Wireless Networks - MULTIMEDIA

The rapid developmentsin computer and communication technologies have made ubiquitous computing areality. From cordless phones in the early days to cellular phones in the nineties and personal digital assistants (PDAs), Pocket PCs, and videophones now a days, wireless communication has been the core technology that enabled personal communication services (PCS), personal communications network (PCN) and personal digital cellular (PDC).

Geographically, wireless networks are often divided into cells. Each mobile phone in a cell contacts its access point, which serves as a gateway to the network. The access points themselves are connected through wired lines, or wireless networks or satellites that form the core network. When a mobile user moves out of the range of the initial access point, a handoff (or handover, as it is called in Europe) is required to maintain the communication.

In 19S5, frequency bands at 902 - 928 MHz, 2.400 - 2.4835 GHz, and 5.725 - 5.850 GHz were assigned to Industrial, Scientific, and Medical applications by the FCC, hence the name ISM bands.

Traditionally, cell size is on the order of kilometers. The introduction of PCS, however, creates the needfor a hierarchical cellular network in which several levels of cells can bedefined:

  1. picocell. Each covers upto 100 meters; useful for wireless / cordless applications and devices (e.g., PDAs) in an office or home.
  2. microcell.Each covers up to 1,000 meters incities or local areas, such as radio access pay phones on the streets.
  3. cell.Each has upto 10,000 meters coverage; good for national or continental networks.
  4. macrocell. Provides worldwide coverage, such as satellite phones.

Fading is a common phenomenon in wireless (and especially mobile) communications, in which the received signal power (suddenly) drops. Multipath fading occurs when a signal reaches the receiver via multiple paths (some of them bouncing off buildings, hills, and other objects). Because they arrive at different times and phases, the multiple instances of the signal can cancel each other, causing the loss of signal or connection. The problem becomes more severe when higher data rates are explored.

A possible geometric layout foran FDMA cellular system with a cluster size of seven hexagon cells

A possible geometric layout foran FDMA cellular system with a cluster size of seven hexagon cells

Analog Wireless Networks

Earlier wireless communication networks were used mostly for voice communications, such as telephone and voice mail. First - generation (1G) cellular phones used analog technology and Frequency Division Multiple Access (FDMA), in which each user is assigned a separate frequency channel during the communication. Its standard was Advanced Mobile Phone System (AMPS) in North America, Total Access Communication System (TACS)and Nordic Mobile Telephony (NMT) in Europe and Asia. Digital data transmission users needed modems to access the network; the typical data rate was 9,600 bps.

AMPS, for example, operates at the 800 - 900 MHz frequency band. Each direction of the two - way communication is allocated 25 MHz, with mobile station transmit (MS transmit) in the band of 824 to 849 MHz and base station transmit (BS transmit) in the band of 869 to 894 MHz. Each of the 25 MHz bands is then divided upfor two operator bands, A and B, giving each 12.5 MHz. FDMA further divides each of the 12.5 MHz operator bands into 416 channels, which results in each channel having a bandwidth of 30 KHz. The frequency of any MS transmit channelis always 45 MHz below the frequency of the corresponding BS transmit channel in communication.

Similarly, TACS operates at the 900 MHz frequency band. It carries up to 1,320 full - duplex channels, with a channel spacing of 25 KHz.

The above figure illustrates a possible geometric layout for an FDMA cellular system. (For clarity, cells from the first cluster are marked with thicker borders). A cluster of seven hexagon cells can be defined for the covered cellular area. As long as each cell in a cluster is assigned a unique set of frequency channels, interference from neighboring cells will be negligible.

The same set of frequency channels (denoted f1 to f7) will be reused once in each cluster, following the illustrated symmetric pattern. The so called reuse factor is K = 7. In an AMPS system, for example, the maximum number of channels (including control channels) available in each cell is reduced to 416 /K = 416 / 7 ≈ 59.

