Modelling considerations for a jump model

The Problem:

Suppose I have a simple jump model for an asset price

$$dS = S(t-)[\mu dt + YdN(t)]$$

where $$N(t)$$ is a Poisson process and $$Y_i$$ are the jump sizes (assume independece of $$N(t)$$ and $$Y_i$$). Also for simplicity assume that $$Y_i$$ can only take two values, lets say $$a$$ and $$b$$. What constraints are there for $$\mu$$, $$a$$ and $$b$$?

I should also add that we can assume the existence of a money market account with constant short rate: $$B(t) = B_0e^{rt}$$

My initial thoughts:

In order to preserve positivity of stock prices I need both $$a$$ and $$b$$ to be greater than $$-1$$. I also need to consider when there might be arbitrage in my model. However I can't work out what additional constraints I need to impose. Can anyone help?

Thanks in advance for any responses.