# Lightening_Transformer - Photonic AI Accelelerator

Implementation of the photonic AI accelerator – Lightening Transformer for accelerating general matrix multiplication for Fully dynamic and full range matrix inputs


## Prerequisites

- VHDL 
- Visual Studio code
- ModelSim

## Setup

1. Install ModelSim on your system.
2. Any code editor or a text editor to edit the VHDL code is sufficient


## Compilation
 Use ModelSim to compile the files

## Output simple test 
 Testbenches are also written using the text editor


## Output log files 
Simulate the output using graphical user interface offered by the modelsim and the graphs are displayed there

## Project directory structure

Lightening_Transformer
- ddot_unit.vhd
- ddot_unit.vhd
- dtpc.vhd
- basic_dtpc_tb.vhd
- README.md


## The dynamic full range matrix computation in Lightening Transformer (LT)
- The basic computation unit in LT is DDOT - Dynamically- Operated Full - range Dot - Product Unit
- The DDOT is capable of computing the Dot product between two vectors
- When two dynamic full-range matrices A & B are considered, the two vector operators of 
    DDOT is the row vector of A and column vector of B
- Multiple DDOT units are arranged in a cross-connect array to get the DPTC 
- DPTC - Dynamically-Operated Photonic Tensor Core - is the Core unit of photonic accelerator
- The DPTC can compute a small full matrix multiplication at once with the internal DDOT units
- Several DPTC units are clustered onto a single tile
- The DPTC units on a tile are operated in parallel, and results (partial sums) are 
    aggregated locally in analog domain itself
- The photonic accelarator has multiple Tile units Nt, each with Nd number of DPTC cores
- Each Tile unit operates on different slice of Matrix A 
- Every Tile unit operates on the same slice of Matrix B
- The results of Nt tile units are aggregated to get the final output


# Architectural level optimization
1. Broadcasting operand by optical interconnect
    - The matrix B (M2) is shared across tile units
    - Slices of M2 are modulated globally and broadcast to every DPTC core on every tile in each cycle

2. Temporal partial sum aggregation in Analog-domain
    - The results of Nd DPTC cores on each tile units are aggregated in Analog domain first by time integral 
        and then converted into digital domain


# Tiling and spatial/temporal mapping algorithm
1. Transpose B to obtain BT[Nq][Nm]  (so we treat B columns as row vectors for dot products)

2. Partition A into Nt row-blocks:
   Each Tile_t gets Np_per_tile = Np / Nt rows of A

3. For each cycle (processing Nh columns of B at a time):
   a. Broadcast BT_slice of size Nh x Nm (Nh rows of BT = Nh columns of B)

4. On each Tile:
   a. Split A_slice into Nd column-blocks per row:
      Each DPTC_d gets Nm_per_core = Nm / Nd columns from A_slice

   b. Split BT_slice across DPTCs (each gets corresponding Nm_per_core cols):
      Each DPTC_d receives a submatrix of size Nh x Nm_per_core

   c. In each DPTC:
      For each row i in A_slice:
         For each row j in BT_slice:
             Compute DDOT(A_row_chunk[i], BT_row_chunk[j]) using vector width N
             Accumulate result in C[i_global][j_global]
5. Repeat for all Nh-wide slices of B

6. Output the result matrix C[Np][Nq]


# Lightening Transformer Hardware implementation

1. Modulation & WDM
    - The input operands are modulated into full-range values with MZM (Mach-Zehnder Modulator)
    - The phase encoding by Eout = Ein cosφ ensures full range encoding
    - The different wavelengths into which the operands are modulated are multiplexed 

2. The DDOT unit is the basic computing unit
    - It has the directional coupler, phase shifter, and optical detectors to perform the operations 
        in analog optical domain

3. The DPTC is the photonic Core
    - The photonic core is formed by optical cross-bar style array of DDOT units 


# Lightening Transformer architecture and its operation

1. Lightening Transformer Overview:
- General Matrix Multiplication between dynamic, Full-range matrices
- Utilizes broadcast opportunity in optical domain, analog temporal accumulation

2. Key Components:
DDOT:
- DDot is Dynamically-Operated Full-range Dot-Product Unit.

DPTC:
- DPTC - Dynamically-Operated Photonic Tensor Core

Photonic accelerator

3. Computation Mechanism
- Operates in optical domain leveraging operand sharing to accelerate the computaion