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The air interface (known as Um in GSM: Uu in UMTS, LTE, 5G) of a mobile communications network is used to communicate between a mobile device and its RAN. In the GSM system, mobile devices use the frequency‐division multiple access (FDMA)/time‐division multiple access (TDMA)‐based radio access methods. In UMTS, UEs use the Wideband Code Division Multiple Access (WCDMA)‐based radio access method, LTE UEs use the Orthogonal Frequency Division Multiplexing Access (OFDMA) and SC‐FDMA‐based radio access methods, and 5G uses the OFDMA‐based radio access method to communicate with the respective RAN. Figure 2.9 illustrates the evolutions of the 3GPP mobile communications systems along with the radio access method used in each evolution from the GSM to the 5G system. The arrow indicates the increased data rates in each evolution.

The air interface and its evolutions can be studied in terms of the access technologies and its modulation technique used, which is shown in Figure 2.9. One of the factors that determine the rates of data transfer or throughputs is the modulation technique used by each radio access method. Table 2.2 shows the modulation techniques, bandwidths, and data rates offered over the GSM, UMTS, LTE, and 5G air interface. From this table, it is observed that even within a particular radio access method, for example, UMTS, the data throughput offered in the downlink (DL) and uplink (UL) directions are different depending on the modulation technique used by it.

Figure 2.9 Illustration: 3GPP systems and air interface evolutions.

Table 2.2 Evolutions of 3GPP systems and their air interfaces.

System/features	Modulation techniques	Bandwidth	Throughputs
GSM (2G)	GMSK	200 KHz	40 kbps
GPRS (2.5G)	GMSK	200 KHz	171 kbps
EDGE (2.75G)	GMSK, 8‐PSK	200 KHz	384 kbps
UMTS (3G)	QPSK	5 MHz	384 kbps
HSDPA (3G) Feature	DL: QPSK, 16 QAM UL: QPSK	5 MHz	DL: 14.4 Mbps UL: 384 kbps
HSUPA (3G) Feature	DL: QPSK, 64 QAM UL: QPSK, 16 QAM	5 MHz	DL: 14.4 Mbps UL: 5.8 Mbps
HSPA+(3G) Feature	DL: QPSK, 64 QAM UL: QPSK, 16 QAM	5 MHz	DL: 21–42 Mbps UL: 11 Mbps
LTE (4G)	DL: QPSK, 64 QAM	1.4, 3,5, 10, 15,20 MHz	DL: 300 Mbps UL: 75 Mbps
LTE‐A (4G)	DL: QPSK, 64 QAM	1.4, 3,5, 10, 15, 20 MHz	DL: 3 Gbps UL: 1.5 Gbps
5G	QPSK, 16/64/256 QAM	5 to 400 MHz	DL: 20 Gbps UL: 10 Gbps

As shown in Table 2.2, the UMTS features and the LTE and 5G system air interface use the different modulation techniques in the UL and DL directions between the UE and RAN. Using GMSK modulation, one modulation symbol can carry 1 bit of data; one QPSK modulation symbol can carry 2 bits of data; one 16 Quadrature Amplitude Modulation (QAM) modulation symbol can carry 4 bits of data; and so on. The number of data bits transmitted per modulated symbol is called the modulation order. The throughput offered by different modulation techniques is shown in the fourth column.

UMTS features High-Speed Downlink Packet Access (HSDPA) and High-Speed Uplink Packet Access (HSUPA) are known as the High-Speed Packet Access (HSPA) method in the DL and UL directions, respectively. The HSPA+ feature is called as the evolution of the HSPA method. These access methods use the same modulation technique, i.e. QAM, but with different modulation orders. HSPA+ and HSUPA use higher (64) QAM than the HSDPA method. However, the HSPA+ method uses multiple-input multiple-output transmission techniques (MIMO) with two antennas for transmission of data from the UTRAN to UEs. More about the modulation techniques and MIMO transmissions are described later in Chapter 19.