LTE PART II: 3GPP RELEASE 8 - The LTE Link-Layer Design

Following the successful standardization of highspeed packet access (HSPA), the 3rd Generation Partnership Project (3GPP) recently specified the universal mobile telecommunications system (UMTS) terrestrial radio-access network — or UTRAN — long term evolution (LTE) to meet the increasing performance requirements of mobile broadband. The result includes a flexible and spectrally efficient radio link protocol design with low overhead, which meets the challenging targets [1] that were set to ensure good service performance in varying deployments. The data rate can vary from more than 300 Mb/s in the downlink and 75 Mb/s in the uplink — for terminals in favorable radio conditions — to a few tens of kb/s at the cell edge, depending on the deployment scenario. The one-way latency target is set to be less than 5 ms between terminal and base station, and the handover mechanism supports real-time applications such as voice. The LTE architecture also should contribute to reducing the cost of network deployment, as discussed in the following section. The high peak rates and the large range of possible data rates of the LTE physical layer, in combination with the strict latency requirements and the new, simplified architecture were the main challenges when designing the link-layer protocols. In this article, we explain in detail the linklayer protocols that handle the data flow over the LTE radio interface, as well as crosslayer interactions that are required to realize the required functionality efficiently. Finally, we discuss the low header-overhead design of the LTE link-layer protocols and provide simulation results that illustrate the retransmission protocol design for high transmission control protocol (TCP) performance.