Which of the following is the term for the unit of measurement of the output power of a radio?

12.5.3 Handoff Process and Algorithms

A basic handoff process consists of three main phases including measurements, decision, and execution phase (see Figure 12.9). The overall handoff process discussed here is related to MAHO strategy. The MS continuously measures the signal strength of the serving and the neighboring cells, and reports the results to the network. From a system performance standpoint, handoff measurement phase is an important task. The signal strength of the radio channel may vary significantly due to fading and signal path loss, resulting from the cell environment and user mobility. Also, an excess of measurement reports by MS or handoff execution by the network increases the overall signaling load, which is not desired.

The decision phase consists of an assessment of the overall quality of service (QoS), of the connection and comparing it with the requested QoS attributes and estimates from neighboring cells. Depending on the outcome of this comparison, the handoff procedure may or may not be triggered.

The execution phase involves handoff signaling and radio resource allocation.

Radio signal strength (RSS) measurements from the serving cell and neighboring cells are primarily used in most of the networks. Alternatively or in conjunction, the path loss, signal-to-interference ratio (SIR), bit error rate (BER), and block error rate (BLER) have been used in certain voice and data networks. The following parameters are generally used in the handoff algorithm:

Upper threshold is the level at which the signal strength of the connection is at the maximum acceptable level with respect to the required QoS.

Lower threshold is the level at which the signal strength of the connection is at the minimum acceptable level to satisfy the required QoS. Thus, the signal strength of the connection must not fall below this level.

Handoff margin is a predefined parameter that is set at the point where the signal strength of the neighboring cell has exceeded the signal strength of the serving cell by a certain amount and/or a certain time.

Some of the traditional handoff algorithms are as follows:

RSS type: The BS with the largest signal strength is selected.

RSS plus threshold type: A handoff is performed if the RSS of a new BS exceeds that of the serving BS and the signal strength of the serving BS is below the lower threshold value.

RSS plus handoff margin type: A handoff is performed if the RSS of a new BS is more than that of the serving BS by a handoff margin.

1.26.4.3.3 Statistical approaches

Commonly, wireless signal-strength indicators used in positioning relate to power, direction and time of a received signal. Several characteristic indicators are widely used for position estimation purposes, such as time of arrival, angle of arrival, and time difference of arrival. Gezici (2008) conducted a comprehensive survey on those wireless position estimation techniques. Statistical approaches are employed to formulate a generic framework for position estimation using one or multiple characteristic indicators, which can be expressed as:

(6)Zi=filatlon+δi,i=1,2,…,N

where Zi is the estimated ith indicator value, fi (lat, lon) is the function for the ith indicator value at a given location with coordinate (lat, lon), δi is a noise parameter, and N represents the number of estimated indicators. The parameters can be estimated based on offline signal sampling data collected at different reference locations, which is similar to previously introduced fingerprinting approach. However, the main difference between fingerprinting and statistical approaches is whether to formulate a generic parameter-based theoretical framework that can be employed for online location estimation in the second step.

Depending on available information on the noise parameter or the indicator probability density function, we can choose parametric statistical tests relying on assumptions about the shape of the distribution (e.g., a Gaussian distribution) and about the form of parameters (e.g., mean, median, and standard deviations), or nonparametric estimation methods (e.g., least squares regression) relying on a fit to empirical data in the absence of any guidance or constraints from theory to estimate the target location.

Before You Start: Basic Concepts and Definitions

Before you can install and/or construct any antenna, there are several terms and calculations with which you should be familiar. While a degree in physics is not necessary, a basic understanding of physics is helpful.

An antenna is simply a passive transducer that radiates energy (gain) into space. Antennas do not actually amplify the signal; they simply change the shape of the energy pattern being radiated. You should be able to select or construct a basic antenna for your use once you understand the basics of antenna design, construction, and operation.

The decibel is the most important unit of measurement when looking at antenna performance. The decibel (or dB) is the basic unit used for radio frequency (RF) power measurement. Table 10.1 lists decibel power levels in relation to wattage levels.