In this configuration, users in two different clusters using the same frequency fn are guaranteed to be more than D apart geographically, where D is the diameter of the hexagonal cell. In a vacuum, electromagnetic radiation decays at a rate of D - 2 over a distance D. However, in real physical spaces on the earth, the decay is consistently measured at a much faster rate of D - 35 to D - 5. This makes the FDMA scheme feasible for analog wireless communications, since interference by users of the same frequency channel from other groups becomes in significant.

Digital Wireless Networks

Second - generation (2G) wireless networksuse digital technology. Besides voice, digital data is increasingly transmitted for applications such as text messaging, streaming audio, and electronic publishing. In North America, the digital cellular networks adopted two competing technologies in 1993: Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). In Europe and Asia, Global System for Mobile communications (GSM), which used TDMA, was introduced in1992.

Below, we introduce TDMA and GSM first, followed by an introduction to spread spectrum and analysis of CDMA.

TDMA and GSM

As the name suggests, TDMA creates multiple channels in multiple time slots while allowing them to share the same carrier frequency. In practice, TDMA is always combined with FDMA — that is, the entire allocated spectrum is first divided into multiple carrier frequency channels, each of which is further divided in the time dimension by TDMA.

GSMwas established by the European Conference of Postal and Telecommunications Administrations (CEPT) in 1982, with the objective of creating a standard for a mobile communication network capable of handling millions of subscribers and providing roaming services through out Europe. It was designed to operate in the 900 MHz frequency range and was accordingly named GSM 900. Europe also supports GSM 1800, which is the original GSM standard modified to operate at the 1.8 GHz frequency range.

In North America, the GSM network uses frequencies at the range of 1.9 GHz (GSM 1900). However, the predominant use of TDMA technology is by operators using the TIA / EIA IS - 54B and the IS - 136 standards. These standards are some times referred to as digital - AMPS or D - AMPS. IS - 54B was superseded in 1996 by the newer IS - 136 standard which employs digital control channels DCCHand other enhanced user services. IS - 136 operates in the frequencies of 800 MHz and 1.9 GHz (the PCS frequency range), providing the same digital services in both. GSM and IS - 136 combine TDMA with FDMA to use the allocated spectrum and provide easy backward compatibility with pure FDMA - mode (analog) mobile stations.

As the following figure shows, the uplink (mobile station to base station) of GSM 900 uses the 890 - 915 MHz band, and the downlink (base station to mobile station) uses 935 - 960 MHz.

Frequency and time divisions in GSM

Frequency and time divisions in GSM

In other words, each is allocated 25 MHz. The frequency division in GSM divides each 25 MHz into 124 carrier frequencies each with a separation of 200 KHz. The time division in GSM then divides each carrier frequency into TDMA frames; 26 TDM A frames are grouped into a traffic channel (TCH) of 120 msec that carries speech and data traffic.

Each TDMA frame is thus approximately 4.615 msec (i.e., 120 / 26 msec) and consists of eight time slots of length 4.615 / 8 & 0.577 msec. Each mobile station is given unique time slots during which it can send and receive data. The send / receive does not occur at the same time slot; it is separated by three slots.

GSM provides a variety of data services. GSM users can send and receive data tousers on POTS, ISDN, and packet - switched or circuit - switched public data networks. GSM also supports Short Message Service (SMS), in which text messages up to 160 characters can be delivered to (and from) mobile phones. One unique feature of GSM is the subscriber identity module (SIM), a smart card that carries the mobile user's personal number and enables ubiquitous access to GSM services.

By default, the GSM network is circuit switched, and its data rate is limited to 9.6 kbps. General Packet Radio Service (GPRS), developed in 1999, supports packet - switched data over wireless connections, sousers are "always connected". It is also referred to as one of the 2.5G (between second - and third - generation) services. The theoretical maximum speed of GPRS is 171.2 kbps when all eight TDMA time slots are taken by a single user. In real implementations, single - user throughput reached 56 kbps in the year 2001. Apparently, when the network is shared by multiple users, the maximum data rate for each GPRS userwill drop.

