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Why WDM is essential in fronthaul networks? - Ultra high CPRI link capacity

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  • 1. 1 NETMANIAS TECH-BLOG Please visit www.netmanias.com to view more posts Why WDM is essential in fronthaul networks? - Ultra high CPRI link capacity March 12, 2014 | By Steve Shin, Harrison J. Son (tech@netmanias.com) | www.netmanias.com Our question in this post is: Why does the CPRI link between BBU (at CO) and RRH (at Cell Site) in C-RAN require a ultra high Gbps bandwidth? Before you read this post, we advise you to read our previous post, "CPRI (1): Emergence of C-RAN and CPRI overview". The CPRI link capacities required between BBU and RRH in the C-RAN architecture are as follows: Table 1. CPRI data rates in function of radio technologies Antenna configuration LTE Bandwidth 10 MHz 20 MHz 2x2 MIMO 1.2288 Gbps (IP rate 75Mbps) 2.4576 Gbps (IP rate 150Mbps) 4x2 (4x4) MIMO 2.4576 Gbps (IP rate 150Mbps) 4.9152 Gbps (IP rate 300Mbps) 8x2 (8x4, 8x8) MIMO 4.9152 Gbps (IP rate 300Mbps) 9.8304 Gbps (IP rate 600Mbps) * Source: CPRI Specification v6.0 (Aug. 30, 2013) In case of a network with LTE Carrier BW of 20MHz and 2x2 MIMO antenna configuration, the maximum IP throughput in the cell is only 150Mbps. So theoretically, a backhaul network with 150 Mbps should be able to handle the traffic. In reality, however, a CPRI link between BBU (at CO) and RRH (at Cell Site) requires a bandwidth as high as 2.4576Gbps. Why such a high bandwidth is needed? Let's see how data traffic is delivered in a standalone base station, where BBU and RRH are kept together, as seen in Figure (a) below (Please note, in the 2X2 MIMO configuration illustration below, only one (instead of two) antenna is shown for the sake of simplicity). CPRI Basic Frame 260.42ns Payload (120bits) Overhead (Control Word= 8 bits) RRH CO BBU Antenna CPRI Rate =2.4576Gbps IP Rate = 150Mbps 20MHz, 2x2 MIMO 260.42ns Max 150Mbps IP Backhaul
  • 2. Netmanias Tech-Blog: Why WDM is essential in fronthaul networks? - Ultra high CPRI link capacity 2 In the LTE Baseband PHY layer, OFDM symbol waveforms (I waveform and Q waveform) are generated once all the signal processing, such as channel coding, modulation, IFFT(inverse FFT), are completed. Through sampling these two waveforms, you get a set of digital sample data (I sample 15 bits and Q sample 15 bits), which then are sent to their associated DAC (Digital to Analog Converter), and converted into analog I/Q signals. Thereafter, once RF processing (modulation/mixing/power amplification) is performed, the radio signal is finally radiated over the air through the antenna. During the sampling process of the OFDM symbol waveforms, the sampling frequency was 30.72MHz (when LTE carrier BW is 20MHz), and the sampling bit-width for I and Q samples was 15 bits (LTE case). Therefore, the resulting I/Q sample data rate was 0.923Gbps (=30bit/30.72Mhz). The massive I/Q samples generated this way are delivered to DAC through the built-in internal digital bus in the base station system. Unlike the standalone base station, in C-RAN, BBU and RRH are kept separated from each other, and connected through the CPRI interface. Because radio parts are located in RRH, baseband I/Q samples must be Baseband I/Q sample 90° fc RF cable Antenna Modulation 0.923Gbps Analog DAC DAC IPBackhaul t t InverseFFT(LTEPHY) Sampling I waveform Q waveform ... Baseband Digital IC 001100101100010I sample data 15 bits 101010111001001 Built-in internal data bus in BS Q sample data 15 bits 90° fc RF cable Antenna Modulation IP Rate: 75Mbps DAC DAC IPBackhaul t t InverseFFT(LTEPHY) I