2.4 Physical Layer
2.4.1 Multiple Access Technologies
Downlink and uplink transmissions in LTE are based on the use of multiple access technolo- gies: specifically, Orthogonal Frequency Division Multiple Access (OFDMA) for the downlink and Single-Carrier Frequency Division Multiple Access (SC-FDMA) for the uplink. Both will be described in the following sections.
188.8.131.52 Downlink: Orthogonal Frequency Division Multiple Access (OFDMA) Orthogonal Frequency Division Multiple Access (OFDMA) is a variant of Orthogonal Frequency Division Multiplexing (OFDM), a digital multi-carrier modulation scheme that is widely used in wireless systems and more recently in cellular. Instead of transmitting an high-rate data stream using a single carrier, OFDM makes use of a large number of orthogonal subcarriers that are transmitted in parallel. These subcarriers are closely spaced
but do not interfere with one another in the frequency domain. Each subcarrier is modulated with a conventional modulation scheme at a low symbol rate, such as Quadrature Phase Shift Keying (QPSK), 16 or 64-Quadrature Amplitude Modulation (QAM), described later in section 2.5. The combination of a great number of subcarriers using lower bandwidths each enables similar data rates to those obtained when using conventional wideband single-carriers with higher bandwidths.
The diagram in Figure 2.10 illustrates the key features of an OFDM signal in frequency and time. In the frequency domain, multiple subcarriers are each independently modulated with data. In the time domain, the long symbols used for OFDM are separated by a guard interval known as Cyclic Prefix (CP), to improve the resilience of the system. The CP is a copy of the ending part of a symbol that is inserted at its beginning. The receiver uses this guard interval to avoid the Inter-Symbol Interference (ISI) between adjacent symbols, caused by multipath reflection delay spread, by sampling the received waveform at the optimum time.
Figure 2.10: Frequency-Time Representation of an OFDM signal (taken from [3GP04]). Nevertheless, OFDM has some disadvantages. Because the subcarriers are tightly spaced, OFDM is more easily affected by frequency errors and phase noise, causing the subcarriers to start losing their orthogonality. OFDM also creates high Peak-to-Average Power Ratio (PAPR) signals which can be problematic to amplifiers, increasing the power consumption.
All subcarriers in OFDM are attributed to a single user at the same time, making only one user able to transmit at a time. If more than one user is trying to transmit using OFDM, they have to take turns. With OFDMA, however, the subcarriers are directly assigned in frequency to different users. That is why, for the downlink,3GPP chose OFDMA.
The result is a more robust system with increased capacity. OFDMA can adjust the modulation and coding for each subcarrier, it has better spectral efficiency and low-complexity modulation [Yan10]. As the users are scheduled by frequency, frequency-selective fading is less prone to happen [Agi09]
A simplified block diagram for the signal generation and reception of OFDMA is illustrated in Figure 2.11. It begins with the mapping of M data bits to their respective modulations, and then the modulated data symbols into the available subcarriers. An Inverse Fast Fourier Transform (IFFT) is performed to convert the data symbols to the time domain, where a CP is inserted for robustness of the system, preventing multipath fading. After going through
the air interface, the symbols enter the receiver and go through a reverse process, which ends in their de-mapping back to M data symbols. Time and frequency synchronization must be accurate, especially for the CP removal step, otherwise a wrong part of the symbol will be dropped.
Figure 2.11: Simplified model of OFDMA signal generation and reception (adapted from [Agi09]).
184.108.40.206 Uplink: Single-Carrier Frequency Division Multiple Access (SC-FDMA) The high PAPR associated with OFDM led 3GPP to look for a different transmission scheme for the LTE uplink. SC-FDMA was chosen because it combines the low PAPR techniques of single-carrier transmission systems, such as GSM, with the multipath fading resistance and flexible frequency allocation of users of OFDMA.
The block diagram for the signal generation and reception of SC-FDMA is illustrated in Figure 2.12. SC-FDMA signal generation begins with a special precoding process but then continues in a identical manner to OFDMA. SC-FDMA data symbols in the time domain are converted to the frequency domain using a Discrete Fourier Transform (DFT). Then, in the frequency domain, they are mapped to the desired location in the channel bandwidth before being converted back to the time domain, using an IFFT. Finally, the CP is inserted, to provide robustness to the system against multipath.
Figure 2.12: Simplified model of SC-FDMA signal generation and reception (taken from [Agi09]).
For the reception of the SC-FDMA signal, the process follows the same steps as for OFDMA, with the addition of performing an Inverse Discrete Fourier Transform (IDFT) that converts the frequency-shifted signal to the time domain, along with the rest of the decoding process related to SC-FDMA.
A graphical comparison of OFDMA and SC-FDMA is shown in Figure 2.13. The CP is shown as a gap; however, it is actually filled with a copy of the end of the next symbol.
Figure 2.13: Comparison of OFDMA and SC-FDMA transmitting a series of QPSK data symbols (taken from [Mor08]).
As illustrated, OFDMA transmits the four QPSK data symbols in parallel, one per sub- carrier, while SC-FDMA transmits them in series at four times the rate, meaning each data symbol occupies four times the OFDMA’s symbols bandwidth.
It is the parallel transmission of multiple symbols that creates the undesirably high PAPR of OFDMA. By transmitting N data symbols in series at N times the rate, the SC-FDMA occupied bandwidth is the same as multi-carrier OFDMA but, most importantly, the PAPR is the same as that used for the original data symbols. As the number of subcarriers increases, the PAPR of OFDMA with random modulating data approaches Gaussian noise statistics, but the SC-FDMA PAPR remains the same as that used for the original data symbols [Agi09]. In each transmission, a scheduling decision is made where each scheduled UE is assigned a certain amount of radio resources in the time and frequency domain [Aru11]. In the next section, the frame structure and radio resource allocation is described.