Transmitter and receiver of Frequency Hopping (FH) spread spectrum

Transmitter and receiver of Frequency Hopping (FH) spread spectrum

Spread Spectrum and CDMA

Spread spectrum isa technology in which the bandwidth of a signal is spread before transmission. In its appearance, the spread signal might be in distinguishable from background noise, so it has distinct advantages of being secure and robust against intentional interference (jamming).

Spread spectrum is applicable to digital as well as analog signals, because both can be modulated and "spread". The earlier generation of cordless phones and cellular phones, for example, used analog signals. However, it is the digital applications, in particular CDMA, that made the technology popular in various wireless data networks.

Following is a brief description of the two ways of implementing spread spectrum: frequency hopping and direct sequence.

Frequency Hopping.Frequency hopping is the earlier method for spread spectrum. The technology of analog frequency hopping was invented by Hedy Lamarr, the actress, in 1940, during the World WarII. The above figure illustrates the main components of the transmitter and receiver for frequency - hopping.

Initially, data (analog or digital) is modulated to generate some baseband signal centered ata base frequency fb. Because of the relatively low data ratein current wireless applications, the bandwidth of the baseband Bb is generally narrow. For example, if the data rate is 9.6 kbps, then (depending on the modulating scheme) the bandwidth Bb wouldnot be higher than 2 x 9.6 — 19.2 KHz. The pseudo random frequency generator produces random frequencies fr within a wideband whose bandwidth is usually on the order of megahertz (MHz).

At the Frequency - Hopping (FH) Spreader, fr is modulated by the baseband signal to generate the Spread Spectrum Signal, which has the same shape as the baseband signal but a new center frequency Since fr changes randomly in the wideband, fc of the resulting signal is "hopping" in the wideband accordingly.

At the receiver side, the process is reversed. As long as the same pseudo random frequency generator is used, the signal is guaranteed to be properly despread and demodulated.

It is important to note that although the FH method uses a wideband spread spectrum, at any given moment during transmission, the FH signal occupies only a small portion of the band — that is, Bb.

Frequency Hopping

Spreading in Direct Sequence (DS) spread spectrum

Spreading in Direct Sequence (DS) spread spectrum

The transmission of the FH spread spectrum signal is rather secure and robust against narrowband jamming attacks, since only a tiny portion of the FH signal can be received or jammed in any narrow band.

If the hopping rate is slower than the data rate, it is called slow hopping and is easier to realize. Slow hopping has been used in GSM and shown to help reducing multipath fading, since each TDMA framewith frequency hopping will likely be sent under a different carrier frequency. In fast hopping, the hopping rate is much faster than the data rate, which makes itmore secure and effective in resisting narrowband interference.

Direct Sequence. Occasionally, when the FH spread spectrum scheme is employed in a multiple - access environment, more than one signal can hop onto the same frequency and thus create undue interference. Although some form of TDMA canalleviate the problem, this still imposes a limitation on the maximum number of users.

A major break throughin wireless technology is the development and adoption of Code Division Multiple Access (CDMA). The foundation of CDMA is Direct Sequence (DS) spread spectrum. Unlike FDMA or frequency hopping, in which each user is supposed to occupy a unique frequency band at any moment, multiple CDMA users can make use of the same (and full) bandwidth of the shared wideband channel during the entire period of transmission! A common frequency band can also be allocated to multiple users in all cells — in other words, providing a reuse factor of K = 1.This has the potential to greatly increase the maximum number of users, as long as the interference from them is manageable.

Transmitter and Receiver of Direct Sequence (DS) spread spectrum

Transmitter and Receiver of Direct Sequence (DS) spread spectrum

code) consists of a stream of narrow pulses called chips, with a bit width of Tr. Its bandwidth Br is on the order of 1 / Tr. Because Tr is small, Br is much wider than the bandwidth Bb of the narrowband signal.

The spreading code is multiplied with the input data. When the data bit is 1, the output DS code is identical to the spreading code, and when the data bit is — I, the output DS code is the inverted spreading code. As a result, the spectrum of the original narrowband data is spread, and the bandwidth of the DS signal is

BDS = Br.

