C-RAN

The Centralized/Cloud Radio Access Network (C-RAN) has attracted tremendous attention from the wireless infrastructure industry in the recent years (see “The Next Generation Network Takes Shape”)  This is due to the substantial benefits introduced by the C-RAN architecture including lower total-cost-of-ownership (TCO), enhanced spectral efficiency and simplified support of multi-standards and future evolution. Perhaps more importantly, this architecture complements the industry’s migration toward Network Functionality Virtualization (NFV) and Self Organized Networks (SON) in terms of network architecture convergence.

Motivation

The ever-increasing use of smart phones and other portable devices is driving exponential growth in mobile broadband data traffic and capacity demands. This presents  significant challenges to the existing wireless network:

  • Gap of Growth Rate between Air-interface Resources and Data Capacity
    Escalating data traffic demands have exceeded traditional air-interface capacity and, as a result, new architectures and approaches are required.
  • Gap of CAGR between Cost and Income Growth
    Operators’ income-per-subscriber is predicted to grow at a modest rate over the next few years while data will grow exponentially. Thus, operators are under pressure to lower cost-per-bit in order to maintain services in a profitable manner.
  • The Green Imperative
    Substantial growth in the number of base stations has resulted in a significant increase in power consumption on the wireless network and, correspondingly, operational capital expenditure (OPEX). Operators need new approaches to reduce total power consumption and OPEX.
  • Underutilization of network resources
    There are load imbalances across regions (residential/commercial) and time intervals (day/night workday/weekend).  With current network deployments, hardware must be over provisioned to support the worst-case loading; consequently the network is, on average, under-utilized.
  • Interferences introduced by High Density Networks
    The high density of Base Transceiver Stations (BTS) deployed in urban areas cause inter-cell interference which limits performance.

Due to these limitations in the existing wireless network, the industry is trying to optimize the network architecture by considering a number of key innovations. Together these enhancements are termed C-RAN.

  • Reducing number of BTS sites
    Decreasing the number of sites reduces the cost (both CAPEX and OPEX) and simplifies future upgrades.
  • Use Coordinated Multiple Processing (COMP)
    Advanced COMP capability will resolve the issue of inter-cell interference in high density areas.
  • Shared processing, load balancing and Self-Organizing Networks (SON) Shared processing allows the network to allocate processing power between cell sites based on need or capacity to improve operational efficiency. Shared processing power also enables coordination of multi-cell sites and, along with multiple-band support, allows the air-interface to adapt to dynamic loading levels. Load balancing helps the network respond to spikes in bandwidth demand and ensures reliable operation of the network by automatically allocating resources. Intelligent, Self-Organizing Networks promise to reduce network operating costs by simplifying network planning, configuration, management and optimization.

Driven by these architectural innovations, C-RAN solutions have the potential to introduce tremendous inherited advantages by physically aggregating BTS processing to a single site.

C-RAN Description

Full Centralized Architecture

Figure 1 illustrates the fundamental rationale of the C-RAN with full centralization. The Remote Radio Unit (RRU) sites stay the same as the conventional radio networks, while the Baseband Unit (BBU) moves from being co-located with the RRU to a centralized location.  The BBU includes Physical Layer (L1) and higher layer functions, mapped onto a large pool of processing resources that accomplish the virtualized NodeB functionalities.

In order to connect multiple traffic streams together in a dynamic fashion, a switch layer called cloud terminator is also introduced. This layer is used to bridge, connect and control different interface protocols to facilitate dynamic load balancing.

LTE places very strict latency requirements on data processing. For example, round trip latency must be < 5 ms and baseband frame processing must be < 1 ms.  As a consequence, the transmission between the RRU sites and the centralized BBU site must be high throughput (≥10Gbps) and requires latency as low as tens of microseconds.

FPGAs are a keystone of CRAN architecture

The key requirements of CRAN are reconfigurability, low and deterministic latency operation, flexible HW accelerators and high speed switching performance which make FPGAs a natural component of any CRAN architecture.