Good write up on RSRP/RSSI values http://rootzwiki.com/topic/25921-this-is-why-your-verizon-nexus-signal-*****/
RSRP:
Reference signal received power (RSRP), is defined as the linear average over the power contributions in Watts of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth. Used to measure the signal of your LTE (GSM/4g) connection. In short, it's what's used to determine the best cell tower your LTE device can connect to at the given time. Anything below say -80db is considered pretty good and you're pretty close to a tower. -80db to -90db is average what you should expect most of the time. -90db or above and you're probably in an "extended network" area for LTE and getting close to a likely handoff. -105db and above you would be likely to see a handoff to 3G if your signal does not get better.
Throughput for your connection measured with LTE is estimated to decline between 30-50% if your signal goes from -75db to -90db for RSRP. Above -95db and your throughput dramatically drops. At around -108db and worse, your throughput for download drops to nearly 3G rates or worse. Note that this doesn't exactly represent how strong your signal is, just the potential of how efficiently it will send that data.
RSSI:
Received Signal Strength Indicator (RSSI), is the linear average of the total received power in Watts. This is used to measure the db signal for CDMA (3g/2g [2g being your 1x and voice]) signals and used in determining the "signal to noise ratio". It was what was shown on all devices as the "signal" under the Android settings before Android 4.0. Basically this is how much noise/interference is in your connection. Not so good for measuring the overall power of it. It should be in the range of like -58db and greater (like -32db). In other words, the closer it is to being positive, the better. If it's in the high 80s or 90s, your signal is probably starting to cause some slight battery drain when idle. RSSI has less to do with how great your network speeds are and more to do with how good your potential battery life will be.