# 5.16: 3G, Third Generation- Code Division Multiple Access (CDMA)


Originally there were multiple 3G cellular radio systems but the one that became dominant uses code division multiple access. This section begins with a precursor to 3G and which is now called 2.5G cellular radio and this is relatively narrowband, $$1.23\text{ MHz}$$-wide, CDMA. The 3G system uses $$5\text{ MHz}$$-wide wideband CDMA (WCDMA).

## 5.9.1 Generation 2.5: Direct Sequence Code Division Multiple Access

CDMA, or more specifically CDMAOne, was initially promoted as being third generation, but the definition now is that data rates of at least $$2\text{ Mbit/s}$$ must be supported in 3G. Thus CDMAOne is now referred to as 2.5G. A depiction of spread spectrum is shown in Figure $$\PageIndex{1}$$, in which a very fast code is superimposed on a slower data sequence and the combined code is used to modulate a carrier. The same fast code is used to extract the baseband signal from the received bitstream. The effect of the fast code is to greatly spread out the baseband signal, transforming perhaps a $$12\text{ kbit/s}$$ baseband bitstream into an RF signal with a bandwidth of $$1.23\text{ MHz}$$.

The key feature of the DS-CDMA system is the use of lengthy codes to spread the spectrum of the signal that is to be transmitted. In the case of voice, an $$8\text{ kbit/s}$$ bitstream, for example, with error correction coding becomes a $$12.5\text{ kbit/s}$$ baseband bitstream that is mixed with a much faster code that is unique to a particular user. Thus the $$8\text{ kbit/s}$$ bitstream becomes a $$1.23\text{ MHz}$$-wide analog baseband signal. This signal is then modulated up to RF and transmitted. On the receiver side, the demodulated RF signal can only be decoded using the original fast code. Use of the original code to decode the signal rejects virtually all interference and noise in the received signal. Despreading distributes the signal and noise components differently. Upon despreading, the noise is still distributed uniformly in frequency while the information-bearing signal is concentrated in a narrow bandwidth, the bandwidth of the baseband signal. Tremendous processing gain is available using this spreading and despreading approach.

The mechanism that increases SNR in DS-CDMA is shown in Figure 5.6.2. The SNR is enhanced by grouping the signal energy in a narrower bandwidth and the noise is reduced, as only the noise in a narrower

Figure $$\PageIndex{1}$$: Depiction of direct sequence CDMA transmission. If code B is code A, then data B is the same as data A with some corruption by noise.

Property Attribute
Bandwidth per channel $$1.23\text{ MHz}$$
Channel spacing $$1.25\text{ MHz}$$ ($$20\text{ kHz}$$ guard band)
Cell radius $$2-20\text{ km}$$
Base-to-mobile frequency $$869–894\text{ MHz}$$
Mobile-to-base frequency

$$824–849\text{ MHz}$$

$$45\text{ MHz}$$ between transmit and receive channels.

Modulation OPSK
Access method

CDMA

$$64$$ radio channels per physical channel

Forward:

$$55$$ traffic channels, $$7$$ paging channels

$$1$$ pilot channel $$1$$ sync channel

Reverse:

$$55$$ traffic channels, $$9$$ access channels

RF Specifications of Mobile Unit
Transmit power control $$1\text{ dB}$$ power control

Table $$\PageIndex{1}$$: Attributes of the CDMAOne system.

bandwidth is important. In despreading a signal with a single-tone interferer, as seen in Figure 5.6.2(c), the interferer is spread as noise while the signal energy is concentrated in the bandwidth of the baseband signal. Since orthogonal codes are used, many radio channels can be supported on the same radio link. CDMA can support approximately $$120$$ radio channels on the same physical channel. Another important feature is that the same $$120$$ channels can be reused in adjacent cells, as the information bitstream of each user can still be extracted. Thus there is no need for clustering as in the 2G systems. The attributes of the cellular 2.5G CDMAOne system, an immediate precursor to the 3G WCDMA system, are given in Table $$\PageIndex{1}$$.

## 5.9.2 Multipath and Rake Receivers

In a line-of-sight CDMA system the code must be aligned with the data stream received so that the stream can be decoded correctly to reveal the original data. If there are multiple paths then the paths will, in general, have different delays and often the delay differences are more than the chip duration (the time allocated to transition from one symbol to another). The CDMA signal is then said to be transmitted over a dispersive multipath channel. This introduces complexity in aligning codes. The problem that arises is shown in Figure $$\PageIndex{2}$$. The original data are shown in (a) and this

Figure $$\PageIndex{2}$$: Decoding of a direct sequence coded stream showing a code aligned with the data stream and a data stream delayed by $$t_{D}$$.

