1.3 Modulation Techniques

1.3 Modulation Techniques

Modulation techniques embed a signal into the carrier frequency. They can be classified into analog and digital modulations. Traditional analog modulations include amplitude modulation (AM) and frequency modulation (FM). In digital modulations, binary 1s and 0s are embedded in the carrier frequency by changing its amplitude, frequency, or phase. Subsequently, digital modulations, called keying techniques, can be amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK).

Some new popular keying techniques include Gaussian minimum shift keying (GMSK) and differential quadrature phase shift keying (DQPSK). GMSK is a type of FSK modulation that uses continuous phase modulation, so it can avoid abrupt changes. It is used in GSM (Groupe Speciale Mobile) systems, and DECT (digital enhanced cordless telecommunications). DPSK is a type of phase modulation, which defines four rather than two phases. It is used in TDMA (time division multiple access) systems in the United States.

A significant drawback of traditional radio frequency (RF) systems is that they are quite vulnerable to sources of interference. Spread spectrum modulation techniques resolve the problem by spreading the information over a broad frequency range. These techniques are very resistant to interference. Spread spectrum techniques are used in code division multiple access (CDMA) systems, and are described in more detail in Section 1.4.

1.3.1 Wireless System Topologies

Two basic wireless system topologies are point-to-point (or ad hoc) and networked topology. In the point-to-point topology, two or more mobile devices are connected using the same air interface protocol. Figure 1.3a illustrates the full mesh point-to-point configuration, where all devices are interconnected. Limitations of this topology are that the wireless devices cannot access the Web, send e-mail, or run remote applications.

In the networked topology, there is a link between wireless devices connected in the wireless network and the fixed public or private network. A typical configuration, shown in Figure 1.3b, includes wireless devices (or terminals), at least one bridge between the wireless and the physical networks, and the numbers of servers hosting applications used by wireless devices. The bridge between the wireless and the physical networks is called the base station or access point.

Figure 1.3: Wireless topologies— (a) point-to-point topology and (b) networked topology.

1.3.2 Performance Elements of Wireless Communications

Wireless communication is characterized by several critical performance elements:

• Range

• Power used to generate the signal

• Mobility

• Bandwidth

• Actual data rate

The range is a critical factor that refers to the coverage area between the wireless transmitter and the receiver. The range is strongly correlated with the power of the signal. A simplified approximation is that for 1 milliwatt of power, the range is one meter in radius. For example, 1 watt of power will allow the range of 1 kilometer in radius. As the distance from the base station increases, the signal will degrade, and data may incur a high error rate. Using part of the spectrum for error correction can extend the range; also, the use of multiple base stations can extend the range.

Mobility of the user depends on the size of the wireless device. Miniaturization of the wireless device provides better mobility. This can be achieved by reducing the battery size and consequently by minimizing power consumption; however, this will cause the generated signal to weaken, giving reduced range. In summary, there should be a trade-off between the range and the mobility: the extended range will reduce the mobility, and better mobility will reduce the range of wireless devices.

Bandwidth refers to the amount of frequency spectrum available per user. Using wider channels gives more bandwidth. Transmission errors could reduce the available bandwidth, because part of the spectrum will be used for error correction.

Actual data rate mostly depends on the bandwidth available to the user; however, there are some other factors that influence it, such as the movement of the transceiver, position of the cell, and density of users. The actual data rate is typically higher for stationary users than for users who are walking. Users traveling at high speed (such as in cars or trains) have the lowest actual data rate. The reason for this is that part of the available bandwidth must be used for error correction due to greater interference that traveling users may experience.

Similarly, interference depends on the position of the cell; with higher interference, the actual data rate will be reduced. Optimal location is where there is direct line-of-sight between the user and the base station and the user is not far from the base station. In that case, there is no interference and the transmission requires minimum bandwidth for error correction.

Finally, if the density of users is high, there will be more users transmitting within a given cell, and consequently there will be less aggregate bandwidth per user. This reduces the actual data rate.

