Uplink (UL or U/L for short) and downlink (DL or D/L for short) are two basic terminologies used in mobile communications. These are two communication links that allow the mobile phone and the mobile network to communicate with each other.
Uplink is the communication link that transmits the signal from the mobile phone to the network base station (eNodeB, gNodeB, etc.). Downlink is the communication link from the base station to the phone. In CDMA networks, the downlink is called a forward channel, and the uplink is called a reverse channel.
When you make a phone call using your cell phone, the phone is busy doing many things. The two most basic things the phone is doing is that it is continuously sending and receiving. The mobile network base station is also doing the same thing. What a mobile phone sends is received by the base station, and what a base station sends is received by the mobile phone.
The mobile phone and the mobile network are connected to each other even when you are not on a phone call. The mobile phone is always providing its status update to the mobile network. In simple terms, it is always telling the mobile network, “I am here”.
Since both the base station (cell tower) and the mobile phone (cell phone) need to communicate with each other, the communication is separated by creating two dedicated links. These links basically determine who is sending, the phone or the network. This is where uplink and downlink communication comes in.
Downlink communication is when a radio network base station transmits the radio signal from its antennas to the antennas of a mobile phone or cell phone. Radio network base stations are BTS, NodeB, eNodeB, gNodeB, etc.
Uplink communication is when a mobile phone or cell phone transmits the radio signal from the phone antennas to the antennas of the radio network base station (BTS, NodeB, eNodeB, gNodeB, etc.).
Uplink and downlink in CDMA networks
The terms uplink and downlink are used by most mobile networks in the world, including second-generation GSM (Global System for Mobile Communications), third-generation UMTS (Universal Mobile Telephone System), fourth-generation LTE (Long Term Evolution) and fifth-generation NR (New Radio).
However, the CDMA-based 2G and 3G mobile networks use different terminologies for these communication links. In CDMA networks, IS-95 (Interim Standard 1995), CDMA2000 and EVDO (Evolution-Data Optimized), the communication link from the base station to the cell phone is called a forward traffic channel or forward channel. The communication link from the cell phone back to the base station is called a reverse channel.
In summary, the downlink communication in CDMA networks (IS-95, CDMA2000 and EVDO) is called a forward traffic channel or forward channel, and the uplink communication in CDMA networks is called a reverse traffic channel or reverse channel.
| Technology | Network-to-Phone communication | Phone-to-Network communication |
|---|---|---|
| GSM (2G) | Downlink | Uplink |
| UMTS (3G) | Downlink | Uplink |
| HSPA (3G) | Downlink | Uplink |
| IS-95 or CDMAOne (2G) | Forward Traffic Channel | Reverse Traffic Channel |
| CDMA2000 (3G) | Forward Traffic Channel | Reverse Traffic Channel |
| EVDO (3G) | Forward Traffic Channel | Reverse Traffic Channel |
| LTE (4G) | Downlink | Uplink |
| NR (5G) | Downlink | Uplink |
Why uplink and downlink?
The terms uplink and downlink are also used in satellite communications. The link from the satellite to the ground is called downlink, and the link from the ground to the satellite station is called uplink. To remember this concept in mobile communications, look at the picture below.
As you can see, base station antennas are generally always at a higher position than a mobile phone; therefore, they need to transmit the radio signal downwards, hence the term downlink. On the other hand, a mobile phone generally always has to send the radio signal in the upward direction, hence the term uplink. Of course, this situation may change if you are in a high-rise building, but the terminologies are based on general situations for the majority of the population.
The uplink and downlink separation – Duplexing
The uplink and downlink communication links are separate, but the way they are separated from each other may differ depending on the mobile cellular technology, e.g. GSM, UMTS, LTE, NR, etc. The technique used to determine this separation is called a duplexing scheme, which is a fundamental concept in mobile communications. The duplexing scheme determines whether the uplink and downlink communication shall be carried out on separate frequency bands or separate timeslots (of the same frequency band).
When the uplink and downlink communication takes place using separate dedicated frequency bands, it is called Frequency Division Duplex (FDD) because the separation or division is based on the frequency bands. When uplink and downlink communication happens on the same frequency band but using different slots, it is called Time Division Duplex (TDD) because the separation or division is based on the timeslots.
I have written a dedicated post on Frequency Division Duplex (FDD) and Time Division Duplex (TDD), which you can check out for more details.
