Unit 1

BS-Base Station

What is antenna?

An antenna can be defined as an electrical conductor or system of conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy. For transmission of a signal, radio-frequency electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment (atmosphere, space, water). For reception of a signal, electromagnetic energy impinging on the antenna is converted into radio frequency electrical energy and fed into the receiver.

In two-way communication, the same antenna can be and often is used for both transmission and reception. This is possible because any antenna transfers energy from the surrounding environment to its input receiver terminals with the same efficiency that it transfers energy from the output transmitter terminals into the surrounding environment, assuming that the same frequency is used in both directions. Put another way, antenna characteristics are essentially the same whether an antenna is sending or receiving electromagnetic energy.

Types of antenna:

Dipoles- Two of the simplest and most basic antennas are the half-wave dipole, or Hertz, antenna (Figure 5.2a) and the quarter-wave vertical, or Marconi, antenna (Figure 5.2b). The half-wave dipole consists of two straight collinear conductors of equal length, separated by a small gap. The length of the antenna is one-half the wavelength of the signal that can be transmitted most efficiently. A vertical quarter wave antenna is the type commonly used for automobile radios and portable radios.

Parabolic Reflective Antenna An important type of antenna is the parabolicreflective antenna,

which is used in terrestrial microwave and satellite applications.A parabola is the locus of all

points equidistant from a fixed line and a fixed pointnot on the line. The fixed point is called

the focus and the fixed line is called thedirectrix (Figure 5.4a).

If a source of electromagneticenergy (or sound) is placed at the focus of the paraboloid, and if

the paraboloidis a reflecting surface, then the wave will bounce back in lines parallel to theaxis

of the paraboloid; Figure 5.4b shows this effect in cross section. In theory, thiseffect creates a

parallel beam without dispersion.

Isotropic Radiator

An antenna will radiate power in all directions but, typically, does not perform equally well in

all directions. A common way to characterize the performance of an antenna is the radiation

pattern, which is a graphical representation of the radiation properties of an antenna as a

function of space coordinates. The simplest pattern is produced by an idealized antenna

known as the isotropic antenna. An isotropic antenna is a point in space that radiates power in

all directions equally. The actual radiation pattern for the isotropic antenna is a sphere with

the antenna at the center. However, radiation patterns are almost always depicted as a two-

dimensional cross section of the three-dimensional pattern. The pattern for the isotropic

antenna is shown in Figure 5.la. The distance from the antenna to each point on the radiation

pattern is proportional to the power radiated from the antenna in that direction. Figure 5.lb

shows the radiation pattern of another idealized antenna. This is a directional antenna in

which the preferred direction of radiation is along one axis.

Fading in Mobile Environment

Perhaps the most challenging technical problem facing communications systemsengineers is

fading in a mobile environment. The term fading refers to the time variationof received signal

power caused by changes in the transmission medium orpath(s). In a fixed environment,

fading is affected by changes in atmospheric conditions,such as rainfall. But in a mobile

environment, where one of the two antennasis moving relative to the other, the relative

location of various obstacles changesover time, creating complex transmission effects.

Types of fading:

Fading effects in a mobile environment can be classified as either fast or slow. Referring to

Figure 5.11, as the mobile unit moves down a street in an urban environment, rapid variations

in signal strength occur over distances of about one-half a wavelength. At a frequency of 900

MHz, which is typical for mobile cellular applications, a wavelength is 0.33 m. The rapidly

changing waveform in Figure 5.13 shows an example of the spatial variation of received signal

amplitude at 900 MHz in an urban setting. Note that changes of amplitude can be as much as

20 or 30 dB over a short distance. This type of rapidly changing fading phenomenon, known as

fast fading, affects not only mobile phones in automobiles, but even a mobile phone user

walking down an urban street.

As the mobile user covers distances well in excess of a wavelength, the urban environment

changes, as the user passes buildings of different heights, vacant lots, intersections, and so

forth. Over these longer distances, there is a change in the average received power level about

which the rapid fluctuations occur. This is indicated by the slowly changing waveform in Figure

5.13 and is referred to as slow fading.

