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Why WDM is essential in C‐RAN fronthaul networks? ‐ Ultra high CPRI link capacity March 12, 2014 | By Steve Shin and Dr. Harrison J. Son (
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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 CRAN and CPRI overview". The CPRI link capacities required between BBU and RRH in the CRAN architecture are as follows:
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Table 1. CPRI data rates in function of radio technologies
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).
View All (264)
5G (8) 802.1X (1) AP (1) AP
In the LTE Baseband PHY layer, OFDM symbol waveforms (I waveform and Q waveform) are generated
controller (1) ARP (3) Akamai (1)
once all the signal processing, such as channel coding, modulation, IFFT(inverse FFT), are completed.
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Why WDM is essential in CRAN fronthaul networks? Ultra high CPRI link capacity | NETMANIAS Through sampling these two waveforms, you get a set of digital sample data (I sample 15 bits and Q
Antenna (1) Authentication (2)
sample 15 bits), which then are sent to their associated DAC (Digital to Analog Converter), and
Backhaul (1) Big Data (1) Bridging (2)
converted into analog I/Q signals. Thereafter, once RF processing (modulation/mixing/power
CRAN (8) CDN (1) CPRI (4)
amplification) is performed, the radio signal is finally radiated over the air through the antenna.
CSI (1) Cache (1) Captive Portal (1)
(when LTE carrier BW is 20MHz), and the sampling bitwidth for I and Q samples was 15 bits (LTE case).
Carrier Ethernet (3) CoMP (5) Contela (1) Control Plane (4)
Therefore, the resulting I/Q sample data rate was 0.9216Gbps (=30bit/30.72Mhz).
DHCP (5) Data Plane (4) EPS
Bearer (3) FTTH (6)
The massive I/Q samples generated this way are delivered to DAC through the builtin internal digital
Fronthaul (8) GTP (2) GUTI (1)
During the sampling process of the OFDM symbol waveforms, the sampling frequency was 30.72MHz
bus in the base station system. Unlike the standalone base station, in CRAN, 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
GiGAtopia (1) Giga Internet (2) Gigabit Internet (2) Google (6)
Google Global Cache (3) HFR
must be delivered to DAC in RRH. As you can see in the Figure (b) below, I/Q sample data (0.9216Gbps)
(1) HLS (5) HTTP Adaptive
are carried in the payload of a CPRI frame. These sample data and the control word added to them form
Streaming (5) ICIC (1) IMSI (1) IP
a CPRI frame (0.983Gbps=16/15*0.9216Gbps). Then, after 8B/10B encoding process, the frame is
Routing (4) Indentifier (1)
forwarded through the CPRI link (1.2288Gbps = 10/8*0.983Gbps) finally to RRH.
Interference Coordination (1) IoT
Now we know as each antenna requires 1.2288Gbps, 2X2 configuration with two antennas requires the
(3)
KT (16) Korea (14)
double CPRI link capacity of 2.4576Gbps.
L3 Switch (4) LG U+ (11)
LTE (23) LTEA
In summary, in a standalone base station, I/Q samples can be simply delivered through the builtin internal digital bus to DAC. However, in CRAN, I/Q samples are generated at BBU in CO, and thus must travel far to reach DAC at
(13) LTEH (2) LTEU (2) LTE
RRH in a cell site. And this requires a CPRI link with higher bandwidth.
Unlicensed (1) MEF (2) MWC 2015
(3) Netflix (1) OSPF (1) OTT (3) On Net CDN (1) Progressive Download (1)
QoS (3) RRA (1) RRH (1) Radio Bank (1) Routing (3) SDF (3) SK
Broadband (2)
SK Telecom
(14) Security (2) Small Cell (1) Switching (2) Tracking Area (1) VLAN (1) Video Streaming (2) WiFi (5)
YouTube (6) eICIC (1) vRAN (1) LTE Subscribers Growth in Korea
>> view all statistics diagrams
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.
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Why WDM is essential in CRAN fronthaul networks? Ultra high CPRI link capacity | NETMANIAS 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.
Related Contents Why should jitter be minimized in CPRI fronthaul? ‐ frequency accuracy Netmanias Interview with HFR ‐ Fronthaul Solutions for LTE/LTE‐A C‐RAN (Cloud RAN) Evolution of Mobile RAN Architecture in LTE/LTE‐A Era (C‐RAN) C‐RAN Classification (1): Coverage‐based Required CPRI link capacity in function of radio technologies CPRI (1): Emergence of C‐ RAN/Fronthaul and CPRI overview NTT Docomo’s Advanced C‐ RAN Architecture SK Telecom: LTE Fronthaul and Backhaul Architecture
Delivering compressed I/Q samples, another possible way of lowering the fiber cost, will be introduced in the next post. Read more: Required CPRI link capacity in function of radio technologies
You can download pdf file at http://www.netmanias.com/en/?m=view&id=oneshot&no=6259
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