Page 247 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 233

Frequency 1 Frame (10 ms) = 10 subframes (1ms) = 20 slots (0.5 ms) Next frame

180 kHz

t
1 Subframe
=
1 symbol = 1 Resource block =
1 resource element 1 slot (0.5 ms) =
(15 kHz) 12 subcarriers × 7 symbols

Figure 4.8 LTE resource grid. Source: Wiley 2009. Reproduced with permission from John Wiley and
Sons Ltd.

whole carrier bandwidth. In this case, the mobile device returns either no channel feedback
or a wideband channel feedback over the whole bandwidth.
And finally, 10 subframes are combined into an LTE radio frame, which has a length
of 10 milliseconds. Frames are important, for example, for the scheduling of periodic
system information (SI), as discussed further below. At this point, it should be noted
that Figure 4.8 is a simplification as only eight RBs are shown in the y‐axis. On a 10 MHz
carrier, for example, 50 RBs are used.

4.3.4 Reference and Synchronization Signals
As described above, the network assigns a number of RBs to a user for each new sub-
frame, that is, once a millisecond. However, not all symbols of an RB can be used to
transmit user data. Which of the symbols are used for other purposes depends on the
location of the RB in the overall resource grid.
To enable mobile devices to detect LTE carriers during a network search and to esti-
mate the channel quality later on, reference symbols, also referred to as reference sig-
nals, are embedded in a predefined pattern over the entire channel bandwidth. Reference
signals are inserted on every seventh symbol on the time axis and on every sixth subcar-
rier on the frequency axis as shown in Figure 4.9. Details are given in 3GPP TS 36.211
[11]. A total of 504 different reference signal sequences exist, which help a mobile device
to distinguish transmissions of different base stations. These patterns are also referred
to as the Physical Cell Identity (PCI). Neighboring base stations need to use different
symbols for the reference signals for the mobile device to properly distinguish them.
Hence, six PCI groups have been defined, each shifted by one subcarrier.