waveform Q waveform ... Q sample data 15 bits I sample data 15 bits 0011...10 01001110...01 1010...01 AxC 30 bits 0011...10 01001110...01 1010...01 CPRI optical link Standalone Base Station (at Cell Site) OFDM symbol waveform 32.5 nsec BBU (at CO) RRH (at Cell Site) 75Mbps 75Mbps 1.228 Gbps (a) Standalone Base Station: Baseband I/Q sample data delivered through the internal data bus (b) C-RAN: Baseband I/Q sample data delivered through the external CPRI link between BBU and RRH AnalogBaseband Digital IC I sample data Q sample data IP Rate: 75Mbps Baseband I/Q sample 01001110...01 CPRIframer,8B/10Bencoding 8B/10Bdecoding,CPRIdeframer 32.5 nsec
  • 3. Netmanias Tech-Blog: Why WDM is essential in fronthaul networks? - Ultra high CPRI link capacity 3 delivered to DAC in RRH. As you can see in the Figure (b) above, I/Q sample data (0.9216Gbps) are carried in the payload of a CPRI frame. These sample data and the control word added to them form a CPRI frame (0.983Gbps=16/15*0.9216Gbps). Then, after 8B/10B encoding process, the frame is forwarded through the CPRI link (1.2288Gbps = 10/8*0.983Gbps) finally to RRH. Now we know as each antenna requires 1.2288Gbps, 2X2 configuration with two antennas requires the double CPRI link capacity of 2.4576Gbps. In summary, in a standalone base station, I/Q samples can be simply delivered through the built-in internal digital bus to DAC. However, in C-RAN, I/Q samples are generated at BBU in CO, and thus must travel far to reach DAC at RRH in a cell site. And this requires a CPRI link with higher bandwidth. Let's assume a network with LTE Carrier BW of 20 MHz, 2x2 antenna configuration, and three RRHs (3-sector) in a cell site. If we do math, each RRH will require the CPRI capacity of 2.4576Gbps, and thus each cell site will need the capacity of 7.3728Gbps. 2.4576 Gbps/RRH (20MHz, 2x2) x 3 RRHs = 7.3728Gbps Then a network with two bands will require: 2.4576 Gbps/RRH (20MHz, 2x2) x 3 RRHs x 2 band = 14.7456Gbps In this case, if each RRH uses one fiber link, each cell site would need six fiber links, which can be very costly. As an effort to lower the fiber cost, now WDM equipment is installed between a CO and cell site, assigning different wavelengths to each RRH. Delivering compressed I/Q samples, another possible way of lowering the fiber cost, will be introduced in the next post. CO BBU 3 RRHs (Band 1) 3 RRHs (Band 2) CO BBU 3 RRHs (Band 1) 3 RRHs (Band 2) 6 fibers per Cell Site 1 fiber per Cell Site WDM (P2P, Ring) ~ 25Km
  • 4. About NMC Consulting Group (www.netmanias.com) NMC Consulting Group is an advanced and professional network consulting company, specializing in IP network areas (e.g., FTTH, Metro Ethernet and IP/MPLS), service areas (e.g., IPTV, IMS and CDN), and wireless network areas (e.g., Mobile WiMAX, LTE and Wi-Fi) since 2002. Copyright © 2002-2014 NMC Consulting Group. All rights reserved. 4 Carrier WiFi Data Center Migration Wireline Network LTE Mobile Network Mobile WiMAX Carrier Ethernet FTTH Data Center Policy Control/PCRF IPTV/TPS Metro Ethernet MPLS IP Routing 99 00 01 02 03 04 05 06 07 08 09 10 11 12 13 eMBMS/Mobile IPTV Services CDN/Mobile CDN Transparent Caching BSS/OSS Cable TPS Voice/Video Quality IMS LTE Backaul Netmanias Research and Consulting Scope Visit http://www.netmanias.com to view and download more technical documents. Future LTE IP/MPLS CarrierEthernet Networks Consulting POC Training Wi-Fi Infrastructure Services CDN Transparent Caching IMS Concept Design DRM eMBMS protocols Analyze trends, technologies and market Analysis Report Technical documents Blog One-Shot gallery We design the future We design the future We design the future