Why are Low Frequency Bands Configured in FDD Mode?
Mandatory Guard Period in TDD Bands makes them highly inefficient if used in lower frequency configuration
In this note, we will discuss why low bands are used in FDD (Frequency Duplex Mode) mode and why not in TDD (Time Duplex Mode). But before we dive into details let’s first discuss the basics.
FDD vs TDD
The following diagram explains the difference.
FDD (Frequency Division Duplex) is like a double road separated by a divider, whereas TDD (Time Division Duplex) is a single road where the traffic is allowed to move in only one direction at a time. In order words, for a TDD BTS to start receiving it has to first Stop Transmission and immediately switch ON its receiver to process incoming signal. Hence all elements of the TDD network need to be synchronised using a GPS clock for the network to function properly.
TDD Frame Structure
A typical LTE TDD Frame looks something like this.
Typical LTE TDD Frame is 10 milliseconds in length which keeps repeating in a cyclic fashion. Each frame is further subdivided into 10 Sub-Frames, each of 1 (One) milliseconds in length. Now these Sub-Frames are further divided into Symbols, and each Sub-Frame consists of 14 Symbols.
Now, the Sub-Frames are of various types - those marked “D” are called “Downlink Sub-Frame” and carry data from the BTS (eNBs) to the UEs (Handsets), and those marked “U” are called “Uplink Sub-Frame” and carry data from the UE to the BTS.
There is also a Frame marked “S” called a “Special Sub-Frame” which consists of three types of symbols - Downlink Piot, Guard Period, and Uplink Pilot. The Downlink and Uplink Pilots add to the capacities of the respective links, but the Guard Period is designed to serve a special purpose - having huge implications on network planning and the overall capacity of the network as explained below.
Guard Period
The Guard Period (GP) is nothing but gaps in the Frame where nothing is transmitted or received. It is just used to prevent the neighboring BTSs (eNBs) from getting choked due to incoming transmissions emanating from neighboring BTSs. The following diagram explains.
Here the BTSs are working in a frequency band, let’s say 2.3 GHz. Now since the working is in the TDD mode both the eNBs have to synchronise. Which means both have to transmit and receive at the same time. Assume that both the BTSs are now in transmit mode, and they switch to receive mode after some time simultaneously. This means that when they make their switch each of their transmitted RF waves is already in the air moving towards each other. Now for this RF wave to travel 22 km it will take approximately 0.0733 milliseconds (at the speed of light). Hence the receiver of both the BTSs needs to be blocked for that duration to prevent it from choking. This if calculated in Symbols translates to 1 (see box above).
Therefore, to protect the BTSs (eNBs) from choking each other a certain number of GPs are added to the TDD LTE Special Sub-Frames. Typical in Indian summers 2 GPs in each special Sub-Frames is sufficient. But in the winter due to the waveguide effect of the atmosphere, upto 9 GPs are required to provide protection upto 200 Kms (1 GP = 22 Kms). Resulting in a capacity loss of 17%.
Capacity Loss Calculation
The loss of capacity on account of GPs can be calculated as follows. As mentioned above, in winter 9 GP symbols are required to enable at protection of signal traveling to a distance of as much as 200 Km.
The total number of Symbols provisioned for the configuration of “DSUDD” as described in the figure above is = 14 x 3 (DL Sub-Frame) + 3 (Spl Sub-Frame) = 45. Now total number of symbols allocated for GP is 9. Hence, the loss of system capacity is estimated as = 9 / (9+45) = 17%.
TDD in Sub GHz Bands
We all know that the capability of the Sub-GHz bands to carry signals is far greater than those at higher frequencies. Therefore, such signals due to the atmospheric waveguide effect can end up traveling as high as 500 kms. Hence, for the same DSUDD configuration, if all the 12 symbols in the “Special Sub-Frame” are reserved for GP, it will give protection of ONLY 252 Km (21 x 12 = 252).
Therefore to protect a distance of 500 km will need 24 Symbols, thereby turning the Frame configuration into DSSUD - resulting in a 50 % loss of capacity of the downlink.
Conclusion
Hence, it is clear from the above discussion that configuring low spectrum bands (2.1 GHz and below) in TDD mode will result in huge capacity loss due to the need for provisioning GP (Guard Period). That is why all low spectrum bands 600 MHz to 2.1 GHz are configured in FDD mode and not in TDD mode. Configuring High-frequency bands in TDD mode is easy as RF signals emanating from BTS get attenuated quickly compared to those at the low spectrum bands.