Easily available light sensor for simple LIFI project

I am using UART TX line to transmit text. The TX pin is driving an LED, The LED is lit for logic 1 and LED is off for logic 0. Then I have a photo resistor right in front of LED. One end of photo resistor is connected to 5 volts and the other is connected to 500Kohm resistor. 500Kohm resistor is grounded on the other side. and I am feeding the voltage across the 500Kohm resistor back to UART RX pin.

When I send thea character at 1200 baud, I was expecting that I will see the data back. But I get a garbage. I did ensure that my uart is working by looping back tx to rx - with this I receive the correct character I sent.

But I am not able to do the same when used with led to photo resistor path.

Is 1200 baud too high for this lifi interface? I do not have osciloscope to determine how well the signal across the 500Kohm resistor looks. I did check the static value across the 500K resistor. Logic high measure 4.2 volts and logic low measured 0.05 volts. So I feel that the voltage divider looks ok. So I am not sure why the arduino uno altserial uart is not able to receive the chanacter I sent. I do see garbage data but it is not the character I sent.

Any input on this will be great.

Are you using CdS LDRs?

They are sloooow

If Cds sensors are slow then which sensor s typically used for such low baud rate lifi communication?

Photodiode/ phototransistor

(Disclaimer: I have absolutely no experience of LiFi)