Table 10.1. Transmit Power in Decibels

We use decibel measurements because signal strengths vary logarithmically, not linearly. A logarithmic scale allows simple numbers to represent large variations in signal levels. You'll see it's also very useful in calculating system gains and losses. In the following sections, we've included brief definitions of all the terms we'll be using in this chapter:

dB Decibel. The basic unit of measurement that represents the ratio of two signal levels.

dBm/dBW Decibel milliWatt. This measurement is used to represent power, with 0 dBm defined as 1 milliWatt. For larger signals, there is also dBW, a reference to 1W. Small signals are represented as negative numbers, (for example, −95 dBm). When referencing commercial Wi-Fi devices, power output is normally given in dBm. Many WLAN PCMCIA cards and some Access Points (APs) have a power output of +17dBm (50mW). There is also usually a Receive Signal Sensitivity Indicator (RSSI) measurement listed in dBm (for example, −95dBm).

dBd Decibel dipole. The output power (gain) an antenna has over a dipole antenna at the same frequency. A dipole (two-pole) antenna is a 1/2 wave antenna used as a reference against all other antennas. It is a reference known as 0 dBd (zero decibel referenced to dipole). The dBd measurement is usually used only with antennas below 1 GHz.

dBi Decibel isotropic. This measurement is used for antennas above 1 GHz. A dipole antenna has 2.14 dB higher gain than the 0 dBi dipole reference. If antenna gain is given in dBd, not dBi, add 2.14 to convert to the dBi rating.

NEED TO KNOW … RF POWER

There are several basic rules that you should know when working with antennas, RF power, and expected signal strength. The “3 dB” rule is perhaps the most important rule when dealing with RF (signal) power. It states that for every 3 dB increase in level, the power is doubled. For every 3 dB decrease, the power is cut in half. Similarly, every 10 dB increase in level is 10 times the power, and every 10 dB decrease in level results in 1/10 the power. This is sometimes referred to as the “rule of 3s and 10s.”

Once you understand the different decibel measurements, it is easy to understand Figures of Merit (FoMs) when working with antennas. FOMs are attributes that describe an antenna's performance characteristics. The FoMs are listed as part of every antenna's specifications. Important FoM attributes like gain and front-to-back ratio are listed in dB or dBm. There are many other RF terms and figures that use decibel references and values (these terms are explained in greater detail later in this chapter). Once you are familiar with FoMs in general, it will be easy to recognize the important features of antennas and choose the best antenna for your application.

Effective Isotropic Radiated Power (EIRP) is defined as the power found in the main lobe of the antenna relative to an Isotropic radiator with 0 dB of gain. The EIRP is calculated by taking the antenna gain (in dBi) plus the power (in dBm) inbound from the transmitter. For example, a 9 dBi antenna fed with 26 dBm of power would have an EIRP of 35 dBm.

9 dBi + 26 dBm = 35 dBm (3.2W)

The chart in Figure 10.1, known as a Smith chart, shows the propagation area of a Yagi antenna. A Smith chart is included with any antenna specification and represents the radiation pattern of the antenna. It also shows the front-to-back ratio, and the “side lobes,” which are the smaller, less powerful radiation patterns on each side of the main lobe.

The top pattern represents the main lobe and transmit gain. The lower pattern the back lobe. The difference (in dB) between the front and back lobe is called the front-to-back ratio.

NOTE … A WORD ABOUT ANTENNA GAIN AND COVERAGE

Since the EIRP is in the main antenna lobe only, antenna selection is critical.

When using a high-gain omni antenna (8–12 dBi), the propagation angle is very flat and narrow. Placing the antenna too high will cause the main lobe to pass over the intended target antenna. The irony here is that height is required to clear obstructions, a.k.a. Line-of-Sight, from the Wireless Point of Presence (WiPoP) path to the receivers. Higher gain omni antennas have a flatter, “pancake” shape, while lower gain omni antennas tend to have a wider “donut” shaped pattern.

It may be necessary to use a unidirectional antenna and “down tilt” that concentrates the energy (signal) in a more focused area. Unidirectional antennas direct energy in one direction by radiating the entire signal in a concentrated area instead of 360 degrees like an omni. Table 10.2 lists antenna types and associated values in dBi (gain). Figures 10.2 through 10.6 are images of these antenna types.

Table 10.2. Typical Antenna Types and Gain Values for Off-the-Shelf Antennas

NOTE … INTERESTING ANTENNA

An interesting antenna type has been developed by cantenna.com. This “super cantenna” resembles a Pringles can antenna, is linearly polarized, and features a gain of 12 dBi and a beam width of 30 degrees. You can learn more about this innovative, low-cost product at www.cantenna.com.

