CDMA BTS Power Budget

CDMA BTS Power Budget Fundamentals

Michael WoodmanseeWireless Network Engineering: Core RFMarch 14, 2003

CDMA BTS Power Budget - 1

BTS Forward Power Budget


BTS Fwd Power Domains

• The BTS forward link has two power “domains”; – the digital domain and – the analog domain.

• These forward gains are proportional to voltage. • The digital domain refers to algorithms involving the digital

gains that control the output level of each Channel Element (CE).

• The analog domain refers to the total BTS output power from all active CEs in the sector. – The analog output power is controlled by the TRM TX analog

attenuator in the upconverter and is referenced to the DPM output antenna connection (PAM output if DPM is not present).


BTS Fwd Power Domain Diagram

Legacy: 2542 = 4000 mW = 36dBm (+ TPTLTargetPowerOffset/16)Metro Cell: 2542 = (0.2 * MaxPAMPwr * %TxPowerAvailable) +

TPTLTargetPowerOffset

Digital Domain (bits2, 1/16dB, 1/16dBm) Analog Domain

SyncGain = 4900 bits2

PagingGain = 21904 bits2

PilotGain = 30976 bits2

Current Traffic in bits2

Call Blocking = 258320 bits2

TxPowerMax/CarrierTotalTxPowerAvailable = 665 dBm*16

10(-M

inPilotToTotalPwrR

atio/160) = 208

Digital Reference=645320 bits2

HandoffB

lockingThreshold = 0 bits 2

ExcessForwardLinkC

apacity =(D

igital Reference -Traffic -O

verhead) bits 2

CallB

lockingThreshold =387000 bits 2

Handoff Blocking = 645320 bits2

Example for:Pilot/Sync/PagingGain = 176/70/148 (full-rate paging)MinPilotToTotalPwrRatio = -211CallBlockingThreshold = 387000HandoffBlockingThreshold = 0TxPowerMax/CarrierTotalTxPowerAvailable = 665TPTLTargetPowerOffset = 0MaxPAMPwr = 671

1500 mW = 31.8 dBm = 9.6%

12510 mW = 40.97dBm = 80%

41.56dBm = 14330 mW = 92%

31250 mW = 44.95dBm = 200%

1060 mW = 30.3dBm = 6.8%

240 mW = 23.8dBm = 1.5%

Total O’Head = 57780 bits2 2800 mW = 34.5dBm = 17.9%

Legacy: 2542 = 4000 mW = 36dBm (+ TPTLTargetPowerOffset/16)Metro Cell: 2542 = (0.2 * MaxPAMPwr * %TxPowerAvailable) +

TPTLTargetPowerOffset

Digital Domain (bits2, 1/16dB, 1/16dBm) Analog Domain

SyncGain = 4900 bits2

PagingGain = 21904 bits2

PilotGain = 30976 bits2

Current Traffic in bits2

Call Blocking = 258320 bits2

TxPowerMax/CarrierTotalTxPowerAvailable = 665 dBm*16

10(-M

inPilotToTotalPwrR

atio/160) = 208

Digital Reference=645320 bits2

HandoffB

lockingThreshold = 0 bits 2

ExcessForwardLinkC

apacity =(D

igital Reference -Traffic -O

verhead) bits 2

CallB

lockingThreshold =387000 bits 2

Handoff Blocking = 645320 bits2

Example for:Pilot/Sync/PagingGain = 176/70/148 (full-rate paging)MinPilotToTotalPwrRatio = -211CallBlockingThreshold = 387000HandoffBlockingThreshold = 0TxPowerMax/CarrierTotalTxPowerAvailable = 665TPTLTargetPowerOffset = 0MaxPAMPwr = 671