1.3.3 Generations of Wireless Systems Based on Wireless Access Technologies

From the late 1970s until today, there were three generations of wireless systems based on different access technologies:

1. 1G wireless systems, based on FDMA (frequency division multiple access)

2. 2G wireless systems, based on TDMA and CDMA

3. 3G wireless systems, mostly based on W-CDMA (wideband code division multiple access)

In Section 1.7, we introduce the future efforts in building the 4G wireless systems.

1.3.3.1 The 1G Wireless Systems

The first generation of wireless systems was introduced in the late 1970s and early 1980s and was built for voice transmission only. It was an analog, circuit-switched network that was based on FDMA air interface technology. In FDMA, each caller has a dedicated frequency channel and related circuits. For example, three callers use three frequency channels (see Figure 1.4a). An example of a wireless system that employs FDMA is AMPS (Advanced Mobile Phone Service).

1.3.3.2 The 2G Wireless Systems

The second generation of wireless systems was introduced in the late 1980s and early 1990s with the objective to improve transmission quality, system capacity, and range. Major multiple-access technologies used in 2G systems are TDMA and CDMA. These systems are digital, and they use circuit-switched networks.

1.3.3.2.1 TDMA Technology

In TDMA systems, several callers timeshare a frequency channel. A call is sliced into a series of time slots, and each caller gets one time slot at regular intervals. Typically, a 39-kHz channel is divided into three time slots, which allows three callers to use the same channel. In this case, nine callers use three channels. Figure 1.4 illustrates the operation of FDMA and TDMA access technologies.

Figure 1.4: FDMA versus TDMA— (a) In FDMA, a 30-kHz channel is dedicated to each caller. (b) In TDMA, a 30-kHz channel is timeshared by three callers.

The main advantage of the TDMA system is increased efficiency of transmission; however, there are some additional benefits compared to the CDMA-based systems. First, TDMA systems can be used for transmission of both voice and data. They offer data rates from 64 kbps to 120 Mbps, which enables operators to offer personal communication services such as fax, voice-band data, and Short Message Services (SMS). TDMA technology separates users in time, thus ensuring that they will not have interference from other simultaneous transmissions. TDMA provides extended battery life, because transmission occurs only part of the time. One of the disadvantages of TDMA is caused by the fact that each caller has a predefined time slot. The result is that when callers are roaming from one cell to another, all time slots in the next cell are already occupied, and the call might be disconnected.

1.3.2.2 GSM

GSM (Groupe Special Mobile or Global System for Mobile Communications) is the best-known European implementation of services that uses TDMA air interface technology. It operates at 900 and 1800 MHz in Europe, and 1900 MHz in the United States. European GSM has been exported also to the rest of the world.

GSM has applied the frequency hopping technique, which involves switching the call frequency many times per second for security.

The other systems that deploy TDMA are DECT (digital enhanced cordless telephony), IS-136 standard, and iDEN (integrated Digital Enhanced Network).

1.3.2.3 CDMA Access Technology

CDMA is a radically different air interface technology that uses the frequency hopping (FH) spread spectrum technique. The signal is randomly spread across the entire allocated 1.35-MHz bandwidth, as illustrated in Figure 1.5. The randomly spread sequences are transmitted all at once, which gives higher data rate and improved capacity of the channels compared to TDMA and FDMA. It gives eight to ten times more callers per channel than FDMA/TDMA air interface. CDMA provides better signal quality and secure communications. The transmitted signal is dynamic bursty, ideal for data communication. Many mobile phone standards currently being developed are based on CDMA.

Figure 1.5: Frequency hopped spread spectrum applied in CDMA air interface.

1.3.3 The 3G Wireless Systems

The 3G wireless systems are digital systems based on packet-switched network technology intended for wireless transmission of voice, data, images, audio, and video. These systems typically employ W-CDMA and CDMA 2000 air interface technologies.

1.3.3.1 Packet Switching versus Circuit Switching

In circuit-switching networks, resources needed along a path for providing communication between the end systems are reserved for the entire duration of the session. These resources are typically buffers and bandwidth. In packet-switching networks, several users share these resources, and various messages use the resources on demand. Therefore, packet switching offers better sharing of bandwidth; it is simpler, more efficient, and less costly to implement. On the other hand, packet switching is not suitable for real-time services, because of its variable and unpredictable delays.