Federal Communications Commission

A common misconception when using ‘unlicensed’ equipment is that there are no rules covering the operation of such gear. While there are no license requirements, the FCC does have some regulations with respect to the maximum power output levels when using unlicensed equipment. Part 15 of the FCC's rules for radio equipment lists the specific power requirements. We discuss the pertinent limitations in this section.

The FCC has relaxed the rules on EIRP limits for Point-to-Point (PtP) systems. This has increased the choices of antennas and extended the range of PtP systems. The EIRP for a 2.4–2.5 GHz PtP system is now 36dBm (an amazing 4 watts!) We must calculate a link budget to determine the total EIRP, and remain in FCC compliance. The FCC allows only 30 dBm (1W) EIRP for Point-to-Multipoint (PtMP) communications. This limits the antenna choices and makes the calculation of system output very important. However, for most off-the-shelf commercial equipment using attached antennas, the output is 50–200 mW. This coupled with a 6 dBi antenna is well below FCC limits. Using the previous charts and remembering the rules will help you calculate power levels and remain in compliance. A good rule to remember for 2.4 GHz PtP systems is that (at maximum power output levels) for every 3 dBi of antenna gain over 6 dBi, the transmitter power output must be reduced by 1 dB. For 2.4 GHz PtMP (at maximum power output levels), every 3 dBi of antenna gain over 6 dBi must be met with a 3 dB reduction in transmitter power.

The 5 GHz band has various output power limits. The limits depend upon the sub-band within the 5 GHz band in which you're operating. The lower portions of the 5 GHz unlicensed band are between 5.15 and 5.25 GHz The output for these devices is fixed at a maximum of 50 mW. The 5.25–5.35 GHz middle sub-band has a power limit of 250 mW.

The 5.725–5.825 GHz upper band is normally used for high bandwidth (T-1, OC-3) transmissions associated with microwave radio. This band has most recently been adopted by many Wireless Internet Service Providers (WISPs) as a high data rate “backhaul” solution. This removes congestion from the 2.4 GHz (DSSS) frequency band and allows much more bandwidth (and more users) to be concentrated for transmission.

The Link Budget is the calculation of the losses and gains (in dB) for the complete RF system, and is determined using a simple formula that combines all the power and gain figures for both sides of a link.

Link Budget = P(t) + TX(G) + Rx(G) + Rx – Path Loss

Where:

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P(t) = power of transmitter (e.g., 17 dBm)

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TX(G) = transmit antenna gain (e.g., 6 dBi)

▪

RX(G) = receive antenna gain (e.g., 6 dBi)

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Rx = Receive Sensitivity of receiver

The numbers are the gain figures used in a link budget. We will also look at the loss or attenuation levels—caused by cables, connectors, and so forth—that must also be factored into the final Link Budget calculation. (A good online calculator can be found at www.afar.net/RF_calc.htm and www.qsl.net/pa0hoo/helix_wifi/linkbudgetcalc/wlan_budgetcalc.html)

Path loss, the amount of loss in dB that occurs when a radio signal travels through free space (air), is also known as Free Space Loss (FSL). FSL can be calculated using the following formula:

FSL (isotropic) = 20Log10 (Freq in MHz) + 20Log10 (Distance in Miles) + 36.6

Additional factors you should consider when determining your link's requirements:

▪

Radiation pattern/propagation angle The propagation angle is given in degrees and denotes how much area in degrees an antenna broadcasts its signal. Example: Vertical angle = 45 degrees, Horizontal angle = 7 degrees. Search the Internet for various antenna manufacturers to find examples of Smith charts that represent various propagation angles.

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Polarity All antennas have a “pole” (short for polarity), which can be horizontal, vertical, or circularly polarized. Polarity indicates the angle of the RF wave's propagation in reference to an H/V/C plane. You must insure that all Wi-Fi systems you want to communicate with have antennas on the same pole. The difference in H/V poles (if for example, one antenna is horizontally polarized and the other is vertically polarized) is a loss of 30 dB.