1500 mW = 31.8 dBm = 9.6%

12510 mW = 40.97dBm = 80%

41.56dBm = 14330 mW = 92%

31250 mW = 44.95dBm = 200%

1060 mW = 30.3dBm = 6.8%

240 mW = 23.8dBm = 1.5%

Total O’Head = 57780 bits2 2800 mW = 34.5dBm = 17.9%

MaxPAMPwr = 671 dBm*16 41.94dBm = 15620 mW = 100%


The Digital Domain (Fwd Power Control)• The output of the channel element is a digital gain, either fixed or controlled by

forward power control algorithm within defined limits. – The overhead channels are fixed values datafilled at the BTS on a per-sector basis. – The traffic channels vary within a defined range as required by forward link power

control. – The range for the traffic channel gains is datafilled relative to pilot power (IS-95 power

control in SBS). For example, with a pilot gain of 216, an upper limit of -1dB pilot and a lower limit of -15dB pilot, the selector will send digital gains in the range 192 to 38.

• Note that the pilot gain is datafilled per-sector at the BTS (PilotGain), and the traffic channel power control reference in digital gain is defined globally at the SBS (FwdPwrCtrlRefGain).

– It is the SBS FwdPwrCtrlRefGain value that is used to calculate the digital gains for the traffic channel using a dynamic range as defined by setting the offsets PTXupper and PTXlower (TxMaxGain and TxMinGain for IS-2000).

• For IS-2000 forward link power control, the mobile compares measured Pilot energy from total noise (Eb/Nt) with the current threshold that it maintains for given forward traffic channel. Based on the comparison of measured Eb/Nt with the current threshold, the mobile determines the power control bits to be sent to request the BTS to power up/down the forward traffic channel.

– The datafill for fast forward link power control is contained in the SBS.


The Digital Domain (Fwd Link Capacity)• The forward link Call and HandoffBlockingThresholds are datafilled in terms

of ExcessForwardLinkCapacity which is a “bits-squared” value. • ExcessForwardLinkCapacity is calculated as follows:

1. Square the pilot gain (e.g. 2162 is 46656)2. Divide by MinPilotToTotalPowerRatio (e.g. 46656 divided by -7.5dB (in linear terms,

10(-7.5/10)) is 262365). The result is called the “digital reference”.3. Sum up the bits-squared over all channel elements4. Subtract item 3 from item 2. The result is the ExcessForwardLinkCapacity.

• If the ExcessForwardLinkCapacity is less than the CallBlockingThreshold, then new calls are blocked with a reorder message sent to the mobile.

• If the ExcessForwardLinkCapacity is less than the HandoffBlockingThreshold, then handoffs are blocked. – Handoffs are blocked with the method of sending the Handoff Direction without the

new sector. – Once blocked, the mobile may attempt the handoff again by sending a new PSMM.

• Note that there is no action if the “digital reference” is crossed, although EFLC will never be reported as a negative number.


The Digital Domain (HW Power Limiting)• The Metro Cell contains two power limiting

mechanisms: – Software power limiting – Hardware (TX channelizer) power limiting

• The hardware power limiting is a much faster control loop that can be turned on or off via the attribute ChannelizerPowerLimitingEnabled.

• Channelizer Power limiting is triggered when the threshold, (TxPowerMax – average Tx gain), is exceeded.– TxPowerMax = MaxPAMPwr * PercentCarrierTxPower %


The Analog Domain (SW Power Limiting)• The software power limiting is a slower response control loop that is automatically active

when hardware power limiting is disabled. – Only available on SFRM. – HW power limiting can not be disabled on MFRM.– When hardware power limiting is enabled, software power limiting is automatically turned off.

• SW Power limiting is triggered when the threshold, TxPowerMax, is exceeded.– TxPowerMax = MaxPAMPwr * PercentCarrierTxPower %

• When power limiting is triggered, the user traffic gains do not meet the power control requirements.

– Forward link power control will act in opposition to the power limiting, possibly causing an unstable situation.

– This degrades the quality of service due to higher FER and increased likelihood of dropped calls. • For this reason, this algorithm should be viewed as an HPA protection mechanism only,

and the blocking thresholds should be set such that Power Limiting is a rare occurrence.– Triggering power limiting should be avoided, especially channelizer power limiting, to maintain

quality.– The BTS should be kept to less than 10% power limiting.


The Analog Domain (TPTL)

• The Transmit Power Tracking Loop (TPTL) adjusts the Tx digital gain to compensate for variations in Tx analog gain from temperature changes or component aging.