The despreading process involves multiplying the DS code and the spreading sequence. As long as the same sequence is used as in the spreader, the resulting signal is the same as the original data. The above figure shows the implementation of the transmitter and receiver for the DS spread spectrum. The data and spreading sequences are modulated into analog signals before being fed to the DSspreader.

There are two ways to implement CDMA multiple access: orthogonal codes or nonorthogonal codes. A mobile station is dynamically assigned a unique spreading code in the cell that is also being used by the base station to separate and despread its signal.

For orthogonal CDMA, the spreading codes in a cell are orthogonal to each other. Most commonly,the Walsh - Hadamard codes are used, since they possess an important property called orthogonal variable spreading factor (OVSF). This states that OVSF codes of different lengths(i.e., different spreading factors) are still orthogonal. Orthogonality is desirable, since as long as the data is spread by orthogonal codes, it can be perfectly separated at the receiver end.

However, this property comes at a price: Walsh - Hadamard codes can have multiple auto correlation peaks if the sequences are not synchronized, so external synchronization is necessary for the receiver to know where the beginning of the DS signal is. Synchronization is typically achieved by utilizing a Global Positioning System (GPS) in the base station. Another disadvantage is that orthogonal codes are concentrated around a small number of carrier frequencies and therefore have low spectralutilization.

Nonorthogonal codes are Pseudo - random Noise (PN) sequences. PN sequences need to have an average bit value of around 0.5 and a single auto correlation peak at the startof the sequence. Thus, PN sequences are self - synchronizing and do not need external synchronization. A special PN sequence often used is the Gold sequence. Gold sequences have three cross - correlation peaks.

The DS spread spectrum makes use of the entire bandwidth of the wideband; hence, it is even more secure and robust against jamming. However, under multiple access, signals will still interfere with each other due to multipath fading, outer cell interference, and other factors. Below we provide a brief analysis of the viability of DS spread spectrum — that is, CDMA.

Analysis of CDMA

When FDMA or TDM A are used for a multiple - access system, bandwidth or time is divided upbased on the worst case — that is, all users accessing the system simultaneously and all the time. This is of course hardly the case, especially for voice communications. CDMA allows users inthe same channel to share the entire channel bandwidth. Since the effective noise is the sum of all other users' signals, it is based on the so called"average case" or "average interference".

At the receiver, the DS input is recovered by correlating with the particular user's designated spreading code. Hence, as long as an adequate level of signal - to - noise ratio is maintained, the quality of the CDMA reception is guaranteed, and universal frequency reuse is achieved.

Let's denote the thermal noise of the receiver as Nt and the received signal power of each user as Pi. The interference to the source signal received at the base station is

source signal received at the base station

where M is the maximum number of users in a cell.

If we assume that the thermal noise Nt is negligible and the received Pi from each user is the same, then

Nt is negligible and the received Pi

The received signal energy per bit Eb, is the ratio of Pi over the date rate R (bps),

ratio of Pi over the date rate R (bps)

and the interference Nb is

interference Nb

where W (Hz)is the bandwidth of the CDMA wideband signal carrier. The signal - to - noise ratio (SNR) is thus

signal - to - noise ratio (SNR)

we have

signal - to - noise ratio (SNR)

or approximately

signal - to - noise ratio (SNR)

The above equation states that the capacity of the CDMA system — that is, the maximum number of users in a cell — is determined by two factors: W/R and Eb / Nb.

W / R is the ratio between the CDMA bandwidth W and user's data rate R. This is the bandwidth spreading factor or the processing gain. This is equivalent to the number of chips in the spreading sequence. Typically, it can be in the range 102 to 107.

Eb / Nb is the bit - level SNR. Depending on the QoS (error rate requirement) and the implementation (error - correction scheme, resistance to multipath fading, etc.), a digital demodulator can usually work well with abit - level SNR in the range 3 to 9 dB.