▪

Vertical/horizontal beam width This is the angle of the RF “beam” referenced to the horizontal or vertical plane. Typically, the higher the gain, the more focused (narrow) the beam. Example: A 24 dBi antenna commonly has an 18-degree beam width, vs. a 9 dBi antenna, which will have a 45- to 60-degree beam width.

▪

Fresnel zone The Fresnel zone is the propagation path that the signal will take through the air. The Fresnel zone can be determined using the formula below. The Fresnel zone is important when installing Line-of-Site equipment, because if the Fresnel zone or any part of it is obstructed, it will have a direct and negative effect on the system connectivity.

Fresnel Zone Calculation = 72.1 * SqrRoot(Dst1Mi * Dist2Mi / Freq (in GHz) * Distance-in-Miles

You can find a good online Fresnel zone calculator at www.radiolan.com/fresnel.html.

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Front-to-back ratio An antenna's front-to-back ratio is typically given in dB and denotes how much signal is projected behind the antenna, relative to the signal projected in front of the antenna (in the main lobes). The lower the front-to-back ratio, measured in dB, the better. The reason is that you don't want excessive signal propagating from the rear of the antenna.

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Link Margin The Link Margin, sometimes called System Operating Margin (SOM), is the minimum difference between the received signal (in dBm) and the sensitivity of the receiver required for error-free operation. In many systems, this is also referred to as the Signal-to-Noise-Ratio (SNR).

Table 10.3 lists Fade Margins for various link distances.

Table 10.3. Fade Margins for Various Link Distances

Distance (Miles)	Conservative Fade Margin (dB)
0.5	4.2
1	7.5
2	10.8
3	12.75
4	14.1
5	15.2
10	18.5
15	20.4

In many newer radios, a Signal to Noise Ratio (SNR) specification is used instead of the RSSI reading/measurement. Motorola's 5 GHz Canopy system requires only 3 dB SNR to achieve connectivity, while Alvarion's EasyBridge 5.8 GHz system expects a minimum 10 dB SNR for connectivity. Several good Web sites provide calculators for Fresnel Zone, Fade Margin, and Path Loss:

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www.zytrax.com/tech/wireless/calc.htm

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www.dataradio.com/mso/tsan002rf.xls

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www.andrew.com/products/antennas/bsa/default.aspx?Calculators/qfreespace.htm

NEED TO KNOW … THE BIGGER THEY ARE, THE FARTHER THEY CALL

Size does matter! It may be necessary to increase the size of your antenna if you find that you can't quite get the desired distance or throughput from your link. Remember the “6 dB” rule when thinking about antennas (size), propagation distance, and path loss. The rule states that each time you double the distance from transmitter to receiver, the signal level decreases by 6 dB.

Attenuation in Cables, Connectors, and Materials

Attenuation is the reduction in signal due to cable length, connectors, adapters, environment, or building materials. Often, indoor wireless systems will suffer extreme attenuation due to metal cross members or rebar within walls. It is important to consider the type of building materials used for either indoor systems or systems where client antennas are mounted indoors while AP antennas are outdoors at a distance. It is also important to take the figures for cable and connector loss into account when calculating your link budget.

Table 10.4 lists common building materials and the expected loss in dB.

Table 10.4. Attenuation Factors for Various Materials

Material	Attenuation Factor/dB Loss
Plasterboard wall	3 dB
Glass wall with metal frame	6 dB
Cinder block wall	4 dB
Office window	3 dB
Metal door	6 dB
Metal door in brick wall	12.4 dB

The most common cables used in unlicensed wireless include:

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RG-58 Commonly used for pigtails and is not recommended for long runs. Loss at 2.4 GHz per 100 feet = 24.8 dB.

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LMR 195 Identical in gauge to RG 58, but with less loss. Loss at 2.4 GHz per 100 feet = 18.6 dB.

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LMR 400 Used most commonly for antenna runs over 6 feet. Loss at 2.4 GHz per 100 feet = 6.6 dB.

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LMR 600 The best, but also the most expensive cable. Loss at 2.4 GHz per 100 feet = 4.3 dB.

The loss quoted for any cable specification is generally per 100 feet. The loss factor is important to remember when installing outdoor systems. For both cables and connectors, the loss factor is commonly listed as “insertion loss.” A good online cable loss calculator can be found at www.timesmicrowave.com/cgi-bin/calculate.pl.