• TPTL maintains a constant calibration set point that relates the transmitted RF power to a corresponding digital power.

• TPTL maps the digital gains to the analog power transmitted over the forward link as follows: – 2542 = W mapping = (0.2 * 10^(MaxPAMPwr/160-3)) *

(PercentCarrierTxPower/1000) * 10^(TPTLTargetPowerOffset/160)– MaxPAMPwr is divided by 3 for MFRM.

• TPTLTargetPowerOffset (TPTLTPO) can be used to increase or decrease the forward link analog power in 1/16 dB steps.– TPTLTPO will scale all overhead and traffic channel gains, call blocking

threshold and handoff blocking threshold.


BTS Power Management Block Diagram

CE(pilot)

CE(traffic)

CE(traffic)

CE(sync)

CE(paging)

PilotGain

SyncGain

PagingGain

Traffic Gainfrom SBS


FwdPwrCtrlRefGainPTXupperPTXstartPTXlowerPrTXerror

Dup

TPTLAlgorithm

HPA

TxPwrFilterDecayConst

TPEFilterDecayExponential

Feed to:SectorTxPowerBTSPerformanceData

PowerLimitingAlogorithm

(WiltBloss)/BBWStepSizeWiltBlossStepPeriod(TxPowerMax)/CarrierTotalTxPowerAvailable

TxAttenNormal

Feed to calculations for:ExcessForwardLinkCapacityCall and Handoff BlockingBTSPerformanceData(CallBlockingThresholdHandoffBlockingThresholdMinPilotToTotalPwrRatio)

ForwardPowerEstimationEnabled

TPTLEnabledTPTLTargetPowerOffset

CE(pilot)

CE(traffic)

CE(traffic)

CE(sync)

CE(paging)

PilotGain

SyncGain

PagingGain



FwdPwrCtrlRefGainPTXupperPTXstartPTXlowerPrTXerror

Dup

TPTLAlgorithm

HPA

TxPwrFilterDecayConst

TPEFilterDecayExponential

Feed to:SectorTxPowerBTSPerformanceData

PowerLimitingAlogorithm

(WiltBloss)/BBWStepSizeWiltBlossStepPeriod(TxPowerMax)/CarrierTotalTxPowerAvailable

TxAttenNormal

Feed to calculations for:ExcessForwardLinkCapacityCall and Handoff BlockingBTSPerformanceData(CallBlockingThresholdHandoffBlockingThresholdMinPilotToTotalPwrRatio)

ForwardPowerEstimationEnabled

TPTLEnabledTPTLTargetPowerOffset


BTS Fwd Power Budget

• The BTS has only a finite amount of RF power available to provide CDMA service, and this available power needs to be used wisely to provide the maximum capacity possible throughout the network.

• The BTSfwdpowerbudget worksheet helps with managing this power.

• The following 3 examples show the effects a TPTLTPO values on the BTS forward power.


Datafill input boxes are green Metro Cell w/ MFRMSelect MFRM carriers (TxPowerDistribution 1 1 Carrier Use these settings if system is entirely Metro Cell only.Select FRM configuration, 11) 1 DPM - Outdoor 0.706 dB Adjust TotalTxPowerAvailable to suit HPA254 gain = 0.2 * MaxPAMPwr = 4.5 Watts NPMTPTLTargetPowerOffset 0 1/16dB IMF uninstalledResulting reference power 3.8 Watts 9)

4) 5)Power Power Sector- Percentag dB relative % of Tot Pwr % of

Key Calculation Datafill Bits square(Watts) (dBm) TxPower of pilot to pilot at Ant Port CallBlockingPilot a 176 30976 1.82 32.60 100.00 0.00 9.60 11.99Sync b 70 4900 0.29 24.59 15.82 -8.01 1.52 1.90Paging c c^2*(1-PRAT*0.5) 148 21904 1.29 31.09 70.71 -1.51 6.79 8.48PRAT (Full = 0, Half = 1) 0Total Overhead d a + b + c 57780 3.39 35.30 570 186.53 2.71 17.91 22.37Typical User Traff CH Pwr (8k=1, 13k=2, e (ADG^2)*VAF*1.15 3 2563 0.150 21.78 8.28 -10.82