Asan example, let's assume the bit - level SNR to be anominal 6 dB (from 10. log Eb / Nb= 6dB); then Eb / Nb ≈ 4. In the IS - 95 A standard, W = 1.25MHz and R = 9.6 kbps.

signal - to - noise ratio (SNR)

This capacity of 32 seems to compare well with the AMPS system. When the reuse factor is K = 7, with a bandwidth of 1.25 MHz, the maximum number of AMPS channels allowed would be only 1,250 / (30 x 7) ≈ 6. However, the above CDMA analysis has assumed no interference from neighboring cells. If this were the case, AMPS could have adopted a reuse factor of K = 1; its maximum number of channels would have been 1,250 / 30 42. So how does CDMA perform if the interference from neighboring cells is taken into consideration?

It turns out that the received interference from all users in neighbor cells is merely about 60% of the interference from users within the cell. Hence, the above equation can simply be modified to include a factor of 1.6 (i.e., 100% + 60%) to reflect the neighbor cell interference:

neighbor cell interference

The above factor of 1.6 can also be called the effective reuse factor, because its role is similar to the reuse factor K in the FDMA systems. It should be apparent that CDMA offers a larger capacity than FDMA because of its better use of the whole bandwidth and the much smaller (1.6 << 7) reuse factor.

The above example now yields

example

a capacity gain of 22 / 6 over AMPS.

Before concluding this brief analysis of CDMA, it must be pointed out that several major simplifications have been made above. Some contributed to enhanced performance, whereas others hampered performance.

  1. We assumed that the received energy Pi from each user is equal. Otherwise, the "near - far" problem dominates, where the received signal from the "near user" is too strong and from the "far user" is too weak, and the whole system's performance will collapse. This requires a sophisticated power control on the transmitter. The modern CDMA power control updates power levels over 1,500 times per second to make sure the received Pi 's are approximately the same.

  2. As a result of the tight power control required, CDMA networks have to implement soft handover. That is, when a mobile user crosses cellboundary, it has to communicate on at least two channels at once, one for each base station in range. It would use the lowest amount of power necessary for its signal to be received properly in at least one base station. This is done to minimize outer cell interference of the mobile.
  3. To reduce mutual interference, antennas are not omni directional. Instead, directional antennas are used, and collectively they are divided into sectors. In a three - way sectorized cell, AMPS capacity is reduced to 1,250 / (30 x 7 x 3) ≈ 2. Remarkably, CDMA capacity is not susceptible to such sectorization. Therefore, its capacity gain over AMPS in sectored cells is even greater. In the above example, it is 22 / 2 « 11.

  4. It was assumed that each user needs the full data rate all the time, which is false in many real applications. For example, voice applications use only 35 - 40% of the capacity. Effective voice coding can readily increase the network capacity by a factor of more than two.

3G Digital Wireless Networks

Third - generation (3G)wireless services feature various multimedia services, such as (low - rate) videoover the Internet. Applications include wireless web surfing, video mail, continuous media on demand, mobile multimedia, mobilee - commerce, remote medical service, and so on. Unlike the current Wireless LAN (WLAN), which is by and large for indoor and private networks, 3G is mostly for public networks. While a large number of 2G wireless networks used both CDMA (such as IS - 95A in North America) and TDMA (among them the most popular ones are GSM and IS - 136), the 3G wireless networks will predominantly use Wideband CDMA (WCDMA).

The 3G standardization process started in 1998, when the ITU called for Radio Transmission Technology (KIT) proposals for International Mobile Telecommunication - 2000 (IMT - 2000). Since then, the project has been known as 3G or universal mobile telecommunications system(UMTS). Regional standards bodies then adopted the IMT - 2000 requirements, added their own, and developed proposals and their evaluations to submit to ITU (TTU - R for radio technologies).

Even as specifications were being developed in the regional standards bodies, which have members from many multinational corporations, it was noted that most bodies tend to adopt similar WCDMA technology. To achieve global standardization and more efficiently hold discussions about the same topic, the Third Generation Partnership Project (3GPP) wase stablished in late 1998 to specify a global standard for WCDMA technology, which was named Universal Terrestrial Radio Access (UTRA). The standards bodies that joined to create the 3GPP forum are ARIB (Japan), ETS1 (Europe), TTA (Korea), TTC (Japan), and T1 (North America). Later in 1999, CWTS(China) joined the group.