Figure $$\PageIndex{3}$$: Rake receiver used to decode two paths, a direct path and a path with excess delay $$t_{D}$$. The integrators eliminate incorrectly recovered data.

is coded using the code in (c) yielding the stream in (d). The stream is just the exclusive or ($$\text{XOR}$$) of the data and code. On receiving, the coded data stream, (d), is despread using the original code (c). If the code and the data stream are aligned, then an $$\text{XOR}$$ operation results in the data being recovered correctly by $$\text{XOR}$$ing (c) and (d) to obtain (b), the original data $$\mathsf{A}$$. However, if the stream is delayed, as shown in (e), then $$\text{XOR}$$ing the delayed stream and the code (which is not delayed) will result in recovered data $$\mathsf{B}$$, (g), which has no relationship to the original data in (a). The solution to this problem is to appropriately delay the despreading code for each propagation-delay path.

When there are reflections of the transmitted signal, versions of the signal followig different paths will arrive at the receiver with different delays. The rake receiver, introduced in the 1950s, recovers the data from each propagation delay path and combines the signal from each path to produce a combined signal with a higher SNR than that obtained using just the line-of-sight component of the signal [20, 21, 22, 23].

A two-path version of the rake receiver is shown in Figure $$\PageIndex{3}$$. The transmitter spreads the data using a code $$C_{1}(t)$$ to produce a stream at $$\mathsf{A}$$ that is transmitted over two paths $$\mathsf{C}$$ and $$\mathsf{B}$$. Path $$\mathsf{C}$$ is delayed with respect to path $$\mathsf{B}$$ by a time $$t_{D}$$ and the signal at the receive antenna, $$\mathsf{D}$$, is a combination of the signal following the two paths. On the top path of the receiver, called a finger, the direct signal (that followed path $$\mathsf{B}$$) is despread by the original code $$C_{1}(t)$$ and, if it is appropriately aligned with the signal that followed path $$\mathsf{B}$$, it produces a signal at $$\mathsf{E}$$ that will contain the low-frequency user data plus a high-frequency component resulting from the wrong code being used to despread the delayed data stream that followed path $$\mathsf{C}$$. The signal at $$\mathsf{E}$$ is lowpass filtered to produce the original data at $$\mathsf{F}$$ as the high frequency component is eliminated. Since the operation is implemented in DSP, an integrator is used over the duration of a data bit. The combined received signal, $$\mathsf{D}$$, is despread using a delayed code, $$C_{1}(t − t_{D})$$, to produce a signal at $$\mathsf{G}$$ that is integrated so that the data at $$\mathsf{H}$$ is the original $$\mathsf{D}$$ due to the component that followed path $$\mathsf{C}$$. The signals at $$\mathsf{F}$$ and $$\mathsf{H}$$, sampled after the duration of a bit, will both contain the original data plus some noise and the noise at the two points will be uncorrelated. When $$\mathsf{F}$$ and $$\mathsf{H}$$ are combined, the signal will combine coherently while the noise will combine incoherently, thus improving the SNR.

The rake receiver can be generalized to have many fingers. After despreading in each finger of the rake receiver, each delayed component is demodulated and the results are combined. The rake receiver is so named because each finger sweeps up information, resembling the tines on a garden rake collecting leaves. Since each component contains the original information, if the magnitude and time of arrival (phase) of each component is computed at the receiver (through a process called channel estimation),

Property Attribute
Number of channels
Bandwidth per channel $$5\text{ MHz}$$
Channel spacing $$5\text{ MHz}$$
Cell radius $$2-20\text{ km}$$
Downlink frequency $$1805-1880$$
Uplink frequency $$1710-1785$$
Modulation (uplink) PAM on I and on Q
Access method CDMA
Symbol rate $$3.84\text{ Msymbols/s}$$
Modulation bandwidth $$3.84\text{ MHz}$$
Symbol duration $$260\text{ ns}$$
Information rate (original)$$†$$ $$200\text{ kbit/s}$$ up- & down-link
Information rate (HSPA+)$$∗$$ $$40\text{ Mbit/s}$$ downlink
Information rate (HSPA+)$$‡$$ $$168\text{ Mbit/s}$$ downlink
Information rate (HSPA+)$$†$$ $$10\text{ Mbit/s}$$ uplink

