Lucent Technologies Bell Labs Innovations
____________________________________________________________________________________
Network Wireless Systems Subject:
CDMA Link Budget Tutorial
date:
April 1, 2000
from:
S. Vasudevan JW45C6000 Room 4c-239 67, Whippany Road Whippany, NJ 07981 Ph:973-581-6862
[email protected]
____________________________________________________________________________________ Objective The objective of this document is to provide a road map for populating a CDMA link budget for specified design objectives, using Lucent equipment. Additionally, explanations are provided for the entries in the CDMA reverse link budget. Introduction The objective of a link budget is to catalog all the losses and gains between the two ends of a communication link, thus yielding the maximum loss in signal strength that can be tolerated between transmitter and receiver. This maximum allowable loss in signal strength is also known as available path loss. It is specified in logarithmic units (decibels) and can in turn be translated into the greatest spatial distance between transmitter and receiver at which reliable communication of the desired quality can still take place. In the context of wireless mobile communication systems, link budgets are a pre-requisite to determining the locations of, as well as spacing between, cell sites in order to ensure reliable and uninterrupted communication as mobiles move through an area of intended radio coverage. Since communication here between the mobiles and base stations is bi-directional, the quality of the link from mobile to base station (uplink or reverse link) as well as from the base station to the mobile (downlink or forward link) must be considered in system design. This document however, focuses mainly on the CDMA reverse link budget and a discussion of the forward link budget is restricted to the checks that need to be made on forward link parameters after obtaining the reverse link available path loss. Definitions of Link Budget Entries The following set of definitions is to be read in conjunction with the appended reverse link budget spreadsheet. Mobile EIRP
This refers to the effective isotropically radiated power from the mobile at the antenna connector. For the PCS CDMA mobile, this value is 21 dBm while it is 23 dBm at cellular frequencies. These values are taken from minimum performance standards for a 200 milliwatt mobile. Antenna Gain This is the gain of the mobile antenna. At both cellular and PCS frequencies, this is a dipole whose gain can be taken to be 2 dB. For new applications such as the proposed CDMA system in the NMT band at 450 MHz, a dipole is not feasible and the antenna gain is expected to be much lower (of the order of -1.5 dB for the helical antenna).
Head/Body Loss This refers to the attenuation of the radio signal during both transmission and reception as the mobile antenna is held to the ear of the user. At PCS and cellular frequencies, this attenuation is mainly due to the head of the user while at lower frequencies (and larger wavelengths) the entire human body could distort the radiation pattern of the mobile antenna. Typical head/body loss values range from 2 to 5 dB. Values to be used in the link budget are typically provided by the wireless network operator based on field measurements or prior experience. It is important to obtain these values from the operator since any design loss raises design cell count. The field measurement technique consists of first setting up a CW transmitter at a typical cell site location. Next the signal strength seen at a mobile antenna held to the ear of a user who walks a test route is logged. The difference between this signal strength and that seen by an isolated reference antenna held at the same height is logged and averaged. The result is an estimate of the head loss to be used in the link budget. Receiver Antenna Gain This refers to the gain of the Rx antenna at the base station. While the actual antennas used in the network may vary from site to site, a nominal, representative value is provided in the link budget based on the frequency of operation and sectorization. The following table provides nominal antenna gain values for PCS and cellular frequencies. The gain units are dBi or gain with respect to an isotropic radiator.
PCS Cellular
Omni 13 10
Three sector 17.5 14
In selecting an antenna of higher (or lower gain) for a specific application, the horizontal and vertical beamwidths should not be compromised. Typical values for vertical and horizontal beamwidths are 60 degrees and 6 degrees. This ensures adequate overlap between cells for soft handoff and also ensures that there is minimal gain variation across the cell. For highway coverage, narrower horizontal beamwidth antennas of 30 degrees may be used. More guidance on antenna selection can be found in the presentation “Antenna Usage: Recommendations and Restrictions” available in the RF Engineering Library on the AMPS Systems Engineering web site.