The 3GPP forum focused on WCDMA air interface, which is aimed at advancing GSM technology and is designed to interface with the GSM MAP core network. At the same time the Telecommunication Industry Association (TIA), with major industry support, had been developing the cdma.2000 air interface recommendation for ITU that is the evolution of the IS - 95standard and is designed to be used on ANSI - 41 (or IS - 41) core network.

As similar work was going on in Asia, following the 3GPP example, the standards organizations decided to form a second forum called Third Generation Partnership Project 2 (3GPP2). The standards bodies that are members are ARIB (Japan), CWTS (China), TIA (North America), TTA (Korea), and TTC (Japan).

The 3GPP and 3GPP2 forums, despite having some similarities in WCDMA air interface proposals, still propose competing standards. However, in the interest of creating aglobal standard, the two forums are monitoring each other's progress and support recommendations by the operators harmonization group.

The two forums have agreed to a harmonized standard referred to as global 3G (G3G)that will have three modes: Direct Spread (DS), Multi - Carrier (MC), and Time Division Duplex (TDD), where the DS and TDD modes are specified as in WCDMA by the 3GPP group, and the MC mode is, as in cdma 2000, specified by 3GPP2. Allair interfaces (all modes) can be used with both core networks. At the end of 1999, ITU - R released the IMT - 2000 specification that for the most part followed the harmonized standard recommendations forWCDMA.

The multimedia nature of the 3G wireless services calls for a rapid development of a new generation of handsets, where support for video, better software and user interface, and longer battery life will be key factors.

A migration (or revolution) path is specified for 2G wireless networks supporting digital communication over circuit switched channels to 3G networks supporting highdata rates over both circuit - switched and packet - switched channels. The evolution path has an intermediate step that is easier and cheaper to achieve (fewer changes to the network infrastructure) called 2.5G (2.5 - generation), which is associated with enhanced data rates and packet data services (i.e., the addition of packet switching to 2G networks).

The following table summarizes the 2G, 2.5G, and 3G standards thathave been (or will be) developed using the IS - 41 core networks (in North America) and GSM MAP core networks (in Europe, etc.).

Evolution from 2G to 3G Wireless Networks

Evolution from 2G to 3G Wireless Networks

The IS - 95 Evolution. IS - 95A and IS - 95B, now known as cdma One, are based on the IS - 41 core network and use narrowband CDMA air interface. As such, all development is geared toward extending the existing CDMA framework to 3G (wideband CDMA) with backward compatibility. This is seen as a major cost efficiency issue and therefore has major industry support, as well as quick adaptability. IS - 95A is a 2G technology and has only circuit - switched channels with data rates up to 14.4 kbps. An extension to it is IS - 95B (2.5G), which supports packet switching and achieves maximum rates of 115 kbps.

IMT - 2000MC mode, originally called cdma 2000, can operate in all bands of the IMTspectrum (450, 700, 800, 900, 1700, 1800, 1900, and 2100 MHz). To ease the deployment of cdma 2000, the evolution framework is divided into four stages, each is backward compatible with previous stages and cdma One.

The cdma 2000 IX (or IX RTT) specification, also known as the high rate packet data air interface specification, delivers enhanced services upto 307 kbps peak rate and 144 kbps on average. This air interface provides two to three times the data capacity of IS - 95B. The IX means that it occupies one times the channels for cdma One — 1.25 MHz carrier bandwidth per channel. As with the IS - 95 air interface, the chip rateis 1.2288 Mcps (megachips per second).

The next step in cdma2000 deployment is cdma2000 1 x EV (EV for EVolution), split into two phases. The air interface tries to support both ANSI - 41 and GSM MAP networks, although priority is given to ANSI - 41. The first phase is called I x EV - DO (Data Only), supporting data transmission only at rates up to 2.4 Mbps. Voice communication is transmitted on a separate channel. Phase 2 is called I x EV - DV (Data and Voice) and enhances the l x EV interface tosupport voice communication as well. It promises an even higher data rate, up to 4.8 Mbps.