Fading effects can also be classified as flat or selective. Flat fading, or non selective fading, is

that type of fading in which all frequency components of the received signal fluctuate in the

same proportions simultaneously. Selective fading affects unequally the different spectral

components of a radio signal. The term selective fading is usually significant only relative to

the bandwidth of the overall communications channel. If attenuation occurs over a portion of

the bandwidth of the signal, the fading is considered to be selective; nonselective fading

implies that the signal bandwidth of interest is narrower than, and completely covered by, the

spectrum affected by the fading.

Rayleigh fading occurs when there are multiple indirect paths between transmitter and

receiver and no distinct dominant path, such as an LOS path. This represents a worst case

scenario. Fortunately, Rayleigh fading can be dealt with analytically, providing insights into

performance characteristics that can be used in difficult environments, such as downtown

urban settings.

Rician fading best characterizes a situation where there is a direct LOS path in addition to a

number of indirect multipath signals. The Rician model is often applicable in an indoor

environment whereas the Rayleigh model characterizes outdoor settings. The Rician model

also becomes more applicable in smaller cells or in more open outdoor environments. The

channels can be characterized by a parameter K, defined as follows:

Handoff

In practice, the distribution of traffic and topographic features is not uniform, and this

presents opportunities of capacity increase. Cells in areas of high usage can be split into

smaller cells. Generally, the original cells are about 6.5 to 13 km in size. The smaller cells can

themselves be split; however, lo5-km cells are close to the practical minimum size as a general

solution (but see the subsequent discussion of microcells). To use a smaller cell, the power

level used must be reduced to keep the signal within the cell. Also, as the mobile units move,

they pass from cell to cell, which requires transferring of the call from one base transceiver to

another. This process is called a handoff.

Handoff: If a mobile unit moves out of range of one cell and into the range of another during a

connection, the traffic channel has to change to one assigned to the BS in the new cell (Figure

10.6f). The system makes this change without either interrupting the call or alerting the user.

Handoff may be network initiated, in which the decision is made solely by the network

measurements of received signals from the mobile unit. Alternatively, mobile unit assisted

handoff schemes enable the mobile unit to participate in the handoff decision by providing

feedback to the network concerning signals received at the mobile unit. In either case, a

number of different performance metrics may be used to make the decision. [HAASOO] lists

the following:

Cell blocking probability: The probability of a new call being blocked, due to heavy load

on the BS traffic capacity. In this case, the mobile unit is handed off to a neighboring cell

based not on signal quality but on traffic capacity.

Call dropping probability: The probability that, due to a handoff, a call is terminated.

Call completion probability: The probability that an admitted call is not dropped before

it terminates.

Probability of unsuccessful handoff: The probability that a handoff is executed while

the reception conditions are inadequate.

Handoff blocking probability: The probability that a handoff cannot be suc- . cessfully

completed.

Handoff probability: The probability that a handoff occurs before call termination.

Rate of handoff: The number of handoffs per unit time.

Interruption duration: The duration of time during a handoff in which a mobile unit is

not connected to either base station.

Handoff delay: The distance the mobile unit moves from the point at which the handoff

should occur to the point at which it does occur.

The principal parameter used to make the handoff decision is measured signal strength from

the mobile unit at the BS. Typically, the BS averages the signal over a moving window of time

to remove the rapid fluctuations due to multipath effects. Figure 10.7a, based on one in

[POLL96], shows the average received power level at two adjacent base stations as a mobile

unit moves from BS A, at LA, to BS B, at LB. This figure is useful in explaining various handoff

strategies that have been used to determine the instant of handoff:

Relative signal strength: The mobile unit is handed off from BS A to BS B when the signal

strength at B first exceeds that at A. If the signal strength at B subsequently falls below that of

A, the mobile unit is handed back to A. In Figure 10.7a, handoff occurs at point L1. At this point,

signal strength to BS A is still adequate but is declining. Because signal strength fluctuates due

to multipath effects, even with power averaging, this approach can lead to a ping-pong effect

in which the unit is repeatedly passed back and forth between two BSs.