Receiver Cable and Connector Losses The receiver cable and connector losses are nominally taken to be 3 dB. When the cable length and diameter (and hence attenuation/ft) are known, the actual cable losses may be substituted in the link budget along with an additional margin of 0.5 dB for connector (and duplexer) losses. Typical cable diameters used are 7/8" and 1 5/8" and corresponding attenuations are 6.15 dB/100m and 3.84 dB/100m. Duplex configuration In a duplex configuration, the transmit and one receive signal use the same cable. Hence one must account for losses in the duplexer where the mixing of the incoming and outgoing signals takes place. Receiver Noise Figure The receiver noise figure is a measure of SNR degradation caused by the base station receiver front end. It is seen as an additional attenuation of the signal before it enters the demodulator past the base station frontend amplifier and filter. There are two cases to consider here. The first is one where the base station receiver input is at the end of the base station cable leading down from the antenna. The second case is one where a low noise amplifier connects to the antenna at the tower top and a modified base station receiver receives the (now amplified) signal at the other end of the tower cabling. In the first case, the base station noise figure and cable losses are additive. In the latter case, a composite base station noise figure calculated on the basis of the cable losses, low noise amplifier and modified base station receiver characteristics replaces the additive combination of cable loss and receiver noise figure. The following table lists the noise figures for various Lucent base station products.
Series II MiniCell Enhanced MiniCell Modular Cell Microcell Micro-mini
Cellular 5 5 N/A 4 4.5 4
PCS N/A 5 4 4 4 4
Composite Noise Figure calculation with Tower-top Low Noise Amplifier (TTLNA) Use the formula for a cascade (in series) of two ports. In this case, the devices are the TTLNA followed by the cable, and modified base station receiver.
NFc = NFLNA +
Lc − 1 NFRc − 1 + 1 GLNA ( )G LNA Lc
where all the quantities are expressed in linear units. Lc is the cable loss (equal to the noise figure of the cable) and the gain and noise figures of the TTLNA and the modified base station receiver are indicated by the subscripts 'LNA' and 'Rc'. The expression can be further simplified to
NFc = NFLNA +
L.NFRc − 1 . GLNA
For the PCS minicell and a cable loss of 3 dB, the composite receiver noise figure is 5.6 dB. For the Enhanced PCS minicell and a cable loss of 3 dB, the composite receiver noise figure is 5.1 dB Currently, the modular cell, the microcell, and micro-mini do not support the use of TTLNA Receiver Noise Density This simply refers to the thermal noise floor at 290 K which is -174 dBm/Hz. Interference Margin Since all the mobiles in a CDMA system transmit over the same frequency carrier, the signals of these mobiles appear as mutual interference at the base station receiver. The extra margin required at the base station in order to ensure adequate performance for a mobile at cell edge is termed the interference margin in the reverse link budget. This margin can be shown to correspond to the rise in the noise floor that is seen at the serving cell site as a result of the operation of mobiles in its vicinity. The interference margin can be simply calculated from the fractional loading of the system (typically 55%):
IM = 1 0 * lo g 10 (
1 ) 1− f
where f is the fraction loading of pole (0.55 in most cases). However, if the system is being designed to specific capacity objectives, knowledge of the pole point (which is the maximum achievable capacity per sector) is necessary. The following table shows the pole capacities
13 kbit vocoder 8 kbit vocoder (EVRC) 8 kbit vocoder with Orange 1.1 ASIC
Omni 27 40 50
Three Sector 24 35 44
Assumptions: Note that voice activity is assumed to be 0.4. If the customer changes the voice activity factor, pole capacity, loading, and interference margin may all have to be changed. The Eb/No requirement is chosen to be 7 dB for the Orange 1.0 ASIC and 6 dB for the Orange 1.1 ASIC. This is the appropriate value for achieving a field FER target of 2% (note the power control target should be run at 1%). Given the required number of channels per sector, one obtains the fractional loading of pole using the table above and hence the interference margin. At 55% loading, the number of usable channels is:
13 kbit vocoder 8 kbit vocoder (EVRC)
Omni 15 22
Three Sector 13 20
8 kbit vocoder with Orange 1.1 ASIC
27
24
Raising system loading above 55% is generally not recommended. More aggressive loadings may be possible in specialized configurations (e.g., EVRC fixed wireless with fixed, directional antennas) that allow significantly higher pole points. Erlang Capacity If the required Erlang capacity per sector is specified as the design objective, the first step is to convert this number into a required number of voice channels from the Erlang B tables at 2% blocking. (Note that we recommend operation of the system at 2% blocking.) Next, the fractional loading of pole can be determined with the aid of the table above and hence the required interference margin in the link budget. The Erlangs corresponding to the previous table of voice channels are:
13 kbit vocoder 8 kbit vocoder (EVRC) 8 kbit vocoder with Orange 1.1 ASIC
Omni 9 14.85 19.25
Three Sector 7.4 13.2 16.6
