his figure also illustrates the greater susceptibil- ity to noise for 64-QAM versus 16-QAM and QPSK modulations: a smaller amount of noise will cause a misinterpretation of one symbol for another. A wireless network can dynamically adjust the modulation method depending on varying channel conditions. If the channel is detected as having a lower signal-to-interference ratio 242 Open Radio Access Network (O-RAN) Systems Architecture and Design 0 180 16-QAM 64-QAM FIGURE 7-5 Modulation schemes. (SIR), the modulation can be dropped down to 16-QAM or QPSK from 64-QAM. The bit rate will be lower, but the connection will have fewer errors. As such, modulation is adjusted by the physical layer to maintain an error ratio below a given level (e.g., 10e-6 bit error rate). Modulation can be individually adjusted per client, ensuring greater flexibility and efficiency. The MAC scheduler controls the modulation used, which impacts the amount of area the data takes up on the OFDMA frame. Base station scheduler algorithms Broadband Wireless Access (BWA) technologies have been studied and implemented since a long time, with 3G, 802.16/WiMAX, LTE, 5G, and 802.11/Wi-Fi standards as examples. Current-generation BWA systems rely on OFDMA and CA as methods to enable multiple user access. Note that a lot of the early research work has been done targeting 802.16/ WiMAX networks as this was an early commercial system that implemented OFDM/OFDMA multiuser access. The air interface typically being the bandwidth bottleneck, bandwidth allocation optimi- zation is an important topic. Scheduling is the job responsible for the allocation of (time, fre- quency) resources across users. Its job is to decide which user can transmit/receive data at which point in time and where in the frequency domain, using a defined modulation and coding scheme (MCS). There are two main approaches to the scheduling problem in wireless communications systems: the Quality of Service (QoS)-centric approach that operates regardless of the user channel state informatio