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Reader Forum: Cloud radio access and small cell networks based on RapidIO

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The wireless radio access network must evolve significantly to satisfy the end user quality-of-experience re-quirements associated with mobile data and video traffic. The changes in the access network in many cases are required by industry standards which evolve to meet end-user requirements. For example, throughput and spectral efficiency in the wireless network is expected to increase by approximately 10-times and four-times respectively between HSPA+ to LTE-Advanced in the coming years. To improve QoE, the wireless protocols are also enhanced to reduce network latency. For example, LTE and LTE-A will support around 10 milliseconds of latency as compared to 150 milliseconds in GSM.

Although end users demand better QoE in terms of more bandwidth and faster responsiveness, the load on the network varies as the users move across different cells during different part of the day. To meet the variable demand, the standards based on LTE and LTE-Advanced incorporated features such as load indication and resource status reporting using the “X2” interface between different eNodeBs at the base-station. Furthermore, quality-of-service management parameters continue to evolve and are expected to be supported in addition to already existing low-latency hand over and interference information exchange over X2 interface between eNodeBs. This paper discusses how RapidIO supports low latency scalable solution in traditional, cloud and small cell network while supporting various data flows and new requirements in the access network.

Radio access network data flows

In traditional and emerging RAN architectures, the base-stations include multiple processing cards which are connected using dedicated CPRI links to the radio units at the top of the tower. With fiber-optic link, it is possible to deliver maximum power to the radio with minimum loss over a reasonable distance. This is particularly beneficial in emerging distributed RAN architecture where a large number of radio units are connected over fiber to the baseband units. Each of the radio nodes, in this architecture, serves a set of users in a small or macro cell configuration.

In both traditional and distributed architectures, the RapidIO protocol connects multiple processing units (e.g., DSPs, SOCs, ASICs, FPGAs) in the channel or baseband cards. The protocol ensures guaranteed delivery with lowest latency of around 100 nanoseconds between any processing nodes.

In a typical RAN architecture, once the signal processing related to a particular radio interface is completed, the data is transported between the baseband and the radio using CPRI protocol. In a distributed architecture, the CPRI protocol may not lead to a cost effective implementation of load management and interference control since by definition CPRI protocol does not provide a standardized low latency packet based switching capability that can be used to distribute traffic across multiple baseband cards from the radios. RapidIO in this case is expected to provide best-in-class performance.

Two major flows in the access network include transmission from mobile to the base station (uplink flow) and reception at the mobile from the base station (downlink flow). Retransmission, in the form of hybrid-automatic-repeat-request (HARQ), might be required in case of transmission error as part of the LTE/LTE-A protocol. The HARQ timing of around four milliseconds is actually much lower than the LTE/LTE-A round-trip latency. If the interconnect fabric between processing nodes support superior flow control and fault tolerance capability, it is further possible to minimize the number of HARQ retransmissions resulting into lower latency. This provides a differentiation point for OEMs and eventually leads to better QoE for the end user.

To support handover, load balancing and interference management, the above major flows further includes exchange of information related to handover, channel quality and load indication between various processing units at the base-station. The exchange of information between baseband units needs to be supported with lowest deterministic latency and guaranteed delivery. This allows the demand on the network and interference between various users to be identified without error and in a timely manner. This also enables low latency hand-over with reliability as the users cross cell boundaries.

RapidIO features for radio access network

The high-performance RapidIO protocol was introduced as a packet-based open data communication standard almost a decade ago. Since then, millions of RapidIO-based devices are shipped around the world from a large number of OEMs and silicon suppliers to meet the networking requirements in 3G/4G wireless base stations in radio access and other type of networks.

The RapidIO protocol and packet formats are specified in a three-layer architectural hierarchy. The protocol supports short-, medium- and long-reach links on or across boards. The standard supports both fiber and cable links. Table 1 summarizes key features in RapidIO applicable to traditional and emerging C-RAN and Small cell access networks.

Key RapidIO protocol features:

RapidIO in next-generation C-RAN and small cell

An example block diagram of C-RAN and small cell based on RapidIO.

To meet the requirements in C-RAN and small cell, in particular, reliable load management, hand over and interference management, OEMs are evolving the base station design by taking advantage of RapidIO’s interconnect features and advancements in SoC, memory, and radio sub-system components. For the interconnect, there are two important functions to consider – RapidIO end-point and RapidIO switching fabric. With integrated EP within SoCs it is possible to offer lowest latency between applications. With a low-latency high-throughput packet based switching protocol, applications can be executed and partitioned across a large number of baseband computing units. To support lowest latency, it is further possible to co-locate baseband processing units for a large number of small cells in one location. In this case the X2 interface is local to the baseband cluster. The exchange of information with deterministic delivery and lowest latency allows the baseband cluster to control data exchange between the right set of radio units and the baseband units at the right time while minimizing or avoiding interference even for the users located at the cell boundary. With a cluster of baseband units, it is possible to virtualize and share the processing units. This allows the average processing capability of the computation units to meet the total capacity of a group of cell at any given time instead for a specific cell all the time.

Base station cluster with X2 interface based on RapidIO to manage load, interference, hand over and to communicate channel quality in C-RAN and small cell.

RapidIO fabrics are being used today in many wireless RAN systems, as RapidIO provides superior QoS with low power consumption. RapidIO fabrics are therefore a natural fit with standards which require communication of load balancing and interference management information, as RapidIO provides standard packet routing, deterministic delivery and low latency. RapidIO based solutions simplify implementation and evolution of the complex interfaces required to allow the cloud radio and small cell access network to evolve.


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