RC3=3, RC4=4) Avg Gain (% of MaxGain 27% 72 <-- Avg Traffic GainQPCH Configuration IZP zones Carriers FBP

NumberOfQPCH 0 1 1 offQPCH_Rate 0=4800, 1=9600 0QPCH_PowerLevelPage 0 - 7 {-5dB to 2dB} 0 0.000 #NUM! 100000 100% 0.00 0.00

MinPilotToTotalPowerRatio f -211

BAND_CLASS datafill "0" or "1" 0

Digital Reference g a^2 / 10^(f/160) 645320 37.88 45.78 738 2083 13.19

CallBlockingThreshold h 387000 387000 22.72HandoffBlockingThreshold i 0 0 0.00

% CarrierTotalTxPowerAvailable

Pilot % of Call/HO block pwr

Calls will block at: j g - h 258320 15.16 41.81 674 80.1 11.99 80.07 100.00Handoffs will block at: k g - i 645320 37.88 45.78 738 200.0 4.80 200.02 249.81

Capacity StuffNumber of connections t (j - d)/e 78.2Sectors per user u 2.2Sector Capacity v t/u 35.6ExcessForwardLinkCapacity (no calls) w g-d 587540 Pilot % Pilot dB downPercentCarrierTxPower (1000, 1437, or 2817) 100% 1000MaxPAMPwr 768 63.10 48.00 768 2.88 -15.40 333.13 416.07CarrierTotalTxPowerAvailable x 20.44 43.10 690 8.90 -10.51 107.92 134.79Power output at antenna connector 18.94 42.77 684 9.60 -10.18 100.00 124.89FwdPwrCtrlRefGain y a * 1.122 197 38809 2.28 33.58

Key results in yellow

Additional Datafill

Pages/hr (System Wide)

% mobiles monitoring the QPCH

% of typical user traffic channel power

800 MHz

IMF uninstalled

RC1 Forward Power Control PTXlower -3584 39 0.0907 19.58PTXupper -512 156 1.43751 31.58PTXstart -1024 124 0.90701 29.58

RC2 Forward Power Control PTXlower -3072 49 0.14375 21.58PTXupper 0 197 2.2783 33.58PTXstart -512 156 1.43751 31.58

RC3 Forward Power Control TxMinGain -72 24 0.03611 15.58TxMaxGain -12 139 1.14185 30.58TxInitGain -28 87 0.45458 26.58


Digital Power PowerDatafill Gain (Watts) (dBm)

Example 1: TPTLTPO = 0

•Using recommended RF datafill.

•Total overhead is 17.9% of total power.

•New calls will block at 80% of total power.

•Low risk of BTS power limiting.


Datafill input boxes are green Metro Cell w/ MFRMSelect MFRM carriers (TxPowerDistribution 1 1 Carrier Use these settings if system is entirely Metro Cell only.Select FRM configuration, 11) 1 DPM - Outdoor 0.706 dB Adjust TotalTxPowerAvailable to suit HPA254 gain = 0.2 * MaxPAMPwr = 4.5 Watts NPMTPTLTargetPowerOffset 56 1/16dB IMF uninstalledResulting reference power 8.5 Watts 9)














Additional Datafill




800 MHz

IMF uninstalled






Example 2: TPTLTPO = 56


•Total overhead is 40% of total power.

•New calls will block at 179% of total power.

•Very high risk of BTS power limiting.

•Average traffic channel consumes twice as much power.

•Datafill changes required to prevent power limiting.

•Capacity will decrease.


Datafill input boxes are green Metro Cell w/ MFRMSelect MFRM carriers (TxPowerDistribution 1 1 Carrier Use these settings if system is entirely Metro Cell only.Select FRM configuration, 11) 1 DPM - Outdoor 0.706 dB Adjust TotalTxPowerAvailable to suit HPA254 gain = 0.2 * MaxPAMPwr = 4.5 Watts NPMTPTLTargetPowerOffset -24 1/16dB IMF uninstalledResulting reference power 2.7 Watts 9)














Additional Datafill




800 MHz

IMF uninstalled






Example 3: TPTLTPO = -24


•Total overhead is 12.7% of total power.

