The design of the LTE physical layer (PHY) is heavily influenced by requirements for high peak transmission rate (100 Mbps DL/50 Mbps UL), spectral efficiency, and multiple channel bandwidths (1.25-20 MHz). To fulfill these requirements, orthogonal frequency division multiplex (OFDM) was selected as the basis for the PHY layer.
OFDM is a technology that dates back to the 1960’s. It was considered for 3G systems in the mid-1990s before being determined too immature. Developments in electronics and signal processing since that time has made OFDM a mature technology widely used in such other access systems as 802.11 (WiFi) and 802.16 (WiMAX) and broadcast systems (Digital Audio/Video Broadcast--DAB/DVB). In addition to OFDM, LTE implements multiple-antenna techniques such as MIMO (multiple input multiple output), which can either increase channel capacity (spatial multiplexing) or enhance signal robustness (space frequency/time coding).
Together, OFDM and MIMO are two key technologies featured in LTE and constitute major differentiation over 3G systems, which are based on code division multiple access (CDMA). This article presents an overview of the LTE physical layer which in itself is a very large and feature-rich topic, particularly as there are different modes of operation (FDD/TDD) and different downlink and uplink access technologies (OFDMA, SC-FDMA), along with options and exceptions for each mode and access technology. To narrow the scope, this paper will focus on essential aspects of the physical layer for FDD mode which is the dominant mode of operation and selected by incumbent mobile operators as it fits well into existing and perspective spectrum assignments. Furthermore, the topic of MIMO is left out and is a subject to a separate whitepaper. It is hoped that this paper will serve as a useful introduction to practitioners involved in designing LTE based-networks and systems such as network engineers, product managers and technical managers.
Multiple access techniques
The OFDM technology is based on using multiple narrow band sub-carriers spread over a wide channel bandwidth. The sub-carriers are mutually orthogonal in the frequency domain, which mitigates inter-symbol interference (ISI) as shown in Figure 1. Each of these sub-carriers experiences 'flat fading' as they have a bandwidth smaller than the mobile channel coherence bandwidth. This obviates the need for complex frequency equalizers, which are featured in 3G technologies.
The information data stream is parallelized and spread across the sub-carriers for transmission. The process of modulating data symbols and combining them is equivalent to an Inverse Fourier Transform operation (IFFT). This results in an OFDM symbol of duration Tu which is termed 'useful symbol length'. In the receiver, the reverse operation is applied to the OFDM symbol to retrieve the data stream--which is equivalent to a Fast Fourier Transform operation (FFT).
The mobile propagation channel is typically time dispersive: multiple replicas of a transmitted signal are received with various time delays due to multipath resulting from reflections the signal incurs along the path between the transmitter and receiver. Time dispersion is equivalent to a frequency selective channel frequency response. This leads to at least a partial loss of orthogonality between sub-carriers. The result is inter-symbol interference not only within a sub-carrier, but also between sub-carriers. To prevent an overlapping of symbols and reduce intersymbol interference, a guard interval Tg is added at the beginning of the OFDM symbol. The guard time interval, or cyclic prefix (CP) is a duplication of a fraction of the symbol end. The total symbol length becomes Ts = Tu+ Tg. This makes the OFDM symbol insensitive to time dispersion.
There are many advantages to using OFDM in a mobile access system, namely:
- Long symbol time and guard interval increases robustness to multipath and limits intersymbol interference.
- Eliminates the need for intra-cell interference cancellation.
- Allows flexible utilization of frequency spectrum.
- Increases spectral efficiency due to orthogonality between sub-carriers.
- Allows optimization of data rates for all users in a cell by transmitting on the best (i.e. non-faded) sub-carriers for each user.