Design Synthesis is the process of taking the functional architecture developed in the Functional Analysis and Allocation step and decomposing those functions into a Physical Architecture (a set of product, system, and/or software elements) that satisfy system required functions.
SMC Systems Engineering Handbook, Figure 17
Synthesis is the process whereby the Functional Architectures and their associated requirements are translated into physical architectures and one or more physical sets of hardware, software, and personnel solutions. It is the output end of the Design Loop. As the designs are formulated, their characteristics are compared to the original requirements, developed at the beginning of the process, to verify the fit. The output of this activity is a set of analysis-verified specifications which describe a balanced, integrated system meeting the requirements, and a database that documents the process and rationale used to establish these specifications.
The first step of Synthesis is to group the functions into physical architectures. This high-level structure is used to define system concepts and products and processes, which can be used to implement the concepts. Growing out of these efforts are the internal and external interfaces. As concepts are developed they are fed back in the Design Loop to ascertain that functional requirements have been satisfied. The mature concepts, and product and process solutions are verified against the original system requirements before they are released as the Systems Engineering Process product output. Detailed descriptions of the activities of Synthesis are provided below. 
Physical architecture is a traditional term. Despite the name, it includes software elements as well as hardware elements. Among the characteristics of the physical architecture (the primary output of Design Synthesis) are the following: 
- The correlation with functional analysis requires that each physical or software component meets at least one (or part of one) functional requirement, though any component can meet more than one requirement,
- The architecture is justified by trade studies and effectiveness analyses,
- A product Work Breakdown Structure (WBS) is developed from the physical architecture,
- Metrics are developed to track progress among Key Performance Parameters (KPP), and
- All supporting information is documented in a database.
The four (4) steps that comprise the SE Process are:
- Step 1: Requirements Analysis
- Step 2: System Analysis and Control
- Step 3: Functional Analysis Allocation
- Step 4: Design Synthesis
AcqLinks and References:
-  Space and Missile Systems Center (SMC) Systems Engineering Primer & Handbook
-  Systems Engineering Fundamentals Chapter 6 – Design Synthesis
- Lisa Guerra of NASA’s Exploration Systems Mission Directorate – System Synthesis Module, Space Systems Engineering, Spring 2008