•New calls will block at 56.7% of total power.

•Very low risk of BTS power limiting.

•Risk reduced call performance if neighboring cells are transmitting at much higher power.


Overhead Channels


Overhead Channel Overview

• The overhead channels consist of a Pilot channel, a Sync channel, and a Paging channel (full or half rate).

• Each overhead channel consumes a Walsh code and transmits power proportional to their digital gain values, independent of each other.

• Changing the Pilot gain for a sector has no effect on the other individual overhead channels of that sector, however, the change will affect the call/handoff blocking levels and intercell interference.– The traffic channels upper, lower, and initial gains (or dynamic range)

are calculated relative to the global FwdPwrCtrlRefGain at the SBS.– The Paging and Sync channel powers can be set independent from

Pilot, but set proportional to pilot so that the mobile can properly demodulate those channels.

– The call blocking level is affected when the digital gain for the Pilot channel is adjusted.


Overhead Channels – Pilot

• The primary goals when datafilling the overhead channel gains is to balance the interaction between: – Increasing access reliability – Lowering interference – Increasing power available for traffic– Providing proper channel estimation

• The pilot defines the handoff borders and strongly influences the coverage area of the forward link. – The early Qualcomm simulations of CDMA systems showed that

a pilot, which is 17 to 20% of the total power available was the optimum amount for best performance of the system.

– However, these simulations assume that the base stations are uniformly loaded between 80 to 100% of full power.


Overhead Channels – Pilot (Cont’d)

• In practice, a wireless network is not uniformly loaded to 100%, for two major reasons:– Non-uniform traffic distributions mean that, even when some sectors are

overloaded, most sectors in the system carry well below maximum load.– A sector that is transmitting close to full power will block 100% of new

calls. Operators’ targets are closer to 1 or 2% blocking. Therefore, operators are unlikely to tolerate a sector that is on transmitting average powers in the 80 to 100% range.

• The evaluation of datafill settings in various markets have shown a pilot channel gain should be approximately 9 to 10%[4] of the HPA power to achieve good channel estimation.– Power ratios lower than this amount may be achievable, but they have

not yet been evaluated in a network. – When there is good channel estimation, reliable coherent detection of

the traffic channel by the mobile is possible.



• Sufficient power must be supplied to the pilot channel at all times so that the mobile may successfully, coherently demodulate the sync, paging, and traffic channels. – Typically, an optimized CDMA network has a best-server Ec/Io of

–12 dB or better throughout the cell coverage area. – The cell handoff boundary area is defined by the pilot Ec/Io.

• Reliable channel estimation as a result of the pilot channel is required for both idle and traffic modes. – In idle mode, the paging channel gain is proportional to the pilot

gain and shared between many users. • All these users react to the channel characteristics equally.



– In traffic mode, the traffic channel gain is referenced to the FwdPwrCtrlRefGain and is controlled for each individual user via the power control sub-channel (PTXupper, PTXlower, and PTXstart).

• FwdPwrCntlRefGain is the forward pilot transmit gain, which sets the forward link power control reference. It does not effect the amount of pilot power at the BTS.

• Each user assigned to a traffic channel will react to channel characteristics differently and will depend on the traffic demodulation and coding scheme used.

• The mobile needs enough traffic channel power to maintain at least 4 to 5 dB Eb/No receive power due to the demodulation and coding schemes defined in the CDMA standard[6]. – This statement is valid for an AWGN channel.– Testing has shown for traffic channels in a mobility environment that the

mobile needs 9 to 10 dB Eb/No.


Pilot vs. Total Output Power

• This figure provides an example of the relationship between pilot power, total power, and bit errors (channel estimation).

