e among all configured BWP settings. This is also an important NR design to ensure reasonable decoder complexity. In addition to computation complexity, the following two issues also pose challenges to area-efficient NR UEs: HARQ buffer size. The total HARQ buffer size can be determined by the product of the peak data rate, transmission time interval (TTI) time duration, and HARQ process number divided by the limited HARQ code rate, 2/3. To deliver multiple Gbps data rates and accommodate up to 16 HARQ processes, the required total buffer size is huge. This calls for sophisticated designs both to compress the HARQ content and to manage the HARQ processes in order to balance the HARQ performance and buffer complexity. Decoder multi-mode sharing. In Universal Mobile Telecommunications System (UMTS) and LTE, turbo coding is used. A multi-mode (i.e. UMTS and LTE) turbo decoder can be developed to minimize the decoder cost. However, NR uses LDPC to achieve the 20-fold increase in peak data rate at a reasonable chip cost. Because of the fundamental difference in the decoding algorithm between the turbo decoder and LDPC decoder, the realization of an efficient multi-mode (UMTS, LTE, NR) decoder requires a sophisticated design in both the decoder algorithm and architecture. 13 5G Modem Design Challenges In summary, this subsection describes how a 20-fold peak data rate can be realized for NR with a reasonable die area. Introducing LDPC codes subject to a limited HARQ code rate for the maximum data volume is the key enabler. The implementation challenges of accommodating huge HARQ buffer size and realizing a multi-mode data decoder are also highlighted. A lesson learned from NR coding design is that the joint consideration of both the power efficiency (in terms of the required power per bit) and the area effi- ciency (in terms of the required die area per bit) can fundamentally reduce the foundry requirement and realize the 20-fold peak data rate in the near future. 13.2.2 MIMO and Network Flexibility Versus UE Complexity