graph-based framework for function splitting in cran

Graph-based Framework for Flexible Baseband Function Splitting

Jingchu LIU*, Sheng ZHOU*, Jie GONG*, Zhisheng NIU*, and Shugong XU+

Network Integration for Ubiquitous Linkage and Broadband (NiuLab)

Tsinghua University, Beijing, China *and

Intel Labs, Beijing, China +

Graph-based framework for flexible baseband function splitting

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Contents

• Fronthaul Bottleneck of C-RAN• Baseband Compression via Function Splitting• Graph-based Framework for BB function splitting• Customized Genetic Algorithm• Simulation Results• Future Work

Jingchu Liu Tsinghua University


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Contents




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FH Bottleneck of C-RAN

• C-RAN and Fronthaul (FH)[1]

• C-RAN centralizes BB processing from radio site• FH carries BB samples (I/Q data) btw. RRH and BBU• Reduce cost, ease implementation of cooperative technology, flexibility

• FH Bottleneck[2]

• Bandwidth: 10Gbps/ant. for typical LTE UL/DL (100Mbps backhaul)• Delay: 3ms maximum processing latency for LTE HARQ -> maximum

500us fronthaul transmission dealy• Synchronization: ~1ppb frequency accuracy, ~1us phase difference

• C-RAN is sound, but FH is (too) expensive

FH = Fronthaul BB = BasebandRRH = Remote Radio Head BBU = Baseband Unit

[1] CMRI. C-RAN: The road towards green RAN[2] CPRI Specification, “interface specification,” 2013.



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BB Compression via Function Splitting

• Transparent BB compression[3]

• Time-domain or frequency domain• Compress at RRH(BBU), reconstruct at BBU(RRH)• Transparent: no changes needed for BBU/RRH

[3] Samardzija, “Compressed Transport of Baseband Signals in Radio Access Networks.,“ TWC 2012Jingchu Liu Tsinghua University


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• Transparent BB compression• Around 3x compression at the cost of distortion (EVM<1.5%)• Enough?



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• Transparent BB compression• Compression via function splitting[4][5]

• Preprocessing at remote site to reduce FH bandwidth• … “move” some BB processing from data center back to radio site

[4] Lorca and Cucala. “Lossless compression technique for the fronthaul of LTE/LTE-advanced cloud-RAN architectures,” in WoWMoM 2013[5] Dotsch, "Quantitative Analysis of Split Base Station Processing and Determination of Advantageous Architectures," Bell Labs Journal, 2013Jingchu Liu Tsinghua University


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• Transparent BB compression• Compression via function splitting

• Why it works?

200+ times



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• Transparent BB compression• Compression via function splitting• Why FLEXIBLE function splitting[6][7][8]

• Various implementation constraint (BW, delay)• Continuous evolvement of PHY technology (massive MIMO)• Unpredictable service requirements (sub-ms latency in 5G)

• Decide splitting option “on-the-fly”!• Need a framework for deciding the “best split”

Research Question:How to decide the best BB function splitting scheme?

[6] P. Rost , et al., “Cloud technologies for flexible 5G Radio Access Networks”, Comm. Mag. May 2014[7] D.Wubben , et al., Benefits and Impact of Cloud Computing on 5G Signal Processing, SPM, Nov 2014[8] J. Liu, et al., “CONCERT: A Cloud-Based Architecture for Next-generation Cellular Systems”, IEEE WCM, Dec 2014



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Graph-Based Framework for BB Function Splitting

• BB processing structure and graph• Basic processing structures are chains• Cooperative processing structures like CoMP may form loops

Typical baseband processing structure of LTE BSJingchu Liu Tsinghua University

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• BB processing structure and graph• Basic processing structures are chains• Cooperative processing structures like CoMP may form loops• General BB processing structures are graphs :

• Each node : BB processing function• Each (directed) link : logical link btw. BB processing fcn.• Node weight : computational complexity of a node• Link weight : fronthaul BW requirement• Sources and sinks terminate BB processing chain (path)



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• BB processing structure and graph• Function splitting and graph clustering

• Graph-clustering: group nodes by “similarity”• Here, places to place BB functions define cluster• Therefore, where to place a function = which cluster to assign a node



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• BB processing structure and graph• Function splitting and graph clustering

• Computational cost of cluster i w.r.t scheme• Fronthauling cost between clusters i, j w.r.t scheme• Path delay constraint




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• BB processing structure and graph• Function splitting and graph clustering• Graph clustering is multi-obj. combinatorial opt.

• Clustering schemes can be represented with integer valued vectors• Overall formulation is 2-obj. opt. with delay constraint



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Customized Genetic Algorithm (GA)

• Natural encoding• Use to represent the chromosome



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• Natural encoding• Linearly combined and delay punished fitness fcn.

• Computational cost + fronthauling cost + delay penalty



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• Natural encoding• Linearly combined and delay punished fitness fcn.• Dispersive crossover

• 2 Parents generate 1 offspring• Genes are randomly passed down from parents



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• Natural encoding• Linearly combined and delay punished fitness fcn.• Dispersive crossover• Graph-based mutation

• ONLY mutate to connected clusters• Graph seeding

• Some modules can not be placed arbitrarily in reality, e.g. antennas• Seed nodes are assigned with fixed cluster indices• DO NOT take part in crossover or mutation



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Simulation Results

• Simulation Setup• BB processing structure, NOTE for CoMP• Link weight (fronthauling cost)



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Simulation Results

• Simulation Setup• Node weights (computational cost)



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Simulation Results

• Simulation Setup• Exponential cost and productive delay fcn.



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Simulation Results

• Simulation Setup• GA parameters



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Simulation Results

• Tradeoff between computational and fronthauling cost



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Simulation Results

• Tradeoff between computational and fronthauling cost



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Simulation Results

• Cooperative processing (like CoMP)



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Simulation Results

• Delay constraint

Delay tolerance decrease



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Future Work

• Realistic simulation parameters• Analytic derivation of optimal split schemes• Information-theoretic interpretation of BB split



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Thanks

• Any problems / suggestions?



34Jingchu Liu Tsinghua University

graph-based framework for function splitting in cran

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