• The bit error rate can be correlated to other measurable performance metrics:– Forward traffic channel gain– Received pilot Ec/Io– Received FER on forward link

• As pilot Ec/Io goes up to the point of “perfect” channel estimation, then the forward traffic Eb/No required to maintain the FER setpoint goes down.

• The lowest point in the curve is the optimal setting of pilot for the network defined.

• The nature of the curve is subject to network design, mobility, and fading.

Pilot to Total Output Power Ratio

Bit

Erro

r Rat

e


Overhead Channels – Paging and Sync

• Reducing the PilotGain at the BTS does not automatically reduce any of the other overhead channels or the traffic channels. – The paging and sync gains must be explicitly reduced to

maintain the desired ratio. • The paging channel strength is dictated by the needs in the

core of the system where it must be strong enough to be decoded reliably. – When the overhead gains are lowered, the overall interference in

the system is lowered which goes some way to maintaining the paging signal to interference level.

• The sync channel is vital for the mobile to be able to acquire the timing necessary to arrive on the paging channel.


Overhead Channels – Paging and Sync (Cont’d)• If the sync gain is reduced to a percentage of power too low with

respect to the total power, then the mobile may not be able to acquire sync on the CDMA channel. – May revert to AMPS or endlessly search CDMA (depends on the mobile

manufacturer/model). • It is recommended that sync channel gain not be less than

approximately 1.9% of the call blocking power level. – Thus, the reduction of power in the overhead channels should be

focused primarily on the pilot and paging channels. – The sync channel uses 1.2 kbps, which allows for a much higher

processing gain than paging and traffic channels it can be transmitted at much lower power relative to the paging and traffic channels.

– Since the sync channel is transmitted at such a low power to begin, trying to “lower” the sync channel even further will not offer much benefit, and the added benefit is not worth the chance of the mobile not being able to acquire system timing.


Forward and Reverse Link Service


Fwd vs. Rvs Service Distance

• The Okumura-Hata propagation model is used to determine the service area of a CDMA cell.

• CDMA is reverse link limited, as the forward link service area is greater than the reverse link service area.[7]

– This means the reverse link will be the determining factor in defining the service area provided by a CDMA cell.

• Use the Nortel reverse link budget.


Fwd vs. Rvs Service Distance (Cont’d)

• The service distance of the forward link as defined by the pilot determines: – Channel estimation

• Nortel recommends for the dominant pilot an Ec/Io of –12 dB to achieve good channel estimation.

– Soft(er) handoff boundary• The mobile controls the soft(er) handoff boundary based on the received

Ec/Io.• Ec/Io as low as –14 dB can provide acceptable coherent detection when in

soft(er) handoff.

• The handoff boundary is proportional to the traffic service area given that the mobile achieves acceptable channel estimation of the traffic channel within the service distance.– The traffic channel gain is defined proportional to the pilot gain so that

the area serviced by traffic is within the handoff boundary and greater than the distance serviced by the reverse link.

– The maximum traffic channel power is recommended to be 1 dB below pilot (applies to RC1).


References

• [1] Core RF Recommended Datafill Spreadsheet – Nortel• [2] BTSfwdpowerbudget_NBSS10.x – Nortel• [3] RF Optimization and RF Performance Update – Nortel• [4] CDMA RF Datafill, Power Management, and Power Control – Nortel• [5] IS-2000 CDMA Air Interface Standard • [6] CDMA: Principles of Spread Spectrum Communication – Andrew J. Viterbi • [7] CDMA RF System Engineering – Samuel C. Yang• [8] IEEE (DS/CDMA Coherent Detection System with a Suppressed Pilot Channel) –

Sadayuki Abeta, et al• [9] IEEE (Channel Estimation Algorithms for DS/BPSK-CDMA Communication Systems) –

Minh-Hung Bui, et al• [10] IEEE (Optimal Power Allocation in CDMA Forward Link Using Dependency between

Pilot and Traffic Channels) – Seung Jong Park, et al• [11] IEEE (On the Capacity of a Cellular CDMA System) – Klein S. Gilhousen, et al• [12] IEEE (Overview of Cellular CDMA) – William C. Y. Lee

Documents

CDMA BTS Power Budget