Multiple Access in OFDM – OFDMA | Radio Interface Basics

LTE uses OFDM as the transmission scheme, as described in the section above. Multiple access is realized with OFDMA with the base station (eNB) taking care of the resources within its cell; this procedure is also called scheduling as the eNB schedules the transmission of user data in the DL and UL direction on the transmission medium used by all users within this cell. The transmission is done on the basis of a shared channel. The control information for granting an UL transmission on the UL-SCH, or informing a UE about data that is transmitted for it on the DL-SCH, is done within the DL control channel with designated control information.  Figure 1 shows the principle of an OFDM shared channel-based multiple user communication.

 
Figure 1: OFDM shared channel-based multiple user communication. Reproduced with permission from Nomor

Resources to be scheduled with OFDMA systems are units of frequency resources and time units describing a time slot for which the scheduled frequency units are valid.

This scheduling procedure not only adds overhead, but also enables the system to be more efficient by introducing a frequency-selective scheduling algorithm with feedback from the UEs regarding current reception quality, rather than use only diversity gains by spreading transmitted data.

Figure 2 compares a shared channel of an OFDMA system using localized and distributed scheduling of user data. In the localized mode, granted areas belonging to one user allocate adjacent frequency resources within one block. The distributed mode is used when frequency diversity is to be used by spreading the user data of the shared channel to distributed non-adjacent frequency resources. Simulations show that systems with frequency-selective scheduling with a fast channel quality feedback report from users can achieve higher cell throughput compared to systems just spreading data over the spectrum in order to achieve frequency diversity.

 
Figure 2: Localized vs. distributed shared channel scheduling

LTE defines both localized and distributed scheduling in the DL direction but only localized scheduling in the UL direction in order to keep the PAPR small in the SC-FDMA symbols of each user.