Relative signal strength with threshold: Handoff only occurs if (1) the signal at the current BS

is sufficiently weak (less than a predefined threshold) and (2) the other signal is the stronger of

the two. The intention is that so long as the signal at the current BS is adequate, handoff is

unnecessary. If a high threshold is used, such as Th1, this scheme performs the same as the

relative signal strength scheme. With a threshold of Th2, handoff occurs at L2. If the threshold

is set quite low compared to the crossover signal strength (signal strength at L1), such as Th3,

the mobile unit may move far into the new cell (L4 ) before handoff. This reduces the quality of

the communication link and may result in a dropped call. A threshold should not be used alone

because its effectiveness depends on prior knowledge of the crossover signal strength

between the current and candidate base stations.

Relative signal strength with hysteresis: Handoff occurs only if the new base station is

sufficiently stronger (by a margin H in Figure 10.7a) than the current one. In this case, handoff

occurs at L3. This scheme prevents the ping-pong effect, because once handoff occurs, the

effect of the margin H is reversed. The term hysteresis refers to a phenomenon known as relay

hysteresis and can be appreciated with the aid of Figure 10.7b.We can think of the handoff

mechanism as having two states. While the mobile unit is assigned to BS A, the mechanism will

generate a handoff when the relative signal strength reaches or exceeds the H. Once the

mobile unit is assigned to B, it remains so until the relative signal strength falls below - H, at

which point it is handed back to A. The only disadvantage of this scheme is that the first

handoff may still be unnecessary if BS A still has sufficient signal strength.

Relative signal strength with hysteresis and threshold: Handoff occurs only if (1) the current

signal level drops below a threshold, and (2) the target base station is stronger than the

current one by a hysteresis margin H. In our example, handoff occurs at L 3 if the threshold is

either Th1 or Th2 and at L 4 if the threshold is at Th3.

Prediction techniques: The handoff decision is based on the expected future value of the

received signal strength.

The handoff decision is complicated by the use of power control techniques, which enable the

BS to dynamically adjust the power transmitted by the mobile unit. This topic is discussed

next.

Effects of Mobile Radio Propagation

Mobile radio communication introduces complexities not found in wire communication or in

fixed wireless communication. Two general areas of concern are signal strength and signal

propagation effects.

Signal strength: The strength of the signal between the base station and the mobile unit

must be strong enough to maintain signal quality at the receiver but not so strong as to

create too much co-channel interference with channels in another cell using the same

frequency band. Several complicating factors exist. Human-made noise varies

considerably, resulting in a variable noise level. For example, automobile ignition noise

in the cellular frequency range is greater in the city than in a suburban area. Other signal

sources vary from place to place. The signal strength varies as a function of distance

from the BS to a point within its cell. Moreover, the signal strength varies dynamically as

the mobile unit moves.

Fading: Even if signal strength is within an effective range, signal propagation effects

may disrupt the signal and cause errors. Section 5.4 discussed fading and various

counter measures.

14.2 Multiple Access

Efficient allocation of signaling dimensions between users is a key design aspect of both uplink and downlink channels, since bandwidth is usually scarce and/or very expensive. When dedicated channels are allocated to users it is often called multiple access4. Applications with continuous transmission and delay constraints, such as voice or video, typically require dedicated channels for good performance to insure their transmission is not interrupted. Dedicated channels are obtained from the system signal space using a channelization method such as time-division, frequency-division, code-division, or hybrid combinations of these techniques. Allocation of signaling dimensions for users with bursty transmissions generally use some form of random channel allocation which does not guarantee channel access. Bandwidth sharing using random channel allocation is called random multiple access or simply random access, which will be described in Section 14.3. In general, the choice of whether to use multiple access or random access, and which specific multiple or random access technique to apply, will depend on the system applications, the traffic characteristics of the users in the system, the performance requirements, and the characteristics of the channel and other interfering systems operating in the same bandwidth.