1.3.3.2 W-CDMA Access Technology

W-CDMA uses a direct sequence (DS) spread spectrum technique. DS spread spectrum uses a binary sequence to spread the original data over a larger frequency range, as illustrated in Figure 1.6. The original data is multiplied by a second signal, called spreading sequence or spreading code, which is a pseudorandom code (PRC) of much wider frequency. The resulting signal is as wide as the spreading sequence, but carries the data of the original signal.

Figure 1.6: Direct sequence spread spectrum applied in W-CDMA air interface.

1.3.4 2.5G Wireless Systems

An intermediate step in employing full packet-switching 3G systems is the 2.5 wireless systems. They use separate air interfaces - circuit switching for voice and packet switching for data - designed to operate in 2G network spectrum. The 2.5G provides an increased bandwidth to about 100 kbps, much larger than 2G systems, but much lower than the expected bandwidth of 3G systems. General Packet Radio Service (GPRS) is the 2.5G implementation of Internet protocol packet switching on European GSM networks. ^[2] It is an upgrade for the IS-136 TDMA standard, used in North America and South America. GPRS combines neighboring 19.2-kbps time slots, typically one uplink and two or more downlink slots per GPRS tower. The rate can potentially reach 115 kbps.

Enhanced Data for Global Enhancement (EDGE) is another packet-switched technology that is a GPRS upgrade based on TDMA. The theoretical pick of this technology is 384 kbps, but the tests show that the practical rates are in the range of 64 to 100 kbps. EDGE is a standard of AT&T Wireless in the United States.

1.3.5 UMTS

Universal Mobile Telecommunications System (UMTS) is a 3G wireless standard that supports two different air interfaces: wideband CDMA (W-CDMA) and time division CDMA (TD-CDMA). W-CDMA will be used for the cellular wide area coverage and high mobility service, while TD-CDMA will be used for low mobility, local in-building services, asymmetrical data transmission, and typical office applications. GSM, IS-136, and PDC (Personal Digital Cellular) operators have all adopted the UMTS standard, but Qualcomm has developed a similar standard, CDMA 2000, which could attract existing IS-95 carriers. Basic concepts of W-CDMA radio access network are described in the Ericsson white paper. ^[3]

Table 1.2 presents the characteristics of three generations of wireless systems, and Figure 1.7 shows the most-possible migration path from 2G to 3G wireless systems.

Table 1.2: Basic Characteristics of Generations of Wireless Systems

Features	1G	2G	2.5G	3G
Air interfaces	FDMA	TDMA CDMA	TDMA	W-CDMA TD-CDMA CDMA 200
Bandwidth		~10 kbps	~100 kbps	~2 to 4 Mbps
Data traffic	No data	Circuit switched	Packet switched	Packet switched
Examples of services	AMPS	GSM IS-136 PDC IS-95	GPRS EDGE	UNITS cdma 2000
Modulation	Analog	Digital	Digital	Digital
Voice traffic	Circuit switched	Circuit switched	Circuit switched	Packet switched (VoIP)

Figure 1.7: Migration path from 2G to 3G wireless systems.

In summary, the target features of 3G wireless systems include:

• High data rates, which are expected to be 2 to 4 Mbps for indoor use, 384 kbps for pedestrians, and 144 kbps for vehicles

• Packet-switched networks, which provide that the users will be always connected

• Voice and data network will be dynamically allocated

• The system will offer enhanced roaming

• The system will include common billing and will have user profiles

• The system will be able to determine the geographic position of the users via mobile terminals and networks

• The system will be well suited for transmission of multimedia and will offer various services such as bandwidth on demand, variable data rates, quality sound, etc.

^[2]Park, J.-H., Wireless Internet access for mobile subscribers based on GPRS network, IEEE Communication Magazine, 40 (4), 38–49, 2002.

^[3]Ericsson, Basic concepts of WCDMA radio access network, White Paper, www.ericsson.com, 2002.