Primary Erlangs It should be noted that the above numbers are primary Erlangs, i.e. the traffic due to users in soft handoff is accretive to these numbers. Total Effective Noise plus Interference Density This number may be seen as measuring the rise in the noise floor as a result of the noise contributions due to interference from other users and the noise generated in the base station receiver. Information rate This refers to the maximum rate at which data is sent over the channel, which is either 14.4 kb/s or 9.6 kb/s. Required Eb/No The Energy per bit/noise spectral density ratio is an alternative means of referring to the required Signal-tonoise ratio or SNR. In the next section, we will relate Eb/No to SNR and explain the sequence of calculations leading up to the determination of receiver sensitivity. Receiver Sensitivity This is the minimum acceptable signal level at the base station receiver (before allowances are made for fading, penetration losses and soft handoff gains) for the desired Frame error rate (FER) performance. Soft Handoff gain The reverse link budget specifies how far out a mobile can go from a cell site and yet achieve the desired Signal-to-noise ratio. In the case of CDMA a mobile at cell edge is in simultaneous communication with two or more cell sites resulting in overall link performance that is better than in the case when only a single cell site is available for communication. This implies that the edge of each cell that abuts other cells can be moved further out than if the cell were isolated. This improvement in coverage is captured by the soft handoff gain entry in the reverse link budget.
The soft handoff gain can essentially be viewed as a credit against the single-link fade margin. The credit can be applied because of the presence of two links that are unlikely to fade simultaneously. The actual value of the soft handoff gain depends on the edge coverage probability. This is because the improvement due to the fact that the mobile has two links to communicate on, depends on the likelihood of the signal quality on at least one of the two being acceptable. When a system is being designed for a lower edge coverage probability, the improvement due to soft handoff is smaller since the likelihood of unacceptable signal quality on both links is increased. For the typical edge coverage probability targets of 90% and 75% and a fading standard deviation of 8 dB, the soft handoff gain values are 4 dB and 3 dB respectively.
Explicit Diversity Gain This value is typically zero since the target Eb/No requirement of 7 or 6 dB presumes the availability of receive diversity. In cases where there is no spatial diversity (due to space constraints at the site, for example) one must increase Eb/No by 5 dB. Log-Normal Fade Margin This is the margin that needs to be provided in the link budget to compensate for the fact that the average path loss at any fixed distance from the base station is a random variable that can exceed the average a significant percentage of the time. The margin required is computed on the basis of the assumption that this random variable is log-normally distributed with a mean equal to the average path loss at cell edge and that it has a standard deviation of 8 dB. For a specified value of standard deviation (assumed or measured), the lognormal assumption allows computation of the fade margin for a specified probability of edge coverage from a standard Gaussian table. The fade margin is calculated on the basis of the specified coverage objective which may be based on either edge or area coverage. The margin is based on a single-link computation. Adjustment for the presence of multiple links is contained in the soft handoff gain (see above). For typical edge coverage objectives of 90% and 75%, the required fade margins are 10.3 and 5.6 dB respectively. The margins correspond to percentiles of the lognormal distribution. For example, 90% of the fades are 10.3 dB or less (presuming 8 dB standard deviation) in a lognormal distribution. Accordingly, a design margin of 10.3 dB allows the mobile to combat 90% of fades at cell edge, resulting in a 90% probability of cell edge coverage. Building/Vehicle Penetration Loss This refers to the attenuation of the signal as it passes through one or more walls of buildings in the desired coverage area. The number is usually supplied by the network operator. Customer input on this point is critical, since these values often raise design cell count significantly, and since no single value will penetrate all locations within all buildings. Operator requirements customarily derive from their insight into local building structure, as well as their ability to purchase additional cells and their projections regarding the fraction of subscribers that are likely to be within buildings. Typical values used for penetration loss are 22, 18, 15, and 10 dB for dense urban, urban, suburban, and rural environments. The penetration loss may be specified as either the maximum value expected or as an average within the actual losses distributed about this value. In the latter case, a log-normal distribution with a standard
deviation of 8 dB is assumed for the random building penetration loss. A composite fade margin based on a combination of outdoor and indoor loss distributions is then used for the fade margin. For example, the composite indoor plus outdoor fading distribution, when each has a standard deviation of 8 dB, has a standard deviation of required fade margin is then 14.6 dB.
