It is important to understand how cellular systems work with multiple users of voice and data. There are several standards worth reviewing, similar to concepts covered for WPAN and WLAN systems. Before LTE was supported by the 3GPP, there were multiple standards for cellular technology, particularly GSM devices and CDMA devices. It should be noted that these technologies are incompatible with each other, from infrastructure to the device:

• Frequency Division Multiple Access (FDMA): Common in analog system systems but rarely used today in digital domains. It is a technique whereby the spectrum is divided up into frequencies and then assigned to users. One transceiver at any given time is assigned to a channel. The channel, therefore, is closed to other conversations until the initial call is finished, or until it is handed off to a different channel. A “full-duplex” FDMA transmission requires two channels, one for transmitting and one for receiving. 
• Code Division Multiple Access (CDMA): Based on spread spectrum technology. CDMA increases spectrum capacity by allowing all users to occupy all channels at the same time. Transmissions are spread over the whole radio band, and each voice or data call is assigned a unique code to differentiate from the other calls carried over the same spectrum. CDMA allows for a soft hand-off, which means that terminals can communicate with several base stations at the same time. The dominant radio interface for third-generation mobile was originally designed by Qualcomm as CDMA2000 and targeted 3G. Because of its proprietary nature, it didn't achieve global adoption and is used in less than 18% of the global market. It manifested in the US with Verizon and Sprint being strong CDMA carriers. 
• Time Division Multiple Access (TDMA): In this model, the capacity of the spectrum increases by splitting each frequency into time slots. TDMA allows each user to access the entire radio frequency channel for the short period of a call. Other users share this same frequency channel at different time slots. The base station continually switches from user to user on the channel. TDMA is the dominant technology for second-generation mobile cellular networks. The GSM organization adopted TDMA as a multi-access model. It resides in four distinct bands: 900 MHz/1800 MHz in Europe and Asia and 850 MHz/1900 MHz in North and South America.

Some devices and modems may still support GSM/LTE or CDMA/LTE. GSM and CDMA are incompatible with each other. GSM/LTE and CDMA/LTE can be compatible, however, if they support the LTE bands. In older devices, voice information is delivered on a 2G or 3G spectrum, which are quite different for CDMA and GSM (TDMA). Data, too, is incompatible since LTE data runs on 4G bands.