Multiple access techniques divide up the total signaling dimensions into channels and then assign these channels to different users. The most common methods to divide up the signal space are along the time, frequency, and/or code axes. The different user channels are then created by an orthogonal or non-orthogonal division along these axes: time-division multiple access (TDMA) and frequency-division multiple access (FDMA) are orthogonal channelization methods whereas code-division multiple access (CDMA) can be orthogonal or non-orthogonal, depending on the code design. Directional antennas, often obtained through antenna array processing, add an additional angular dimension which can also be used to channelize the signal space: this technique is called space division multiple access (SDMA). The performance of different multiple access methods depends on whether they are applied to an uplink or downlink, and their specific characteristics. TDMA, FDMA, and orthogonal CDMA are all equivalent in the sense that they orthogonally divide up the signaling dimensions, and they therefore create the same number of orthogonal channels. In particular, given a signal space of dimension 2BT, N orthogonal channels of dimension 2BT/N can be created, regardless of the channelization method. As a result, all multiple access techniques that divide the signal space orthogonally have the same channel capacity in AWGN, as will be discussed in Sections 14.5-14.6. However, channel impairments such as flat and frequency-selective fading affect these techniques in different ways, which lead to different channel capacities and different performance in practice.

14.2.1 Frequency-Division Multiple Access (FDMA)

In FDMA the system signaling dimensions are divided along the frequency axis into non-overlapping channels, and each user is assigned a different frequency channel, as shown in Figure 14.2. The channels often have guard bands between them to compensate for imperfect filters, adjacent channel interference, and spectral spreading due to Doppler. If the channels are sufficiently narrowband then even if the total system bandwidth is large, the individual channels will not experience frequency-selective fading. Transmission is continuous over time, which can complicate overhead functions such as channel estimation since these functions must be performed simultaneously and in the same bandwidth as data transmission. FDMA also requires frequency-agile radios that can tune to the different carriers associated with the different channels. It is difficult to assign multiple channels to the same user under FDMA, since this requires the radios to simultaneously demodulate signals received over multiple frequency channels. FDMA is the most common multiple access option for analog communication systems, where transmission is continuous, and serves as the basis for the AMPS and ETACS analog cellular phone standards [2, Chapter 11.1]. Multiple access in OFDM systems, called OFDMA, implements FDMA by assigning different subcarriers to different users.

14.2.2 Time-Division Multiple Access (TDMA)

In TDMA the system dimensions are divided along the time axis into nonoverlapping channels, and each user is assigned a different cyclically-repeating timeslot, as shown in Figure 14.3. These TDMA channels occupy the entire system bandwidth, which is typically wideband, so some form of ISI mitigation is required. The cyclicallyrepeating timeslots imply that transmission is not continuous for any user. Therefore, digital transmission techniques which allow for buffering are required. The fact that transmission is not continuous simplifies overhead functions such as channel estimation, since these functions can be done during the timeslots occupied by other users. TDMA also has the advantage that it is simple to assign multiple channels to a single user by simply assigning him multiple timeslots.

A major difficulty of TDMA, at least for uplink channels, is the requirement for synchronization among the different users. Specifically, in a downlink channel all signals originate from the same transmitter and pass through the same channel to any given receiver. Thus, for flat-fading channels, if users transmit on orthogonal timeslots the received signal will maintain this orthogonality. However, in the uplink channel the users transmit over different channels with

different respective delays. To maintain orthogonal timeslots in the received signals, the different uplink transmitters must synchronize such that after transmission through their respective channels, the received signals are orthogonal in time. This synchronization is typically coordinated by the base station or access point, and can entail significant overhead. Multipath can also destroy time-division orthogonality in both uplinks and downlinks if the multipath delays are a significant fraction of a timeslot. TDMA channels therefore often have guard bands between them to compensate for synchronization errors and multipath. Another difficulty of TDMA is that with cyclically repeating timeslots the channel characteristics change on each cycle. Thus, receiver functions that require channel estimates, like equalization, must re-estimate the channel on each cycle. When transmission is continuous, the channel can be tracked, which is more efficient. TDMA is used in the GSM, PDC, IS-54, and IS-136 digital cellular phone standards [2, Chapter 11].

