Frequency Division Multiplexing (FDM) is a transmission technology wherein multiple signals are simultaneously transmitted over a single transmission path, such as radio signals transmitted over a wireless radio transmission path in a cellular wireless radio transmission system between radio access equipment and user equipment. To each radio signal a carrier frequency or sub-carrier frequency is allocated, modulated by user data (text, voice, video, etc.) to be exchanged by user equipment operative in the radio transmission system.
In wireless radio transmission, radio access equipment is also called radio base station and radio user equipment is also called mobile equipment or user terminal. Transmission from wireless radio access equipment to radio user equipment is referred to as a forward link or downlink, and transmission from radio user equipment to radio access equipment is referred to as a reverse link or uplink.
Orthogonal FDM (OFDM) is a radio transmission scheme which modulates user data at a number of sub-carrier frequencies that are spaced apart with the exact minimum frequency spacing needed to make them orthogonal so that they do not interfere with each other. This means that cross-talk between the sub-carriers is eliminated. The orthogonality also allows a high spectral efficiency and efficient modulator and demodulator implementation using digital Discrete Fourier Transform (DFT) techniques, such as Fast Fourier Transform (FFT) techniques.
The distribution of the user data in OFDM over a plurality of sub-carriers allows for low symbol rate modulation schemes (i.e. where the symbols are relatively long compared to the channel time characteristics) making OFDM to suffer less from InterSymbol Interference (ISI) caused by multipath effects. Since the duration of each symbol is relatively long, it is feasible to insert a guard time interval, also referred to as Cyclic Prefix (CP), between the OFDM symbols, thereby reducing or eliminating ISI. The CP consists of the end of the OFDM symbol copied as a guard interval, and the CP is transmitted followed by the complete OFDM symbol.
Accordingly, the benefits of OFDM are, in general, high spectral efficiency, resiliency to RF interference and reduced multi-path distortion.
In the Long Term Evolution (LTE) or Evolved Universal Terrestrial Radio Access (E-UTRA) radio transmission scheme, which is currently under development with the 3rd Generation of Partnership Project (3GPP), downlink radio transmission is based on OFDM. The uplink is based on SC-FDMA (Singe Carrier—Frequency Division Multiplexing), which also can be regarded as DFT pre-spread OFDM.
LTE is also expected to offer significant performance improvements by using, for example, advanced antenna techniques, such as Multiple-Input Multiple-Output (MIMO) techniques. In a further evolution of LTE, called LTE advanced, radio access equipment connects to multiple geographically spread radio antennas of a Distributed Antenna System (DAS) for transmitting to and receiving radio signals from User Equipment (UE).
In DAS a plurality of system or network antennas is placed at relatively large distances apart and connects to the radio access equipment. An antenna of the DAS serves a particular geographical area, referred to as a cell or a sub-cell. A number of antennas of a DAS serving adjacent cells or sub-cells connect to a particular radio access equipment or radio base station.
In LTE, for example, on the transmitter side, the user data are coded, interleaved, scrambled and modulated to symbols using any of a known modulation technique such as Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM), for example 16 QAM or 64 QAM. In the downlink, the symbols are mapped to a specified frequency interval, which is referred to as a number of carrier frequencies or sub-carrier frequencies. OFDM transmit signal generation involves a transformation from the frequency domain to the time domain, for which an Inverse Fast Fourier Transform (IFFT) operation is performed, and insertion of the CP. Typically, one IFFT is used for each transmit antenna of radio access equipment.
At the receiving side, the radio signals are subjected to a time-frequency domain conversion, for example using a digital Fast Fourier Transform (FFT) technique, which in fact is the inverse of the IFFT, in order to extract the user data from the frequency domain representation.
In an OFDM based transmission scheme, strict frequency and timing requirements on both downlink and uplink have to be maintained. The received signals have to be cyclic, in order to eliminate inter-carrier interference between different UE which are allocated to different carrier frequencies and to eliminate inter-carrier interference between different carrier frequencies which are allocated to the same UE. This also enables a simple carrier by carrier demodulator to be used at the receiver in a multi-path radio channel environment.
In an OFDM DAS, at the uplink, dependent on design constraints, in order to be regarded cyclic, the radio signals from UE have to arrive at each of the network antennas of a DAS connected to radio access equipment serving the UE within a particular receiver time window. At the downlink; the signals transmitted from several antennas of the DAS should arrive at the UE within a particular receiver time window.
However, in a typical terrestrial cellular radio transmission network, radio signals may arrive at the antennas at different system timings due to various propagation path lengths between the radio access equipment and the UE. In OFDM, radio signals received with timing differences within the CP time interval are regarded to be cyclic.
Accordingly, the received signals are not cyclic to a receiver FFT when applying OFDM to a DAS and when the difference in distance between the different network antennas and the UE is significant relative to the length of the cyclic prefix, that is the CP times the propagation speed of the transmitted radio signals.