Mostly, people find what is right approach to upgrade SAP from one release to another release. It's important before we quickly jump on the technical overview of S/4HANA Upgrade from 1511 to 1909. Let's understand SAP S/4HANA is not successive release of SAP ERP, running on traditional DB. SAP S/4HANA 1909 powered by SAP HANA, Interesting to talk about When First SAP HANA release was commence in the 2011.
Let's Get started Quickly
People have frequent question about, what are the Business benefits over and above existing release of SAP S/4 . So is for other, Some of the Line of Business Innovations
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Let's Get Started with Core of SAP S/4HANA 1909

Upgrade SAP HANA Compliance 2.0


As We all knows , Every SAP Product has its Main stream maintenance , unlike any sap product. SAP S/4HANA 1511 mainstream maintenance expires Dec-2020 . Kindly refer PAM
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Now Let's Try understand, as we all know , Upgrade is one of the Sequential , process, Like one has to brainstorm on various aspects before planning any upgrade,
Migrating to new H/W , Operating System, Underlying Database/ Compliance (S/4 HANA 1909 , supports SAP HANA 2.0) as compared to SAP HANA 1.0 , and SAP S/4 HANA Upgrade from 1511 to 1909
Reference for SAP HANA
2871484 - SAP supported variants of Red Hat Enterprise Linux Version
SAP S/4HANA 1909, Being as SAP S/4HANA , Embedded Analytics (unlike Traditional Database, two Separate SYSTEMS Where OLTP and OLAP ) Processes run on two different System , With Power of SAP HANA runs In-Memory , has OLTP and OLAP processing on same instance , no additional Extraction required , ready to use reporting. is something is incomparable.
Business Process Automation:
Basically, Innovation driven of Business process automation, to give customer and end user seamless experience when working on SAP S/4HANA.

When we say SAP S/4HANA , it mainly comprise of :
- SAP S/4HANA 1909 , comprise of Automation, Machine Learning, Prediction & Simulation
- SAP fiori intuitive user experience (UI5)
- Innovation over, various line of business
SAP fiori mainstream
- add-on product (SAP fiori front end) for SAP NetWeaver Application Server for ABAP
- SAP fiori Front end comprise of UIs, technology, gateway components including launchpad
- SAP FES 6.0, long term maintenance version
- enterprise-ready web apps
- Task oriented apps, context-sensitive navigation
- Customer centric , apps


As you can see SAP Fiori Front End Server 6.0 is long term maintenance ,
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SAP UX strategy & Fiori apps
User Experience, broadly user experience using website or application. User Experience is broadly categories into three design principles
a) technology
b) Business
c) Human Value
SAP UX strategy Mainly consists of
a) New: Consumer grid user experience
b) ReNew: applied to existing application
c) Enable: ability to improve user experience , like business critical applications
References:
Overall, As we all knows SAP S/4 HANA 1511, mainstream maintenance is expiring by END of 2020. I've just highlighted, some key areas, one should consider, mainly impact overall all upgrade strategy. SAP deliver SAP Intelligent ERP, keeping customer needs and demand.
Thanks you for your support . Your support Always motivates , with regards to sharing Information , available. Kindly do write back , which are other topics ,you like to be publish. I will try my best to write and publish/write on those topics.
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King Sharma