Figures 10.7 through 10.11 are examples of connector types used in unlicensed wireless systems. In most cases, it is assumed that the loss per connector is between .2 and 1.0 dB. Many people use .5 dB of loss per connector as a general rule of thumb. If a connector is suspect and produces more loss, it is either of poor design or is faulty.

Figure 10.7. “N” Type

Figure 10.8. SMA

Figure 10.9. MMCX

Figure 10.10. TNC

Figure 10.11. Reverse Polarity (R/P) TNC

System Grounding and Lightning Protection

Since an antenna is a metal object with a corresponding wire connection and is elevated several feet in the air, it unfortunately makes an excellent lightning rod. It is always recommended that you use both an earth ground and a lightning arrestor when installing antennas outdoors. The earth ground should be connected to the antenna mast and the antenna tower to ground electrical charges (lightning). It is also recommended to use a lightning arrestor to protect radio equipment. The insertion loss of a good lightning arrestor is commonly a maximum of 1.5 dB.

Figure 10.12 shows a typical lightning arrestor.

Figure 10.12. Common Lightning Arrestor for 2.4 GHz

WARNING: HARDWARE HARM

The labeling on the lightning arrestor denotes the antenna port connection and the equipment (radio) port connection. Connecting the device in reverse may result in damage to equipment and systems. It is also quite probable that the system will not work or performance will be severely degraded.

The lightning arrestor should be located between the radio equipment and the antenna. Figure 10.13 is an example of a small unidirectional antenna with jumper cable plus a lightning arrestor and pigtail assembly. This could be mounted on a pole, on the side of an eave, or in conjunction with an outdoor box containing the radio.

Figure 10.13. Lightning Arrestor Mounting Scenario

WARNING: HARDWARE HARM

It is always recommended that proper grounding techniques and lightning protection devices be used when installing any antenna system outdoors. Always use caution when installing antennas, especially when using extended masts or building tower sections.

Signal Strength

There are three basic ways to increase signal strength when receiving a signal: amplification, antenna tuning, and antenna orientation. Using an amplifier will amplify noise as well, including those you may not want, and may simply result in a louder version of the same noisy signal. You can also tune the length of the antenna to the frequency of interest. This can be done by adjusting the length of the antenna, or changing to an antenna tuned for the range you are interested in. Finally, using a directional antenna that is designed to focus the signal energy can increase the signal strength.

Wait for the Client to Associate to Your Access Point

If all goes well and the signal strength of your access point is stronger than the target networks access point, you should see the wireless client connect to your access point. When a wireless client associates to your access point, you need to assign it an IP address (see Figure 9.19). Dnsmasq will provide an IP address to the client using the DHCP allocations defined in the /etc/dnsmasq.conf file. The client will use the IP address of your access point as the gateway and primary DNS server.

To monitor incoming connections to your access point, you can start a network sniffer (Global Regular Expression Parser [GREP] for DHCP requests from the Dnsmasq log file (logs to syslog in /var/log/messages). Using the command below, you can see that the client you sent a deauthentication flood to is now connected to your access point.

4.3 Dependence on signal strength A

We also examine the dependence on the signal strengthA in CR. Figure 19.13 shows A dependence of maxτCτ (a) and MI(F; S) (b) in the strong chaotic (d= −16), the weak chaotic (d=−11), and the periodic (d=5) cases. The strong and weak chaotic states have higher values of maxτCτ and MI(F; S) than the periodic state in the signal strength region (A≲1). Especially, as shown in Figure 19.13(a), maxτCτis higher and τ is smaller in the weak chaotic state than those in the strong chaotic state in (0.03<A<0.7). This result indicates that the weak chaotic state has higher sensitivity and a prompter response against the weak signal than the strong chaotic state.

2.9.2 Frequency diversity

You can get a similar differential in signal strength over two or more signal channels by transmitting on separate frequencies. For the same location of transmitting and receiving antennas, the occurrences of peaks and nulls will differ on the different frequency channels. As in the case of space diversity, choosing the strongest channel will give a higher average signal-to-noise ratio than on either one of the channels. The required frequency difference to get near independent fading on the different channels depends on the diversity of path lengths or signal delays. The larger the difference in path lengths, the smaller the required frequency difference of the channels.

