Page 241 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 227

4.3 FDD Air Interface and Radio Network

The major evolution in LTE compared to previous 3GPP wireless systems is the
completely revised air interface. To understand why a new approach was taken, a quick
look back at how data was transmitted in previous generation systems is necessary:
GSM is based on narrow 200 kHz carriers that are split into eight repeating timeslots
for voice calls. One timeslot carries the data of one voice call, thus limiting the number
of simultaneous voice calls on one carrier to a maximum of eight. Base stations use
several carriers to increase the number of simultaneous calls. Later on, the system was
enhanced with GPRS for packet‐switched data transmission. The decision to use 200
kHz carriers, however, remained the limiting factor.
With UMTS, this restriction was lifted by the introduction of carriers with a band-
width of 5 MHz. Instead of using dedicated timeslots, CDMA, where data streams are
continuous and separated with different codes, was used. At the receiving end, the
transmission codes are known and the different streams can hence be separated again.
With HSPA, the CDMA approach was continued but a timeslot structure was intro-
duced again to improve user data scheduling. The timeslots, however, were not opti-
mized for voice calls but for quickly transporting packet‐switched data traffic.
With today’s hardware and processing capabilities, higher datarates can be achieved
by using an increased carrier bandwidth. UMTS, however, is very inflexible in this
regard as the CDMA transmission scheme is not ideal for wider channels. When the
carrier bandwidth is increased, the transmission steps need to become shorter to take
advantage of the additional bandwidth. While this can be done from a signal processing
point of view, this is very disadvantageous in a practical environment where the radio
signal is reflected by objects, and the signal reaches the receiver via several paths. As a
result, the receiver sees not just one signal per transmission step but several, each arriv-
ing at a slightly different time. When transmission speed increases, which results in a
decrease in the time of each transmission step, the negative effect of the delayed signal
paths increases. As a consequence, CDMA is not suitable for carrier bandwidths beyond
5 MHz. Multicarrier operation has been defined for UMTS to mitigate the problem to
some degree at the expense of rising complexity.
The following sections now describe how LTE enables the use of much larger band-
widths in the downlink and the uplink directions.

4.3.1 OFDMA for Downlink Transmission
In the downlink direction, LTE uses Orthogonal Frequency Division Multiple Access
(OFDMA). Instead of sending a data stream at a very high speed over a single carrier as in
UMTS, OFDMA splits the data stream into many slower data streams that are transported
over many carriers simultaneously. The advantage of many slow but parallel data streams
is that transmission steps can be sufficiently long to avoid the issues of multipath trans-
mission on fast data streams discussed above. Table 4.3 shows the number of subcarriers
(i.e. data streams) used, depending on the bandwidth used for LTE. Not included in the
count is the center carrier, which is always left empty. The more bandwidth that is available
for the overall LTE carrier, the more the number of subcarriers used. As an example, a total
of 600 subcarriers are used with an overall signal bandwidth of 10 MHz. In other words,
the overall datarate can be up to 600 times the datarate of each subcarrier.