8 2 + 8 2 or 11.3 dB. For an edge coverage probability of 90%, the
Design Objectives A typical design objective would be specified as a combination of coverage and capacity objectives. Coverage Objective Coverage objective is typically specified as a target coverage probability at cell edge. Typical numbers are 90% and 75% edge coverage. Achieving 90% edge coverage implies that at 90% of the locations at edge, a call can be initiated and kept up. In order to achieve this probability of coverage at cell edge it is necessary to build in a margin into the reverse link budget. This number is specified as the log-normal fade margin and is 10.3 dB for 90% edge coverage and 5.6 dB for 75% edge coverage. Due to difficulties associated with measuring coverage at cell edge (since the boundary is not clearly defined), the coverage objective is often specified as an area coverage requirement. Using path loss models, one can relate area coverage to edge coverage and hence to fade margin requirement. We currently map 95% area coverage to 90% edge coverage and 90% area coverage to 75% edge coverage. These mappings differ somewhat from those found in the literature. These values presume a completely noise-limited receiver, whereas in CDMA cochannel interference at the receiver cannot be ignored. Capacity Objective The capacity objective may be specified as a required number of Erlangs or channels per sector, or as an Erlang density for each morphology. Keeping the constraints on loading per carrier in mind, appropriate equipment choices can be made, interference margin in the reverse link budget can be calculated, and carrier requirements at each site can be determined. Trading capacity for coverage By using the correct interference margin in the reverse link budget based on capacity inputs, any extra dBs available for coverage are automatically indicated in the available path loss. Reverse Link Performance Objective The Eb/No requirements are predicated on a 1% FER design target. Experience has indicated that the Erlangs associated with the 1% target can be achieved while maintaining an average (across coverage area) measured FER of 2%. RIGHT HERE Overall Reverse Link Budget Calculations In this section, we work out the calculation of receiver sensitivity and make the correspondence between this and the computations in the reverse link budget. The required Signal to noise ratio at the base station receiver may be related to the required Eb/No (or Energy per bit to Noise power spectral density) as follows:
SNR =
Eb R N 0W
where R and W are the system information rate and carrier bandwidth respectively. The noise in the SNR consists of the thermal noise, interference from other users as well as noise injected by the base station amplifier. The above expression may be simplified and rewritten as a minimum signal requirement:
S req =
Eb + R + N 0 + NFRc + IM N0
where all terms are expressed in logarithmic units and the dependence on W, the carrier bandwidth, has disappeared. This equation is represented by the computations leading up the calculation of receiver sensitivity in the reverse link budget.
Forward Link Check The forward link check consists of making sure that, given the forward power constraints, all mobiles at cell edge can meet the required forward link Eb/No targets at a distance allowed by the reverse link budget available path loss. The forward link check is critical. A system design employing a reverse link budget will not be viable if the available forward link power cannot meet forward link Eb/No targets within the footprint dictated by the reverse link. Forward Power output and Eb/No requirement The following table lists the forward power output at the antenna connector (J4) to be used in the forward link spreadsheets to verify that acceptable FER targets are being met at cell edge on the downlink.
