTS (E-UMTS) using E-UTRA, and it adopts OFDMA in downlink and adopts SC-FDMA in uplink. LTE-advanced (LTE-A) is an evolution of LTE.
The wireless communication system 10 includes one or more base stations (BSs) 11. The BSs 11 provide communication services to respective geographical areas (commonly called cells) 15a, 15b, and 15c. Each of the cells may be divided into a plurality of areas (also called sectors). User equipment (UE) 12 may be fixed or mobile and may also be called another terminology, such as a mobile station (MS), a mobile terminal (MT), a user terminal (UT), a subscriber station (SS), a wireless device, a personal digital assistant (PDA), a wireless modem, or a handheld device. The BS 11 commonly refers to a fixed station communicating with the UEs 12, and it may also be called another terminology, such as an evolved NodeB (eNB), a base transceiver system (BTS), or an access point.
UE commonly belongs to one cell. The cell to which the UE belongs is called a serving cell. A BS providing the serving cell with communication service is called a serving BS. Another cell neighboring the serving cell exists because a wireless communication system is a cellular system. The cell neighboring the serving cell is called a neighbor cell. A BS providing the neighbor cell with communication service is called a neighbor BS. The serving cell and the neighbor cell are relatively determined on the basis of UE.
This technology may be used in downlink and uplink. In general, downlink refers to communication from the BS 11 to the UE 12, and uplink refers to communication from the UE 12 to the BS 11. In downlink, a transmitter may be part of the BS 11, and a receiver may be part of the UE 12. In uplink, a transmitter may be part of the UE 12, and a receiver may be part of the BS 11.
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In a cooperative communication system, UE may receive signals from a plurality of nodes. The signals received from the plurality of nodes may have different delays, and some of the signals may be received through an RS. For this reason, a reception signal having a longer propagation delay may have a stronger intensity. Accordingly, in order to prevent ISI, the length of the CP needs to be longer in the cooperative mode in which cooperative communication is performed than in the normal mode. Furthermore, the length of the CP needs to be flexibly changed according to a mode.
Accordingly, in a cooperative communication system, each node can change the length of the CP and the amount of transmission power according to the transmission mode, such as the normal mode or the cooperative mode. A proposed method of demodulating a downlink signal is described below in connection with embodiments.
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For example, ISI with a previously received signal may be prevented by setting an FFT section 703 on the basis of the demodulation timing of the first node (i.e., a serving node) in demodulating the normal mode signal. Furthermore, ISI may be prevented by setting an FFT section 704 on the basis of the demodulation timing of an OFDM signal foremost received, from among OFDM signals received from a plurality of nodes, in demodulating the cooperative mode signal. In
A BS 800 includes a processor 810, memory 820, and a radio frequency (RF) unit 830. The processor 810 implements the proposed functions, processes, or methods or all of them. The layers of a wireless interface protocol may be implemented by the processor 810. The memory 820 is connected to the processor 810 and configured to store various pieces of information for driving the processor 810. The RF unit 830 is connected to the processor 810 and configured to transmit or receive a radio signal or to transmit and receive radio signals.
UE 900 includes a processor 910, memory 920, and an RF unit 930. The processor 910 implements the proposed functions, processes, methods, or all of them. The layers of a wireless interface protocol may be implemented by the processor 910. The memory 920 is connected to the processor 910 and configured to store various pieces of information for driving the processor 910. The RF unit 930 is connected to the processor 910 and configured to transmit and/or receive a radio signal.
The processor 810, 910 may include application-specific integrated circuits (ASICs), other chipsets, logic circuits, or data processors or all of them. The memory 820, 920 may include read-only memory (ROM), random access memory (RAM), flash memory, memory cards, storage media, or other storage devices or all of them. The RF unit 830, 930 may include a baseband circuit for processing a radio signal. When the above embodiment is implemented in software, the above scheme may be implemented using a module (process or function) for performing the above function. The module may be stored in the memory 820, 920 and executed by the processor 810, 910. The memory 820, 920 may be placed inside or outside the processor 810, 910 and connected to the processor 810, 910 using a variety of well-known means.
Cell coverage may be determined depending on whether cooperative communication is performed in accordance with the proposed method of demodulating a downlink signal. Accordingly, a plurality of nodes within a cooperative communication system can control the length of a CP through proper scheduling, and thus radio resources can be efficiently used. Furthermore, in the cooperative mode, although a signal received from a serving node, from among downlink signals received by an MS, is received with delay via an RS and both a normal mode signal and a cooperative mode signal are received, the downlink signals can be efficiently demodulated while preventing inter-symbol interference (ISI).
Consequently, ISI can be reduced in a cooperative communication system.
In the above exemplary systems, although the methods have been described on the basis of the flowcharts using a series of the steps or blocks, the present invention is not limited to the sequence of the steps, and some of the steps may be performed at different sequences from the remaining steps or may be performed simultaneously with the remaining steps. Furthermore, those skilled in the art will understand that the steps shown in the flowcharts are not exclusive and other steps may be included or one or more steps of the flowcharts may be deleted without affecting the scope of the present invention.
The above embodiments include various aspects of examples. Although all the possible combinations for describing the various aspects may not be described, those skilled in the art may appreciate that other combinations are possible. Accordingly, the present invention should be construed as including all other replacements, modifications, and changes which fall within the scope of the claims.
Number | Date | Country | Kind |
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10-2011-0064163 | Jun 2011 | KR | national |