improve scheduler performance in typical general-purpose processor (GPP) software-defined radio implementations, thus enabling the reader to achieve real time performance for the algorithms discussed. This text includes a long list of literature references. This is done on purpose to allow the reader to investigate algorithmic enhancements and system performance optimizations that are outside the scope of this text. Orthogonal frequency division multiple access Orthogonal frequency division multiplexing (OFDM) was developed specifically for wireless applications because it offers significant advantages over legacy spread-spectrum solutions such as direct-sequence spread spectrum used for example in 802.11b. These advantages include excellent resistance to selective fading and robustness to temporal dispersion. OFDM is also used in nonwireless applications, such as DSL (DMT or Discrete Multi-Tone is very similar to OFDM) and powerline networking (e.g., HomePlug) for similar reasons. OFDM helps resolve wireless applications issues by breaking one high-speed data stream into lower speed data streams, which are then transmitted simultaneously in par- allel. Each lower speed stream is used to modulate a subcarrier. This creates a multicar- rier transmission by dividing a wide frequency band (or channel) into narrower frequency bands (or subchannels) (Fig. 7-1). Frequency is shown on the vertical axis and time on the horizontal axis. Both single- carrier and OFDM modes occupy the same channel bandwidth and, under ideal conditions, give the same data rates. It is the robustness of OFDM under less than ideal conditions that gives it a practical advantage over the wideband single-carrier mode. Open Radio Access Network (O-RAN) Systems Architecture and Design. DOI: https://doi.org/10.1016/B978-0-323-91923-4.00004-5 © 2022 Elsevier Inc. All rights reserved. 238 Open Radio Access Network (O-RAN) Systems Architecture and Design frequency FIGURE 7-1 OFDM conceptualized. OFDM, orthogonal frequency division multiplexing. In OFDM