The last stage in the evolution to 3G is the recommended MC mode in IMT - 2000. It is referred to as cdma 2000 3X (or 3X RTT), since it uses a carrier spectrum of 5 MHz (3 x 1.25 MHz channels) to deliver a peak rate of atleast 2 - 4 Mbps. The chip rate is also tripled to 3.686 Mcps.

Typical 3G datarates are 2 Mbps for stationary indoor applications, and 384 kbps and 128 kbps for slow - and fast - moving users, respectively.

The GSM Evolution. The GSM radio access network (RAN) uses the GSM MAP core network. The IMT - 2000 DS and TDD modes are based on the WCDMA technology developed for the GSM MAP network. GSM is TDMA - based and therefore less compatible with the WCDMA technology than IS - 95. Hence the 3G WCDMA standard doesnot achieve backward compatibility with current - generation GSM networks. Moreover, each evolution toward 3G requires support for another mode of operation from mobile stations.

GSM is a 2G network providing only circuit - switched communication. General Packet Radio Service (GPRS) is a 2.5Genhancement that supports packet switching and higher date rates. As with CDMA2000 IX, EDGE (Enhanced Data rates for Global Evolution or Enhanced Data GSM Environment) supports upto triple the data rate of GSM and GPRS. EDGE is still a TDMA - basedstandard, defined mainly for GSM evolution to WCDMA. However it is defined in IMT - 2000 as UWC - 136 for Single Carrier Mode (IMT - SC) and, as such, is a 3G solution. It can achieve a data rate up to 384 kbps by new modulation and radio techniques, to optimize the use of available spectrum.

Eventually, the 3G technology (also referred to as 3GSM) will be adapted according to the WCDMA modes EvIT - 2000 recommendations. WCDMA has two modes of operation: Direct Sequence (DS) [also called Frequency Division Duplex (FDD)] and Time Division Duplex (TDD). FDD mode is used in the paired frequencies spectrum allocated where the uplinkand downlink channels use different frequencies. However, for the unpaired frequencies, it is necessary to transmit both uplink and downlink channels at the same frequencies. This is achieved by using time slots, having the uplink use a different time slot than the downlink. It also requires a more complicated timing control in the mobile than in TDMA.

Key differences in WCDMA air interface from a narrowband CDMA air interface are

  1. To support bitrates up to 2 Mbps, awider channel bandwidth is allocated. The WCDMA channel bandwidth is 5 MHz, as opposed to 1.25 MHz for IS - 95 and other earlier standards.

  2. To effectively use the 5 MHz bandwidth, longer spreading codes at higher chip rates are necessary. The chip rate specified is 3.84 Mcps, as opposed to 1.2288 Mcps for IS - 95.

  3. WCDMA supports variable bitrates, from 8 kbps up to 2 Mbps. This is achieved using variable - length spreading codes andtime frames of 10 msec, at which the user data rate remains constant but can change from one frame to the other — hence Bandwidth onDemand (BoD).

  4. WCDMA base stations use asynchronous CDMA with Gold codes. This eliminates the need for a GPS in the base station for global time synchronization, as in IS - 95 systems. Base stations can now be made smaller and less expensive and can be located indoors.

Wireless LAN(WLAN)

A wireless local area network (WLAN) links two or more devices using some wireless distribution method (typically spread - spectrum or OFDM radio), and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network. Most modernWLANs are based on IEEE 802.11 standards, marketed under the Wi - Fi brandname.

Wireless LANs have become popular in the home due to ease of installation, and in commercial complexes offering wireless access to their customers; often for free. Large wireless network projects are being put up in many major cities: New York City, for instance, has begun a pilot program to provide city workers in all five boroughs of the city with wireless Internet access.

IEEE 802.11. IEEE 802.11 was the earlier standard for WLAN developed by the IEEE 802.11 working group. It specified Medium Access Control (MAC) and Physical (PHY) layers for wireless connectivity in a local area within a radius of several hundred feet. PHY supported both Frequency Hopping (FH) spread spectrum and Direct Sequence (DS) spread spectrum. The ISM frequency band used was 2.4 GHz. Moreover, (diffused) infrared light was also supported for indoor communications in the range of 10 - 20 meters.