Table $$\PageIndex{2}$$: Attributes of the 3G WCDMA system. 3G operates in many frequency bands and just one representative band, Band $$3$$, is considered here. The uplink modulation in WCDMA is unusual applying data to the I channel of an I/Q modulator and control data to the Q channel. Here the I channel (and Q channel) has two or four levels classified as pulsed amplitude modulation (PAM). $$†$$per user. $$∗$$shared, non-MIMO. $$‡$$shared, $$2$$-cell carrier aggregation, MIMO. The 3G standards do not specify minimum information transmit rates per user.

then all the components can be added coherently to improve the information reliability. This method of combining is called maximal-ratio combining (MRC), a method of receiver diversity combining in which

1. the signals from each channel are added together,
2. the gain of each channel is made proportional to the rms signal level and inversely proportional to the mean square noise level in that channel, and
3. different proportionality constants are used for each channel.

It is also known as ratio-squared combining and predetection combining. MRC is optimum combining for independent AWGN channels.

CDMA, WCDMA, and WLAN (WiFi) units use multipath signals and combine them to increase the SNR at the receivers. In contrast, the narrowband 2G systems cannot discriminate between the multipath arrivals, and multipath has negative impact. The rake receiver in Figure $$\PageIndex{3}$$ has two fingers, but the practical limit on the number is based on the expected delay spread of the multipaths divided by the chip duration. Only delays that are an integer multiple of a chip duration are needed in a rake receiver [21, 22]. A trade-off must be made in terms of the amount of circuitry and the DC power available. In the original CDMA cell phone system (circa 2000), three fingers were used in handsets and four to five in basestations. Many more are used today.

## 5.9.3 3G, Wideband CDMA

Third-generation radio, (3G), is coordinated by the Third Generation Partnership Project (3GPP). This is a collaborative agreement of standards development organizations and other related bodies for the production of a complete set of globally applicable technical specifications for mobile communication systems (see http://www.3gpp.org). Initially the efforts of 3GPP were directed at establishing the 3G standards but the scope has expanded and it develops evolving standards for the transition to 4G and now 5G systems. Some of the attributes of 3G are given in Table $$\PageIndex{2}$$.

The 3G mobile systems support variable data rates depending on demand and the level of mobility. Switched-packet radio techniques are required to support this bandwidth-on-demand environment. Here the physical channel is shared (i.e., packet switched) rather than the user being assigned a physical channel for exclusive use (referred to as circuit switched).

The drive for 3G systems was partly fueled by the saturation of 2G systems in many places and a desire to increase revenues by supporting high-speed data. Prior to the rollout of 3G systems, the increased demand primarily resulted from an increased consumer base rather than the emergence of significant data traffic. The increased subscriber base was addressed by 2.5G systems, which have some of the 3G concepts but only partially implemented. The driving concept of 3G was the development of a standard that supports high-speed data, global roaming, and supports advanced features including two-way motion video and internet browsing.

Third-generation cellular radio is defined by the International Telecommunications Union (ITU) [24] and is formally called International Mobile Telecommunications 2000 (IMT-2000). The basic requirements are for a system that supports data rates up to $$2\text{ Mbit/s}$$ in fixed environments ranging down to $$144\text{ kbit/s}$$ in wide area mobile environments. In 1999 the ITU adopted five radio interfaces for IMT-2000:

1. IMT-DS direct-sequence CDMA, more commonly known as WCDMA;
2. IMT-MC multi-carrier CDMA, more commonly known as CDMA2000, the successor to CDMAOne (specifically international standard IS-95);
3. IMT-SC time-division CDMA, which includes time division CDMA (TD-CDMA) and time division synchronous CDMA (TD-SCDMA);
4. IMT-SC single carrier, more commonly known as EDGE; and
5. IMT-FT frequency time, more commonly known as DECT.

The dominant choice for 3G is WCDMA. In October 2007 the ITU Radiocommunication Assembly included WiMAX-derived technology, specifically orthogonal frequency division multiple access (OFDMA, see Figure 5.4.1(d)) and MIMO, in the set of IMT-2000 standards as the sixth radio interface. 3GPP [25] provides a migration strategy for cellular communications through a process called long-term evolution LTE and through a number of releases each building on prior infrastructure and adding capabilities.

EDGE has intermediate data speeds between those of GSM and WCDMA. The following terms are also used to describe networks using the 3G WCDMA specification: Universal Mobile Telecommunication System (UMTS) (UMTS, in Europe), UMTS Terrestrial Radio Access Network (UTRAN), and Freedom of Mobile Multimedia Access (FOMA, in Japan). UMTS is the 3G successor of the GSM standard, with the air interface now using WCDMA. The terminology used in UMTS, listed in part in Table $$\PageIndex{3}$$, is based on the terminology used in GSM, with subtle differences. UMTS was first deployed in Japan in 2001. The term WCDMA describes the physical interface and protocols that support it, while UMTS refers to the whole network. A large number of frequency bands are designated for 3G, see Tables $$\PageIndex{4}$$ and $$\PageIndex{5}$$.

