## 3 Overview of Video Delivery in 4G Mobile Cellular Networks
### Radio Access Network Support for Mobile Video Delivery over LTE  
**4G LTE Radio Access Network:** RAN comprises large number of eNBs establishing radio connections to user devices (UEs). Figure 3 illustrates RAN user plane protocol stack for data delivery between eNB and UE. IP data flow towards UE passes through the Packet Data Conversion Protocol (PDCP) for header compression and ciphering, and Radio Link Control (RLC) protocol for segmentation/concatenation into suitably sized RLC packets that match the MAC frame size. If used in acknowledged automatic repeat request (ARQ) mode, RLC handles error-free and in-sequence RLC packet delivery between the eNB and UE. MAC layer introduces hybrid ARQ (HARQ) protection where, if MAC frame is not received correctly, up to three additional incremental redundancy MAC frames are transmitted. Finally, each MAC frame is allocated a single PHY transport block (PHY TB) placed into an OFDM-based time-frequency resource grid available to the eNB [43].  
PHY TBs of concurrent IP data flows are scheduled onto PHY resource blocks (PHY RBs) within every transmission time interval (TTI) of 1 ms duration. A single PHY RB is a resource allocation unit of 1 TTI time duration and 12 OFDM carriers (180 kHz). The total number of PHY RBs per TTI depends on the bandwidth allocated to eNB (e.g., 50 PHY RBs for 10 MHz downlink channel). The information carrying capacity of PHY TBs depends on the number of PHY RBs allocated to the UE, adaptive modulation and coding scheme (MCS) applied and the multiple antenna (MIMO) mode used. Note that the user plane downlink and uplink data flows are multiplexed as part of the hierarchy of logical, transport and physical channels defined at LTE MAC and PHY layer. For example, at the lowermost layer, user data is carried by the physical downlink/uplink shared channel (PDSCH/PUSCH), along with a number of other physical control channels and channel reference signals. For a detailed overview of LTE RAN interface, we refer to more detailed exposition in [43].  
**Radio Access Network Support for Video Streaming/Downloading:** eNB/UE interface represents the critical link in end-to-end video delivery from content servers to mobile devices, both in terms of channel capacity and variability. Optimization of resource allocation is a key factor for efficient usage of available radio spectrum. In LTE, MAC scheduler is responsible for allocation of PHY RBs to active UEs based on feedback on their channel quality indicators (CQI), and is usually based on proportional-fair (PF) scheduling [43; 50]. However, standard MAC schedulers are typically oblivious to video traffic. As a tempting idea, large number of studies explored cross-layer optimized design with MAC schedulers directly using perceived video-quality information [11; 51; 52; 53]. Although such a schemes offer performance gains, due to complexity of cross-layer implementation, they are rarely applied in practical systems.  
**Radio Access Network Support for Video Multicasting/Broadcasting:** Besides CN upgrade, eMBMS requires additional radio resources for eMBMS service. Thus a new set of logical, transport and physical channels have to be configured at the RAN interface. For example, physical multicast channel (PMCH) is introduced to carry user plane eMBMS data to eMBMS users in the cell. Resource allocation, coding and modulation configuration at PMCH can be further optimized for efficient LTE video broadcasting [54; 55; 56], although these prospects are rarely applied at mobile operators.