Overview of OFDM and OFDMA [LTE Self-training]

Multiplexing: combine a number of independent signals into  composite signal, which is suitable to be delivered over a common channel. To deliver over the same channel, these signals must be separated to avoid interference. Most common ways for separation is time-based or frequency-based.

Orthogonal Frequency Division Multiplexing (OFDM)

• modulation and multiplexing tech used for wireless or wired
• divide entire channel into multiple low-rate orthogonal sub-carriers (narrow subbands) to carry data, these sub-carriers then composite a high data rate comms system
• comparing against conventional FDM, OFDM allows overlapped among sub-carriers, since orthogonal will ensure the sub-carriers separation at the receiver. Consequently, OFDM has better spectrum utilization.
• Robust to fading (flat fading and selective frequency fading) and interference
• frequency response over individual subbands is relatively flat as subband are smaller than coherence bandwidth of the channel, therefore, equalization is simpler than a single carrier system
  

Orthogonal Frequency Division Multiple Access (OFDMA)

• Multi-user version of OFDM, assigning multiple sub-carriers to the invidivual user
• Simpler than the contention-based multiple access (e.g. collision avoidance)
• Further improves OFDM robustness in terms of fading and interference
  Frequency-Division Multiplexing is well described in chapter 2.5 in book <<Communication Systems>> by Simon Haykin and chapter 12 in book <<Wireless Communications>> by Andrea Goldsmith