7.7.3 Minimum discernible signal (MDS) and dynamic range

We saw that there is a minimum signal strength that a given receiver can detect, and there is also some upper limit to the strength of signals that the receiver can handle without affecting the sensitivity. The range of the signal-handling capability of the receiver is its dynamic range.

The lowest signal level of interest is not the sensitivity, but rather the noise floor, or minimum discernible signal, MDS, as it is often called. This is the signal power that equals the noise power at the entrance to the demodulator. It can be found through Eq. (7.1) or (7.15), minus the last term, S/N.

The reason for using the MDS and not the sensitivity for defining the lower limit of the dynamic range can be appreciated by realizing that interfering signals smaller than the sensitivity but above the noise floor or MDS, such as those that arise through IMD, will prevent the receiver from achieving its best sensitivity.

The upper limit of the dynamic range is commonly taken to be the level of interfering signals that create a third-order spurious signal with an equivalent input power equal to the MDS. This is called two-tone dynamic range, TTDR. It is determined from the input intercept IIP3 with the following relation:

(7.20)TTDR=23⋅ IIP3−MDS

While Eq. (7.20) is the definition preferred by high-level technical publications, you may also see articles where dynamic range is used as the difference between the largest wanted signal that can be demodulated correctly and the receiver sensitivity. This definition doesn’t account for the effect of spurious responses and is less useful than Eq. (7.20). Another way to define dynamic range is to take the upper limit as the 1-dB compression point and the lower level as the MDS. This dynamic-range definition emphasizes the onset of desensitization. It may be called single-tone dynamic range.

The TTDR of the receiver in Fig. 7.3A using an IIP3 of − 5 dBm and MDS of (− 106.9 − 8.5) = − 115.4 is, from Eq. (7.20)

TTDR=23⋅−5−−115.4=73.6dB

We note here that the intercept point IIP3 to use in finding dynamic range is not necessarily that of the LNA, since the determining intercept point may be in the following mixer and not in the LNA. The formula for finding the intercept point of three cascaded stages is

(7.21)IIP3=11IIP31+G1IIP32+ G1⋅G2IIP33

where G1 and G2 are the numerical gains in of the first two stages and IIP31, IIP32, and IIP33 are the numerical input intercepts expressed in milliwatts of the three stages. Then the total input third-order intercept in dBm will be 10 log(IIP3).

Note that in calculating IMD of a receiver from its components in the RF chain, you must base the IMD on interfering signals outside of the IF passband and take account of the resulting strength of those interfering signals as they pass through bandpass filters.

It should be evident from this description that adding a preamplifier to improve (reduce) noise figure to increase sensitivity may adversely affect the dynamic range because its gain reduces overall IP3, and using an input attenuator to control IMD and compression will raise the noise figure and reduce sensitivity. The design of a receiver front end must take the conflicting consequences of different measures into account in order to arrive at the optimum solution for a particular application.

3.11 Fade Margin

As we discussed earlier, the local mean signal strength in a given area at a fixed radius, R, from a particular base station antenna is lognormally distributed [7]. The local mean (i.e., the average signal strength) in dB is expressed by a normal random variable with a mean Sm (measured in dBm) and standard deviation σs (dB). If Smin is the receiver sensitivity, we determine the fraction of the locations (at d = R) wherein a mobile would experience a received signal above the receiver sensitivity. The receiver sensitivity is the value that provides an acceptable signal under Rayleigh fading conditions. The probability distribution function for a log-normally distributed random variable is:

(3.40)p(S)=1σs2πe−[(S−Sm)2 /(2σs2)]

The probability for signal strength exceeding receiver sensitivity PSmin(R) is given as

(3.41)PSmin(R)=P [S≥Smin]=∫Smin∞p(S )dS=12−12erf(Smin−Smσs2)

Note: See Appendix D for erf, erfc and Q functions.

25.4.1 Repeaters

All network connections suffer from a reduction in signal strength (attenuation) and digital pulse distortion. Thus, for a given cable specification and bit rate, each connection will have a maximum length of cable that can be used to transmit the data reliably. Repeaters can be used to increase the maximum interconnection length, and may do the following:

Clean signal pulses.

Pass all signals between attached segments.

Boost signal power.

Possibly translate between two different media types (such as fibre to twisted-pair cable).