Orthogonal Frequency Division Multiple Access

Description

Orthogonal Frequency Division Multiple Access (OFDMA) extends the Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme into a flexible multiple access protocol. While OFDM defines how data is modulated onto multiple subcarriers for a single user, OFDMA defines how the time-frequency resources of an OFDM system are partitioned and shared among multiple concurrent users. The fundamental resource unit is the Resource Block (RB), which consists of a group of contiguous subcarriers for a duration of one scheduling interval (e.g., one slot). The scheduler in the base station (eNodeB in LTE, gNB in NR) dynamically assigns these RBs to different users based on factors like channel quality, QoS requirements, and traffic load.

In operation, the transmitter (base station for downlink) multiplexes data for multiple users within the same OFDM symbol. Each user's data is mapped to the specific subcarriers assigned to them. The composite signal for all users is then generated via an IFFT, transmitted, and received by all users in the cell. Each user's receiver performs an FFT on the entire received signal but only decodes the subcarriers within the RBs allocated to it, ignoring the others. This is possible due to the orthogonality of the subcarriers. For the uplink in 5G NR, OFDMA is also used, requiring precise time and frequency synchronization among all transmitting user equipments (UEs) to maintain the orthogonality at the base station receiver.

OFDMA provides several key advantages for cellular networks. It enables fine-grained, two-dimensional (time and frequency) resource allocation, allowing the scheduler to exploit multi-user diversity by assigning resources to users on their best frequencies. It supports scalable bandwidth allocation, from a single RB to the entire system bandwidth, adapting to each user's instantaneous data needs. Furthermore, it seamlessly integrates with advanced technologies like MIMO spatial layers, where different layers can be assigned to different users (Multi-User MIMO). The flexibility of OFDMA, especially with the variable numerology introduced in 5G NR, is critical for supporting diverse services from massive IoT to enhanced mobile broadband and ultra-reliable low-latency communications.

Purpose & Motivation

OFDMA was developed to address the inefficiencies of static or code-based resource sharing in previous cellular generations. In 3G UMTS, WCDMA allocated the entire bandwidth to a user via spreading codes, which was inefficient for bursty data traffic and limited multi-user scheduling granularity. The goal was to create a multiple access scheme that could efficiently support a large number of users with varying and dynamic data rate requirements, which is characteristic of packet-switched internet traffic.

Its introduction with LTE (Release 8) was driven by the need for higher spectral efficiency, lower latency, and better support for packet data services. OFDMA solves these problems by allowing dynamic, per-TTI (Transmission Time Interval) allocation of frequency resources. This enables frequency-domain packet scheduling, where users are served on sub-bands where their channel conditions are strongest, maximizing system throughput. It also allows for very small minimum resource allocations, making it efficient for low-data-rate devices and reducing scheduling latency. For 5G NR, extending OFDMA to the uplink (replacing the SC-FDMA used in LTE uplink) provided greater scheduling flexibility and was enabled by improved UE power amplifier efficiency, further optimizing the system for the extreme demands of new use cases.