Radio Interface Techniques in 3GPP Systems

In wireless cellular systems, mobile users share a common medium for transmission. There
are various categories of assignment. The main four are FDMA (frequency division multiple
access), TDMA, CDMA, and OFDMA (orthogonal frequency division multiple access). Each
of the technologies discussed earlier in the chapter utilizes one of these techniques. This is
another reason for distinguishing the technologies.

1.2.1 Frequency Division Multiple Access (FDMA)
In order to accommodate various devices on the same wireless network, FDMA divides the
available spectrum into sub-bands or channels. Using this technique, a dedicated channel can
be allocated to a user, while other users occupy other channels or frequencies.
FDMA channels can suffer from higher interference. They cannot be close together due to
the energy from one transmission affecting the adjacent or neighboring channels. To combat
this, additional guard bands between channels are required, which also reduces the system’s
spectral efficiency. The uplink or downlink receiver must use filtering to mitigate interference
from other users.

1.2.2 Time Division Multiple Access (TDMA)
In TDMA systems the channel bandwidth is shared in the time domain. It assigns a relatively
narrow spectrum allocation to each user, but in this case the bandwidth is shared between a
set of users. Channelization of users in the same band is achieved by a separation in both
LTE Network Architecture and Protocols 7
frequency and time. The number of timeslots in a TDMA frame is dependent on the system.
For example, GSM utilizes eight timeslots.
TDMA systems are digital and therefore offer security features such as ciphering and
integrity. In addition, they can employ enhanced error detection and correction schemes
including FEC (forward error correction). This enables the system to be more resilient to noise
and interference and therefore they have a greater spectral efficiency than FDMA systems.

1.2.3 Code Division Multiple Access (CDMA)
The concept of CDMA is slightly different to that of FDMA and TDMA. Instead of sharing
resources in the time or frequency domain, the devices are able to use the system at the same
time and using the same frequency. This is possible because each transmission is separated
using a unique channelization code.
UMTS, cdmaOne, and CDMA2000 all use CDMA as their air interface technique. However,
the implementation of the codes and the bandwidths used by each technology is different. For
example, UMTS utilizes a 5MHz channel bandwidth, whereas cdmaOne uses only 1.25MHz.
Codes are used to achieve orthogonality between the users. In the HSDPA system, for
example, the channel carrying the data to the user has a total of 16 codes in the code tree.
If there are multiple users in the system at the same timeslot of scheduling, the users will
share the 16 codes, each with a different part of the code tree. The more codes assigned to
the HSDPA user, the higher the data rate becomes. There are limitations on the code tree and
hence capacity is tied to the code allocation. Voice users and control channels get the highest
priority in code assignment, and then the data users utilize the remaining parts of the tree.
WCDMA systems are also interference limited since all users are assigned within the same
frequency in the cell. Hence, power control and time scheduling are important to limit the
interference impacting the users’ performance.

1.2.4 Orthogonal Frequency Division Multiple Access (OFDMA)
OFDMA uses a large number of closely spaced narrowband carriers. In a conventional FDM
system, the frequency spacing between carriers is chosen with a sufficient guard band to ensure
that interference is minimized and can be cost effectively filtered.
In OFDMA, the carriers are packed much closer together. This increases spectral efficiency
by utilizing a carrier spacing that is the inverse of the symbol or modulation rate. Additionally,
simple rectangular pulses are utilized during each modulation symbol. The high data rates are
achieved in OFDM by allocating a single data stream in a parallel manner across multiple
subcarriers.
The frame structure and scheduling differences between CDMA and OFDMA are discussed
in the next chapter.