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.In a hard handover (usually withfrequency change) the mobile switches to the alternative cell that gives thebest reception, while in a soft handover (no frequency change and commonRF Design Overview127in CDMA) the mobile always makes use of the base station with the bestreception.This leads to a beneficial handover gain that is of the order of 2.5to 3 dB for hard handovers and 3.5 to 4 dB for soft handovers [4].A specialcase of the soft handover is the softer handover that occurs between two sec-tors of a cell.This differs from the basic soft handover by the reduction in the number of distinct power control commands and in the signaling requiredbetween base station components, and it is also faster.The trade-off between these factors is determined by calculating a radiolink budget that can then be used to relate the possible cell size to BTSantenna height using standard design models.6.3 Radio Link BudgetThe link budget determines the maximum path loss that is allowable toachieve the received SNR required for adequate quality.Any antenna has again that is formally defined by the expressionGain = 4p × Ae/l2(6.4)where Ae is the effective area or capture area of the antenna and l is the wavelength of the signal; so large antennae provide more gain.A crude picture of transmission and reception is shown in Figure 6.3 to allow calculation of the power received from a distant transmitter.In this diagram the powers of transmitter and receiver are P(T ) andP(R), respectively, with antennae gains G(T ) and G(R), while the cable/connector losses are L(T ) and L(R).In addition, there is assumed to be aP(R)P(T)L(P)G(R)G(T)L R( )L T( )Figure 6.3 Power factors.128QoS in Integrated 3G Networkstotal loss on the path between antennae of L(P).The received power is thengiven in logarithmic decibel format byP(R) = P(T ) − L(T ) + G(T ) − L(P) + G(R) − L(R)(6.5)where P(R) and P(T ) are the logarithms of PR and PT in (6.2).These indi-vidual factors then have to be further subdivided and related to various mar-gins to work out what is required.Because of the lower power of the mobile as compared to the base sta-tion, the radio budget for the uplink is more critical than that for the downlink in determining the cell size.As a result, in the budget calculations the mobile is assumed to be the transmitter and the BTS the receiver.The behavior of the BTS receiver is more complicated than that forthe mobile, and calculations have to make explicit allowance for noise, interference, and fast fading in addition to the basic parameters above, althoughthese factors also affect the mobile.The fast fading requires a fading mar-gin to be left in the closed power control loop of CDMA that is greatest forslow-moving users.In addition to the signal, there is also white noise, N, that results fromfinite temperature of the equipment.This is given by Boltzmanns lawN = kTBwF(6.6)where Bw is the receiver noise bandwidth and F is the noise figure charac-terizing the receiver.At a typical temperature of 17°C, kT amounts to−174 dBm/Hz, and the noise figure F is typically about +5 dBm [5].ForCDMA Bw is the chip rate (e.g., 3.84 Mcps for WCDMA-FDD) and con-tributes a figure of 10 log (Bw) in decibel units, which is roughly 66 dB for WCDMA, leading to a receiver noise power of about −103 dB in this case.The ratio PR/N is the SNR that determines the link budget.This is moreconveniently expressed as the ratio of the energy per modulated symbol tothe noise (Ec/N0) where(Ec/N0) = PRBw/(NRc)(6.7)where Ec is the white noise power spectral density and Rc is the transmission rate of modulated symbols, which is 3.84 Mcps for WCDMA-FDD.In logarithmic form (Ec/N0) is given byRF Design Overview129(Ec/N0) (dB) = P(T ) + G(T ) + G(R) − 10 log (kT ) (dB/Hz) − 10 log Rc− 10 log F − L(R) − L(T )(6.8)where the factor 10 arises from the use of decibels as opposed to bels.The interference parameter for the BTS appears as the interferencemargin to allow for interference from other users and is usually approximated as an additional white noise term.The size of this factor varies accordingto the loading of the cell by users, ranging from 0 at very light loadings to roughly 3 to 5 dB at 50% to 60%.This combined with the receiver noisedensity gives the receivers effective noise plus interference density, which is likely to be of the order of −100 dBm.CDMA provides another unique type of gain in addition to the anten-nae gains.This is a processing gain that results from the spreading of the signal over multiple frequencies in accordance with the spreading factor.Thedegree of this spreading, and hence of the processing gain, depends on the bit rate according to the formulaProcessing gain = 10 × log (chip rate/bit rate)(6.9)where the bit rate is that of the signal before channel coding and spreading.In the case of WCDMA-FDD, this becomes 10 × log (3,840/bit rate inKbps), and at low bit rates, such as the 12.2 Kbps of AMR voice, amounts toaround 25 dB.This allows lower powers to be used for CDMA mobiles for agiven bit rate than for earlier GSM varieties.The receiver sensitivity is given by boosting the noise plus interferencedensity by the processing gain and subtracting the required signal strength
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