WLAN can be used either as a replacement or an extension to the wired LAN. Similar to Ethernet, the basic access method of 802.11 is Carrier Sense Multiple Access with Collision Avoidance (CSMA / CA).The data rates supported by 802.11 were 1 Mbps and 2 Mbps.

The 802.11 standards also address the following important issues:

  1. Security. Enhanced authentication and encryption, since WLAN is even more susceptible to breakins.
  2. Power management. Saves power during no transmission and handles doze and awake.
  3. Roaming. Permits acceptance of the basic message format by different access points.

IEEE 802.11b. IEEE 802.11b is an enhancement of 802.11. It still uses DS spread spectrum and operates in the 2.4 GHz band. With the aid of new technology, especially the Complementary Code Keying (CCK) modulation technique, it supports 5.5 and 11 Mbps inaddition to the original 1 and 2 Mbps, and its functionality is comparable to Ethernet.

In North America, for example, the allocated spectrum for 802.11 and 802.1 lb is 2.400 - 2.4835 GHz. Regardless of the datarate (1,2,5.5, or 11 Mbps), the bandwidth of a DSspread spectrum channel is 20 MHz. Three nonoverlapped DS channels can be accommodated simultaneously, allowing a maximum of 3 access points in a local area.

IEEE 802.11b has gained public acceptance and is appearing in WLANs everywhere, including university campuses, airports, conference centers, and so on.

IEEE802.11a. IEEE 802.11a operates inthe 5 GHz band and supports data rates in the range of 6 to 54 Mbps. Instead of DS spread spectrum, it uses Orthogonal Frequency Division Multiplexing (OFDM). It allows 12 nonoverlapping channels, hence a maximum of 12 access points in a local area.

Because 802.11a operates in the higher frequency (5 GHz) band, it faces much less Radio Frequency (RF) interference, such as from cordless phones, than 802.11 and 802.11b. Coupled with the higher data rate, it has great potential for supporting various multimedia applications in a LAN environment.

High Performance Radio LAN (HTPERLAN / 2) is the European sibling of IEEE 802.1 la. It also operates in the 5 GHz band and is promised to deliver a data rate of up to54 Mbps. Wesel provides a good description of HIPERLAN.

IEEE 802.11g and others.IEEE 802.11g, an extension of 802.11b, is an attempt to achieve data rates up to 54 Mbps in the 2.4 GHz band. As in 802.11a, OFDM will be used instead of DS spread spectrum. However, 802.11g still suffers from higher RF interference than does 802.11a, and as in 802.11b, has the limitation of three access points in a local area.

IEEE 802.11g is designed to be downward compatible with 802.11b, which brings a significant overhead for all 802.11b and 802.1 lg users on the 802.llg network.

Another half - dozen 802.11 standards are being developed that deal with various aspects of WLAN. The Further Exploration section of this chapter has WWW URLs for these standards. Notably, 802.1 le deals with MAC enhancement for QoS, especially prioritized transmission for voice and video.

BluetoothBluetooth (named after the tenth - century king of Denmark Harold Blue ­ tooth) is a proprietary open wireless technology standard for exchanging data over short distances(using short - wavelength radio transmissions in the ISM bandfrom 2400 – 2480 MHz) from fixed and mobile devices, creating personal area networks (PANs) with high levels of security. Created by telecoms vendor Ericsson in1994, it was originally conceived as a wireless alternative to RS - 232 datacables. It can connect several devices, overcoming problems of synchronization.

Bluetooth is managed by the Bluetooth Special Interest Group, which has more than 16,000 member companies in the areas of telecommunication, computing, networking, and consumer electronics. The SIG oversees the development of the specification, manages the qualification program, and protects the trademarks. To be marketed as a Bluetooth device, it must be qualified to standards defined by the SIG. A network of patents is required to implement the technology and are licensed only for those qualifying devices; thus the protocol, whilst open, may be regarded as proprietary.


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