Re: biomedical sensor network
I don't know whether Zigbee is same as XBee or XBee Pro. I have used XBee Pro (bought from rhydolabz) and interfaced with PIC18F's UART. XBee/Pro needs configuration using XCTU software. Please Zip and post your MPLAB project files and also the Proteus file and mention which compiler you are using. I can help you with mikroC PRO PIC code. You can easily port it to other C compilers.
nRF24L01 will be cheaper but I have no experience with nRF. I have only used nRF successfully with Arduino.
XBee/Pro are 3.3V device. Maybe Zigbee are also 3.3V device. If yes, and if you have interfaced Zigbee to PIC16F877A without voltage level translators then maybe your Zigbee is already damaged.
Even nRF24L01 is a 3.3V device. If your RF module is a 3.3V device then see if PIC16LF877A or any other 3.3V PIC is available.
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Maybe you are using this module.
**broken link removed**
If yes then it is a 3.3V device. I have used this module but I don't remember much. The module has two different voltages. One is higher and it is for doing configuration. The other is for after config. There is some AT commands to configure the module pair. Just configure both modules as transreceivers. Some body had interfaced this module to 6V PIC and its UART was damaged and it was giving out some AT response continuously. So, never connect it directly to a 5V PIC. Use a 3.3V PIC.