ttleneck of 5G mobile networks. One of the potential technologies targeted to enable future cellular network deployment scenarios and appli- cations is the support for wireless backhaul and relay links enabling flexible and very dense deployment of new radio cells without the need for densifying the transport network proportionately. 64 Chapter 1 The data rates on the fronthaul links are substantially higher compared to the data rates on the radio interface due to CPRI I/Q sampling and additional control information. The cen- tralized BBU and the distributed RRHs exchange uncompressed I/Q samples; therefore, effi- cient compression schemes are needed to optimize such wideband transmission over capacity-constrained fronthaul links. Possible solutions include digital RF signal sampling rate reduction, use of nonlinear quantization, frequency-domain subcarrier compression, or I/Q data compression. The choice of the most suitable compression scheme is a trade-off between achievable compression ratio, algorithm and design complexity, computational delay, and the signal distortion it introduces, as well as power consumption. Reducing signal sampling rate is a low-complexity scheme with minimal impact on the protocols. Nonlinear quantization improves the signal-to-noise ratio (SNR). Logarithmic encoding algorithms such as u-law or A-law37 can also be used to achieve higher transport efficiency on the fronthaul links. Implementation of the orthogonal frequency division multiplexing (OFDM) processing blocks at the RRH allows further reduction in the required fronthaul capacity. CPRI requires a round-trip latency of 5 us, excluding propagation delay [75]. More impor- tantly, the total delay including propagation delay is limited by the air-interface hybrid auto- matic repeat request (HARQ) timing, since HARQ acknowledgments have to be received at the DL/UL transceiver within certain duration and baseband processing would take certain time depending on the air-interface technology. As a result, typically around 200 us is av