Mobile backhaul evolution: from GSMto LTE-Advanced

Abstract

Mobile backhaul describes the connectivity between cellular radio base stations and the associated mobile network operator's (MNO's) core network. Previously, this was known as `transmission' but during the 1990s, the term backhaul was adopted. This chapter will review the development of mobile backhaul for the global system for mobile communications (GSM), universal mobile telecommunications system (UMTS) and long-term evolution (LTE), including LTE-Advanced radio access technologies. Original GSM terrestrial transmission interfaces were defined as time division multiplexing (TDM), within Europe and elsewhere these were based on 2.048 Mbps E1 circuits. These circuits could be multiplexed via the plesiochronous digital hierarchy (PDH) and synchronous digital hierarchy (SDH) standards to realise higher order transmission systems. The introduction of UMTS brought a new requirement to support asynchronous transfer mode (ATM) technology within the mobile backhaul domain. ATM is a fixed length cell switching technology which is carried over TDM transmission systems such as PDH and SDH. The evolution of mobile networks from predominately voice-centric to increasingly data-centric operation resulted in the development of High Speed Downlink Packet Access technologies which, together with advanced uplink technologies, resulted in the need to significantly scale the capacity of mobile backhaul networks. To address the need for scalability mobile backhaul evolved from n × E1 and TDM systems to Carrier Ethernet, there was however a few challenges to address to support this migration, firstly, the end points on base stations and network controllers were all TDM based and secondly, the E1 circuits provided a deterministic synchronisation signal via the native line code which ensured the base station operated within its allocated radio frequency channels. To address the first requirements, there was widespread adoption of pseudo-wire technology, whereas the second challenge was addressed by the introduction of Synchronous Ethernet or alternatively by the deployment of a local or packet-based synchronisation reference. Overtime, the base station and network controllers migrated to native Ethernet interfaces and therefore supported end-to-end Carrier Ethernet transmission with an Internet Protocol (IP) transport network layer. LTE was introduced with native Ethernet and IP support, the LTE radio interface offers significantly higher peak and average data rates than previous generations of cellular radio access networks. To support the deployment of LTE technology, the need for combined GSM, UMTS and LTE backhaul and the growing trend towards network sharing between MNOs, resulted in 1 Gbps Carrier Ethernet backhaul solution being deployed. This increase in backhaul capacity requirements effectively ruled out copper twisted-pair-based technologies in favour of ever more optical fibre and high-capacity microwave and millimetre wave radio backhaul technologies. The ongoing development of LTE-Advanced features, such as carrier aggregation, continues to drive the capacity of backhaul networks, whereas new products and services ensure constant evolution of performance with lower latency and reduced packet error loss rates becoming the norm.