{{distinguish, Transmission time
TTI, ''Transmission Time Interval'', is a parameter in
UMTS
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the Inte ...
(and other digital telecommunication networks) related to encapsulation of data from higher layers into frames for transmission on the
radio link layer
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
. TTI refers to the duration of a transmission on the radio link. The TTI is related to the size of the data blocks passed from the higher network layers to the radio link layer.
To combat errors due to
fading
In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a ...
and interference on the
radio link
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
, data is divided at the transmitter into blocks and then the bits within a block are encoded and
interleaved. The length of time required to transmit one such block determines the TTI. At the receiver all bits from a given block must be received before they can be deinterleaved and decoded. Having decoded the bits the receiver can estimate the
bit error rate
In digital transmission, the number of bit errors is the number of received bits of a data stream over a communication channel that have been altered due to noise, interference, distortion or bit synchronization errors.
The bit error rate (BER) i ...
(''BER''). And because the shortest decodable transmission is one TTI the shortest period over which BER can be estimated is also one TTI. Thus in networks with
link adaptation Link adaptation, comprising adaptive coding and modulation (ACM) and others (such as Power Control), is a term used in wireless communications to denote the matching of the modulation, coding and other signal and protocol parameters to the condit ...
techniques based on the estimated BER the shortest interval between reports of the estimated performance, which are used to adapt to the conditions on the link, is at least one TTI. In order to be able to adapt quickly to the changing conditions in the radio link a communications system must have shorter TTIs. In order to benefit more from the effect of interleaving and to increase the efficiency of error-correction and compression techniques a system must, in general, have longer TTIs. These two contradicting requirements determine the choice of the TTI.
In UMTS Release '99 the shortest TTI is 10
ms and can be 20 ms, 40 ms, or 80 ms. In UMTS Release-5 the TTI for
HSDPA
High Speed Packet Access (HSPA) is an amalgamation of two mobile protocols—High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA)—that extends and improves the performance of existing 3G mobile telecommunicat ...
is reduced to 2ms. This provides the advantage of faster response to link conditions and allows the system to quickly schedule transmissions to mobiles which temporarily enjoy better than usual link conditions. As a result the system most of the time transmits data over links which are better than the average conditions, because of this the bit rates in the system most of the time are higher than what the average conditions would allow. This leads to increase in system capacity.
In
1xEV-DO technology, a slot, which is not quite the same thing as the TTI, but which still fulfills a somewhat similar function, is 1.667 ms. In
1xEV-DV it has a variable length of 1.25 ms, 2.5 ms, 5 ms, and 10 ms.
Mobile telecommunications standards