The 3GPP timeline is summarized in Figure $$\PageIndex{4}$$. The CDMA2000 and WCDMA paths become the single LTE path beyond 3G. The CDMA2000 (the IS-2000 standard) path builds on the original CDMA system defined by the IS-95 standard and commonly known as CDMAOne (the IS-95 standard). CDMA2000 1xEV-DO is the first evolution of CDMA2000 that meets the ITU basic specification for 3G. Evolution-data optimized (EV-DO), combines CDMA and TDMA for higher data throughput.

Term Description
AuC Authentication center
GGSN Gateway GPRS support node
GMSC Gateway MSC
HLR Home location register
ISDN Integrated services digital network
MSC Mobile switching center
Node B Basestation
PSTN Public switched telephone network
SGSN Serving GPRS support node
UE User equipment
USIM Universal subscriber identity module
VLR Visitor location register

Table $$\PageIndex{3}$$: UMTS terminology.

$$(\text{MHz})$$ $$(\text{MHz})$$ NA LA EMEA ASIA Oceania Japan
$$1$$ $$1920-1980$$ $$2110-2170$$
$$2$$ $$1850-1910$$ $$1930-1990$$
$$3$$ $$1710-1785$$ $$1805-1880$$
$$4$$ $$1710-1755$$ $$2110-2155$$
$$5$$ $$824-849$$ $$869-894$$
$$6$$ $$830-840$$ $$875-885$$
$$7$$ $$2500-2570$$ $$2620-2690$$
$$8$$ $$880-915$$ $$925-960$$
$$9$$ $$1749.9–1784.9$$ $$1844.9–1879.9$$
$$10$$ $$1710-1770$$ $$2110-2170$$
$$11$$ $$1427.9–1447.9$$ $$1475.9–1495.9$$
$$12$$ $$698-716$$ $$728-746$$
$$13$$ $$777-787$$ $$746-756$$
$$14$$ $$788-798$$ $$758-768$$
$$15-18$$ reserved
$$19$$ $$832.4–842.6$$ $$877.4–887.6$$
$$20$$ $$832-862$$ $$791-821$$

Table $$\PageIndex{4}$$: Spectrum assignments for 3G, [25, Release 99].

The WCDMA and LTE evolution is defined by releases beginning with an initial release in 2000 known as Release 99 (Rel-99) [26]. This saw the beginning of the Third Generation Partnership Project (3GPP) specifying and controlling the evolution of cellular communications through 3G, 4G, and now 5G. The releases are designed to protect the installed investment in cellular systems while providing a migration path.

## 5.9.4 Summary

WCDMA 3G and 4G are widely deployed. While 3G/WCDMA it will be gradually replaced by 4G and 5G it has particular aspects that could see the system remain for many years.

Figure $$\PageIndex{4}$$: Timeline for implementation of 3G, 4G and 5G. DL indicates the downlink data rate; UL indicates the uplink data rate; BW indicates the channel bandwidth. Development supports Internet protocol (IP) and voice over IP (VoIP). The two 3G paths become a single long term evolution (LTE) path. LTE is the concept of smoothly evolving through 4G and into 5G utilizing existing infrastructure and adding capability [25]). Begining with Release 99 (Rel-99) the timeline is controlled by 3GPP . The dates refer to the first significnat commercial availability of the standard and the official release dates can be found at http://www.3gpp.org.

Band Uplink $$(\text{MHz})$$ Downlink $$(\text{MHz})$$
$$33$$ $$1900-1920$$ $$1900-1920$$
$$34$$ $$2010-2025$$ $$2010-2025$$
$$35$$ $$1850-1910$$ $$1850-1910$$
$$36$$ $$1930-1990$$ $$1930-1990$$
$$37$$ $$1910-1930$$ $$1910-1930$$
$$38$$ $$2570-2620$$ $$2570-2620$$
$$39$$ $$1880-1920$$ $$1880-1920$$
$$40$$ $$2300-2400$$ $$2300-2400$$

Table $$\PageIndex{5}$$: Spectrum assignments for TDD 3GPP [25, Release 8].

5.16: 3G, Third Generation- Code Division Multiple Access (CDMA) is shared under a CC BY-NC license and was authored, remixed, and/or curated by Michael Steer.