Series II MiniCell Compact MiniCell Enhanced MiniCell Modular Cell Microcell Micro-mini
Cellular 9 W – 33 W 4.5 W 20 W N/A 20 W 10 w 16 w
PCS N/A 8/16 w N/A 8/16 w 16 w 8w 16 w
For full mobility systems using the 8 kb vocoder, the Eb/No requirements are 6.8 dB at PCS and 7.3 dB at cellular frequencies. Reverse Link Amplifier Backoff In cases where the forward Eb/No requirement is not met, the available reverse link path loss may be reduced until this requirement is met. The approach is simply to decrement the available path in the reverse link until the Eb/No target is met in the forward link spreadsheet. If too large a reduction in path loss is necessitated, it may be preferable to take a capacity penalty and reduce the number of users on both forward and reverse links since forward Eb/No-s are much more sensitive to capacity changes than to reductions in path loss. This penalty, if applied, is sometimes referred to as the “design tradeoff adjustment factor”, since it essentially entails adjustments that trade off capacity against coverage.
Reverse Link Budget Spreadsheet An Example of Reverse Link Budget for PCS 8 kbps CDMA MiniCells for various morphologies using customer specifications DU U SU Rural Without Without With With TTLNA TTLNA TTLNA TTLNA Item (a) Maximum Transmitted power per traffic channel
Units dBm
Value 23
Value 23
Value 23
Value 23
(b) Transmit Cable, connector, combiner, and body losses
dB
2
2
2
2
(c) Transmitter Antenna Gain
dBi
0
0
0
0
dBm
21
21
21
21
dBi dB
17.5 3
17.5 3
17.5 3
17.5 3
(g) Receiver Noise Figure (h) Receiver Noise Density (i) Receiver Interference Margin
dB dBm/Hz dB
5 -174 3.01
5 -174 3.01
5.6 -174 3.01
5.6 -174 3.01
(j) Total Effective Noise plus Interference Density=(g+h+i)
dBm/Hz
-165.99
-165.99
-165.4
-165.4
(k1) Information Rate (10log(Rb)) for Digital or (k2) NAMPS traffic channel 3dB bandwidth
dB
39.8
39.8
39.8
39.8
(l1) Required Eb/No for Digital systems or (i2) Required C/I for NAMPS
dB
7
7
7
7
dBm dB dB
-119.17 4 0
-119.17 4 0
-118.53 3 0
-118.53 3 0
(d) Transmitter EIRP per traffic channel (ab+c) (e) Receiver Antenna Gain (f) Receiver Cable and Connector Losses
(m) Receiver sensitivity (j+k+l) (n) Hand-off Gain (o) Explicit Diversity Gain
Comments
Antenna Gain is assumed as 2dBi, and it is included in item (a)
For the suburban and rural configurations, the cable loss is included in item (g), the receiver noise figure
=10log(1/(1-x/28)) where x is the number of channels per carrier (14 for 50% pole loading), 28 is the sectorized pole capacity derived by assuming 0.5 of voice activity factor, 7 dB Eb/No requirement and 0.85 of other sector to serving sector interference ratio.
41.6 for 13 kbps systems and 39.8 for 8 kbps systems
(p) Log-normal Fade Margin
dB
13.9
13.9
7.9
7.9
(p') Building/Vehicle Penetration Loss (q) Maximum Path loss {d-m+(e-f)+o+n-pp'}
dB dB
22 122.8
18 126.8
14.0 138.1
10.0 142.1
(r) Max Path Loss w/o penetration margin
dB
144.8
144.8
152.1
152.1
The fade margin is a composite number including margin for both outdoor (8 dB std. dev.) and indoor/in-vehicle fading (8/4 dB std. dev.). The values correspond to 90% edge coverage for dense urban and urban, and 75% edge coverage for suburban and rural.