14.2.3 Code-Division Multiple Access (CDMA)

In CDMA the information signals of different users are modulated by orthogonal or non-orthogonal spreading codes. The resulting spread signals simultaneously occupy the same time and bandwidth, as shown in Figure 14.4. The receiver uses the spreading code structure to separate out the different users. The most common form of CDMA is multiuser spread spectrum with either DS or FH, which are described and analyzed in Chapters 13.4- 13.5.

Downlinks typically use orthogonal spreading codes such as Walsh-Hadamard codes, although the orthogonality can be degraded by multipath. Uplinks generally use non-orthogonal codes due to the difficulty of user synchronization and the complexity of maintaining code orthogonality in uplinks with multipath [5]. One of the big advantages of non-orthogonal CDMA in uplinks is that little dynamic coordination of users in time or frequency is required, since the users can be separated by the code properties alone. In addition, since TDMA and FDMA carve up the signaling dimensions orthogonally, there is a hard limit on how many orthogonal channels can be obtained. This is also true for CDMA using orthogonal codes. However, if non-orthogonal codes are used, there is no hard limit on the number of channels that can be obtained. However, because non-orthogonal codes cause mutual interference between users, the more users that simultaneously share the system bandwidth using non-orthogonal codes, the higher the level of interference, which degrades the performance of all

the users. A non-orthogonal CDMA scheme also requires power control in the uplink to compensate for the near-far effect. The near-far effect arises in the uplink because the channel gain between a user’s transmitter and the receiver is different for different users. Specifically, suppose that one user is very close to his base station or access point, and another user very far away. If both users transmit at the same power level, then the interference from the close user will swamp the signal from the far user. Thus, power control is used such that the received signal power of all users is roughly the same. This form of power control, which essentially inverts any attenuation and/or fading on the channel, causes each interferer to contribute an equal amount of power, thereby eliminating the near-far effect. CDMA systems with non-orthogonal spreading codes can also use MUD to reduce interference between users. MUD provides considerable performance improvement even under perfect power control, and works even better when the power control is jointly optimized with the MUD technique [6]. We will see in Sections 14.5-14.6 that CDMA with different forms of multiuser detection achieves the Shannon capacity of both the uplink and the downlink, although the capacity-achieving transmission and reception strategies for the two channels are very different. Finally, it is simple to allocate multiple channels to one user with CDMA by assigning that user multiple codes. CDMA is used for multiple access in the IS-95 digital cellular standards, with orthgonal spreading codes on the downlink and a combination of orthogonal and non-orthogonal codes on the uplink [2, Chapter 11.4]. It is also used in the W-CDMA and CDMA2000 digital cellular standards [4, Chapter 10.5].

14.2.4 Space-Division

Space-division multiple access (SDMA) uses direction (angle) as another dimension in signal space, which can be channelized and assigned to different users. This is generally done with directional antennas, as shown in Figure 14.5. Orthogonal channels can only be assigned if the angular separation between users exceeds the angular resolution of the directional antenna. If directionality is obtained using an antenna array, precise angular resolution requires a very large array, which may be impractical for the base station or access point and is certainly infeasible in small user terminals. In practice SDMA is often implemented using sectorized antenna arrays, discussed in Chapter 10.8. In these arrays the 360o angular range is divided into N sectors. There is high directional gain in each sector and little interference between

sectors. TDMA or FDMA is used to channelize users within a sector. For mobile users SDMA must adapt as user angles change or, if directionality is achieved via sectorized antennas, then a user must be handed off to a new sector when it moves out of its original sector.

Mobile Communication

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Comparision of SDMA CDMA FDMA TDMA

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u1note(in other notes)

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remaining questions from the question paper