1. UL/DL Considerations of BDA fed Active DAS
Steve Blum
Senior Solutions Engineer
Jan, 2014

2. Donor Site Noise Rise Avoid Donor site de-sense from DAS/BDA Noise
Signal and Noise
Noise Power – Path Loss
Donor Antenna
DAS Remote
DAS Antenna A 1
BDA
DAS Interface

3. Generic DAS Remote converts and amplifies RF Transport medium
Head end with RF interface to signal source and conversion for transport

4. DAS UL Noise Figure
Noise Figure is the degradation in Signal to Noise from the input to output
Passive DAS 16 dB Loss
-90 signal
-113 noise
SNR = 23
-106 signal
-113 noise
SNR = 7
-90 signal
-113 noise
SNR 23
Active DAS with 16 dB NF
-90 signal
-97 noise
SNR 7

5. Noise Power
DAS UL Noise Power = -174 dBm (thermal noise floor, 50 ohms, 1kHz) + 10log(Bandwidth of RX) + NF + Gain
For CDMA = NF + Gain
Coax adds to NF but subtracts from gain, net 0 impact to noise power

6. Combined Noise Power
Noise Figure = Base noise figure + 10log (number of remotes)
NF = log(4) or 6 = 16 dB NF
Gain = 10
Noise Power = = -87 dBm

7. Mitigating DAS Noise UL Attenuation will reduce Noise
Small Impact to NF/SNR
-90 Signal
-113 Noise
S/N = 23
Stages NF
Gain
Cascade Calculations
DAS
16.0
10.0
16.32
10.00
UL Atten
15.0
-15.0
-5.00
Passive
0.0
BDA
Coax
Antenna
Stage 8
Noise
-80/-87 S/N = 7
-95/-101.7
S/N = 6.7
-15 dB
NF 16, Gain 10

8. Noise Power ERP
Noise Power ERP = DAS Noise - Coax + BDA Noise - Coax + Antenna Gain
Noise Power ERP
= -28 dBm
BDA
DAS Remote A 1
DAS Interface
Antenna gain
2 Coax
BDA Gain
Cascaded Calculation
Stages NF
Gain
Cascade Calculations
DAS
16.0
10.0
16.00
10.00
UL Atten
0.0
Passive
1.0
-1.0
16.03
9.00
BDA
5.0
60.0
69.00
Coax
-5.0
64.00
Antenna
0.5
Stage 8
Noise
-27.97
The Noise Figure is pretty much set by the DAS so the only way to reduce Noise Power is to reduce the gain. But where we do that is important and we can analysis this with the cascaded calculation
1 Coax
Noise Power

9. Reducing UL Noise
Cascaded Calculator to evaluated effects of UL attenuation
Goal is to reduce attenuation with minimal impact to NF
15 dB UL attenuation on DAS
15 dB UL attenuation on BDA

10. Optimizing UL Attenuation
Splitting Attenuation reduces UL overdrive concerns with minimal degradation to NF

11. DAS Remote Gain Adjust Adjusting Remote UL Gain impacts NF
Normal, 10 dB Atten
Adjusting Remote UL Gain impacts NF
Low, 1 dB Gain, 13 dB NF
Normal, 11 dB Gain, 10 dB NF
High, 21 dB Gain, 7 dB NF
+6 to NF for 4 remotes
Low, no UL Atten
High, 20 dB Atten
Keep in mind that UL IMD can become a concern with higher UL gain at the remote

12. Balancing Remotes
High Gain on 1 remote will dominate the noise floor

13. DL Gain Considerations
BDA
DAS Remote A 1
DAS Interface
DAS Remote adds gain to distribute RF
DAS Net Gain = DAS Remote Composite Pout – BDA Composite Pout
DAS HE attenuates DL for transport

14. Balancing DL Gain DAS Remote gain stays the same
BDA
DAS Remote A 1
DAS Interface
DAS Remote gain stays the same
BDA DL gain reduced = lower Pout
Net effect = No gain change
With lower Pin, DAS HE reduces attenuation

15. Balancing DL Gain? Net DL Gain = BDA Gain + BDA Pout – DAS Remote Pout
BDA = 60 dB Gain and 30 Pout
Remote = 33
Net gain = 63
Net UL Gain
Remove donor coax and antenna
Lower power DAS
BDA = 60 dB Gain and 30 Pout
Remote = 20
Net gain = 50
Either adjust the UL gain/attenuation to match DL or reduce DAS remote Pout

16. BDA – DAS Oscillation When Gain is higher than isolation
Donor Antenna
Isolation
Attenuate and/or isolate the specific antenna
DAS Remote
DAS Antenna A 1
BDA
At -5 isolation, the system will oscillate activating AGC and causing alarms. At 15, generally everything will be fine but 20 allows for no problems
DAS Interface

17. Mixed Power Remotes
DAS common head end with a mix of remote options

18. Summary UL Noise from a BDA fed DAS may de-sense the donor base site
Noise is determined by the cascaded NF + gain
Find best place to adjust gain while minimizing NF increase
Increasing the DAS remote gain can reduce NF but may create IMD concerns
Adjusting BDA DL gain to match UL gain adjustment will not balance the gain
Adjusting DL net gain requires reducing remote Pout
Preventing oscillation will require attenuating/isolating the offending antenna (DL/UL)
DAS with mixed remotes may require independent head end components