TTG vs RTG: Understanding Time Gaps in WiMAX and LTE

This article explains the concepts of TTG (Transmission Time Gap) and RTG (Receiver Time Gap) used in WiMAX and LTE systems.

What are TTG and RTG?

• TTG: Stands for Transmission Time Gap.
• RTG: Stands for Receiver Time Gap.

TTG Gap Explained

The TTG gap is used as a transmit/receive transition gap.

As illustrated in the figure below, it appears after the downlink subframe (of frame N) and before the uplink subframe of the same frame “N”. This parameter is defined for both the base station and the subscriber station in a WiMAX system.

During this gap, neither the base station nor the subscriber station transmits any data or information. For example, if we consider this specification for the base station, it represents the time required for the base station to switch from transmit mode to receiver mode.

The TTG gap ensures that there is no data loss during switching.

• It provides sufficient time for the Base Station RF carrier to ramp down before settling.
• It allows enough time for the antenna switch to transition from transmit to receive mode.
• It enables Base Station RF and baseband modules to prepare for receiving WiMAX bursts transmitted from subscriber stations.

For the Subscriber station, this gap is the time needed to switch from receive mode to transmit mode. The value of the TTG gap is specified in the standard in units of Physical Slots.

$1 PS = 4 / Fs$

Where $Fs$ is the sampling frequency, which depends on the Bandwidth and sampling factor.

RTG Gap Explained

The RTG gap functions as a receive/transmit transition gap.

It is positioned after the uplink subframe (of frame N) and before the subsequent downlink subframe (i.e., Frame: N+1) in the WiMAX frame structure. During this period, equivalent to the RTG gap, both the BS and SS refrain from transmitting any data.

This gap enables the Base Station (BS) to transition from receive mode to transmit mode. It also allows the Subscriber Station (SS) to switch from transmit mode to receive mode. The RTG gap ensures that no data is lost during the switching process.

• It allows enough time for the Base Station RF carrier to ramp up before settling.
• It allows the antenna switch enough time to actuate.
• It allows the Subscriber station’s RF and baseband modules to get ready for the reception of WiMAX bursts transmitted from the base station.

For WiMAX OFDM specifications, TTG/RTG should be $> 5 \mu S$ and $<= 100 \mu S$.

Therefore, a typical choice is to use TTG and RTG values of about $80 \mu S$ (i.e., 1 OFDM symbol).

This configuration works effectively for a bandwidth of 3.5MHz and an $Fs$ of 4MHz. The specification of the TTG/RTG gap varies for OFDMA specifications. Consult the respective WiMAX standard specifications for detailed information.

LTE Considerations

LTE’s TDD frame structure incorporates a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS) in a special frame. These elements are similar to the gaps present in WiMAX.