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FRONT-HAUL COMPRESSION FOR EMERGING CRAN AND SMALL CELL NETWORKS
April 29, 2013
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©2013 Integrated Device Technology, Inc.
OVERVIEW
This document discusses how performance optimized compression offers significantly higher capacity
between the Radio and the Baseband units in traditional RAN, emerging C-RAN and Small Cell Access
Networks. Topics discussed include the following:
•
Introduction
•
Link Requirements and Deployment Scenarios
•
Performance Requirements
•
Performance Example
•
Summary
Key Words: Compression, Front-Haul, Base-Station, Cloud-RAN, Small Cell, RapidIO.
1. INTRODUCTION
The vision of Small Cell and Centralized Radio Access Network (C-RAN) has recently generated significant interests among the research community, the device suppliers and the OEMs (Project C-RAN). The
emerging architectures are expected to provide significant energy savings through a range of
schemes, such as, network resource sharing, traffic off-loading, and interference management. The
network architects can support a variety of network topologies based on the C-RAN concept. In particular, when deployed together with a macro- and micro-cellular network, the small cell architectures
based on C-RAN are expected to provide significant CAPEX and OPEX savings, which in turn, could be
translated to savings for the end-users.
2. LINK REQUIREMENTS AND DEPLOYMENT SCENARIOS
To enhance the network capacity and the QoE, both a traditional and the emerging architectures shall
support the higher data rate requirements offered by the LTE and LTE-Advanced protocols. These protocols offer significantly higher spectral efficiency through a range of PHY and MAC layer techniques,
such as, Carrier Aggregation (CA), MIMO, Co-ordinated multi-point (CoMP), Interference Cancellation
etc. In particular, LTE-A offers aggregation of up-to five 20-MHz LTE Carriers. When combined with
MIMO, the CA techniques may lead up-to 100s of Gigabit/sec between the Radio and the Baseband
units (i.e., the front-haul). Once the quantized I and Q samples are available, the receiver could apply a
Front-Haul Compression for Emerging C-RAN and Small Cell Networks
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wide variety of interference cancellation, MIMO decoding, and CoMP algorithms to enhance the SNR
in the network. Table 1 illustrates an example throughput calculation for a 3 sector LTE-A system with
five 20 MHz carriers.
Table 1.
An example link rate calculation for a 3 sector cell with LTE-Advanced.
Parameters
Settings
Units
Sectors
3
LTE Carriers
5
Bandwidth
100
MHz
MIMO
2x2
Tx-Rx
Bits-per-I/Q
15
Bits
Protocol
LTE-A
Throughput
13.8
Gbps
To transport data in the front-haul, the operators may use the existing fiber or cable connections. Alternatively, the operators may rely on the emerging techniques, such as, front-haul over wireless links.
The deployment decision is typically influenced by the infrastructure constraints. For example, in dense
urban areas if it is difficult to deploy new fibers or requires zero foot-print solution, a wireless link
might be more suitable. On the other hand, areas that already support fiber links, the operators may
take advantage of the existing infrastructures.
Regardless of the deployments, the equipment suppliers must provide cost-effective power efficient
solution so that the operators can gracefully migrate to higher capacity systems. Such efficient solutions typically result from the utilization of low-cost optical connectors, savings in number of the links
and improvements in spectral efficiency in the front-haul network. One possible way to achieve these
savings is through data compression. For example with 2:1 front-haul compression in C-RAN and Small
cell networks, it is possible to support a data rate of up-to 4.9152 Gbps while staying with an optical
connector that supports only 2.5 Gbps. With low data rate optical connectors and less number of links,
it is possible to reduce both cost and energy consumption. Furthermore, with such configuration, it is
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easy to transport 15-bit I and 15-bit Q samples for up-to 3 sectors with 2x2 MIMO and two LTE component carriers (one 10 MHz and one 20 MHz) in a system. This in-turn enables opportunity to apply
advanced interference cancellation and load management techniques based on the quantized I and Q
samples that leads to system level cost and energy savings in the radio access network. Figure 1 illustrates an example system architecture based on C-RAN and Small Cell with I2Q Compression.
3. PERFORMANCE REQUIREMENTS
In addition to enhancing the spectral efficiency, the wireless protocols maintain certain signal quality
(e.g., EVM) so that a particular QoE could be maintained in the network. On the other hand, depending
on the modulation schemes, the EVM requirements may vary. Table 2 summarizes the LTE-A EVM requirements for various modulation schemes. Similar requirements exist for other wireless protocols,
such as WCDMA, GSM etc.
Table 2.
EVM Requirements for various modulation schemes in LTE-Advanced [Section
6.5.2 3GPP TS 36.104]
Modulation Schemes
EVM Requirements [%]
QPSK
17.5%
16-QAM
12.5%
64-QAM
8%
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Traditional RAN, Emerging C-RAN and Small Cell System Architecture with I2Q
Compression.
Figure 1.
Radio Card Processing
FPGA/ASIC/SOC
CPRI/OBSAI
I2Q Compression
SFP/
SFP+
Decompressor
/Compressor
User Core
Downstream
Functions
RRH: Remote Radio Head
Baseband Processing
FPGA/ASIC/SOC
User Core
Upstream
Functions
Compressor/
Decompressor
CPRI/OBSAI
I2Q Compression
RRH
SFP/
SFP+
RRH
RRH
Baseband Cluster
Macro Cell with
Small Cell Overlay
Baseband Card
Baseband Card
Baseband Card
FPGA/
ASIC
SOC
RapidIO
SOC
SOC
RRH
Transport Quantized I and Q Samples
(Link between Radio and Baseband)
RRH
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4. PERFORMANCE EXAMPLE
To provide a superior QoE, the Compression technology used in the front-haul network should meet
the EVM requirements specified by the wireless protocols. Furthermore, the chosen Compression
methods should keep enough margins for other modules in the signal chain so that the overall EVM
performance could be achieved by the operators while achieving a higher throughput in the network.
Figure 2 illustrates a typical 3GPP E-TM3.1 downlink signal spectrum and Table 3 summarizes the
EVM performance of the I2Q data compression technology with typical E-TM3.1 downlink signal. In this
example, a 20 MHz LTE-A signal is compressed and decompressed with an EVM of less than 1% RMS
for a compression ratio of 2:1. This leaves enough margins for rest of the modules in the signal chain
to meet the overall EVM requirements in the system.
Figure 2.
LTE Downlink 20-MHz Signal Spectrum and Performance with a Compression ratio
of 2:1.
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Table 3.
IP Performance with typical LTE Downlink Signal.
Parameters
Values
Downlink-Signal
E-TM 3.1
Compression Technique
I2Q Data Compression
Average EVM
0.52%
Bandwidth
20 MHz
Protocol
LTE-A
5. SUMMARY
A variety of Compression techniques are being currently considered by a number of organizations and
operators to gracefully migrate towards a system with larger capacity in traditional and emerging radio
access networks. With superior EVM performance, it is possible to support data compression in wireless networks while keeping enough margins for other modules in the signal chain. Based on the available I and Q samples in a compression enabled solution, the system architects and the OEMs will have
opportunity to optimize the system performance through a range of advanced signal processing and
network resource sharing techniques in the emerging C-RAN and small cell networks.
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