calculation & comparison of tdd ratio

Here is below I state the equation of BTS capacity of download and upload, remember below equation is not related to AAA package, it just only for BTS air interface (R1) DL/UL calculation.

# 10Mhz channel bandwidth per sector

# In DL have 30 sub channel & UL have 35 sub channel

# In DL 28 subcarrier where is 24 data subcarrier 4 pilot sub carrier & UL 24 subcarrier where is 16 data subcarrier 8 pilot subcarrier

# TDD frame is 5ms = 200frames/second

Consideration 1:

In respect to 68/31%, TDD symbol ratio 31:15

3 symbol each for control and sync, data symbol 28 for DL and 12 for UL

Data subcarrier in DL = 30 sub channel X 24 data sub carrier/sub channel X 28 data symbol = 20160 data sub carrier

Data sub carrier in UL = 35 sub channel X 16 data sub carrier/sub channel X 12 data symbol = 6720 data sub carrier

Modulation QPSK has 2 modulation order, 16QAM has 4 and 64QAM has 6 modulation order, so one sub carrier can carry 2 bits, 4 bits & 6 bits

There is code rate like 1/2, 2/3, 3/4, 5/6.

If QPSK ½ = 2X1/2 =1bit

16QAM ½ = 4X1/2 = 2bit

64QAM ½ = 6X1/2 = 3bit

Calculation only air interface Modulation is showing below:

Download:

QPSK ½ = 1 bit X 20160 data sub carrier X 200 frames/second = 3.84mbps

64QAM 2/3 = 4 bits X 20160 data sub carrier X 200 frames/second = 15.38mbps

Upload:



Consideration 2:

In respect to 62/38%, TDD symbol ratio 29:18

3 symbol each for control and sync, data symbol 26 for DL and 15 for UL

Data subcarrier in DL = 30 sub channel X 24 data sub carrier/sub channel X 26 data symbol = 18720 data sub carrier

Data sub carrier in UL = 35 sub channel X 16 data sub carrier/sub channel X 15 data symbol = 8400 data sub carrier

Modulation QPSK has 2 modulation order, 16QAM has 4 and 64QAM has 6 modulation order, so one sub carrier can carry 2 bits, 4 bits & 6 bits

There is code rate like 1/2, 2/3, 3/4, 5/6.

If QPSK ½ = 2X1/2 =1bit

16QAM ½ = 4X1/2 = 2bit

64QAM ½ = 6X1/2 = 3bit

Calculation only air interface Modulation is showing below:

Download:



Upload:



Also there is another TDD ration 56/44% = 26/21 symbol, it will make almost equal DL-UL, but in data arena people use download much more than upload.

And if we make this, DL/UL will hit saturation equally and quickly.

From above two calculation, you can easily find by TDD ratio we can control only air interface DL/UL BTS throughput capacity.

Now come to relation between AAA package & BTS throughput capacity on TDD ratio:

If we want to make a package DL:UL = 2:1, it can be made easily from AAA package creation. But it will create impact on BTS upload capacity very quickly.

In above two consideration any of them will hit upload capacity quickly cause difference is not too much ( 2nd consideration will also hit download capacity quickly).

Also we can add more sites to increase upload capacity, but it will also increase interference.

So, it is a tradeoff between capacity, interference, and subscriber, opex.

As per above we can make 2:1 DL/UL for all user or corporate user or some user easily from AAA package creation by exiting BTS thought configuration.

Or we can change the BTS configuration to increase upload capacity.

Finally, total network BW is can be calculated by average subscriber usage like in our network average usage 280kbps, but BTS capacity cannot be calculated in this phenomena.

Every sector have mix of usage, consider one sector has 31:15 TDD ratio with below two types package creation:

DL/UL = 4:1 in 31:15 TDD ratio:

3 – 1mbps, 6 – 512kbps, 15 – 256kbps & 30 – 128kbps = 13.5mbps total assign AAA package of download, so upload will be 3 – 256kbps, 6 – 128kbps, 15 – 64kbps, 30 – 32kbps = 3.34mbps

Already in above general case we touched pick capacity of sector throughput on both DL & UL and no user can’t make new session on this sector.

If 40% concurrent usage, DL = 5.4mbps & UL = 1.34mbps.

DL/UL = 2:1 in 31:15 TDD ratio:

3 – 1mbps, 6 – 512kbps, 15 – 256kbps & 30 – 128kbps = 13.5mbps total assign AAA package of download, so upload will be 3 – 512kbps, 6 – 256kbps, 15 – 128kbps, 30 – 64kbps = 6.75mbps

Already in above general case we touched pick capacity of sector throughput on DL, but UP cross it’s limit, so it is impossible to have those user in this sector, to serve this upload capacity we need two upload cells which was also not possible, we have to think DL & UL together, if we change the TDD ratio, it will impact on DL as well to much, which is much critical than upload.

If 40% concurrent usage, DL = 5.4mbps & UL = 2.7mbps.

Remember: concurrent user will increase day by day if user increase in network.

Above all consideration, BTS has capacity limit and it can play little bit on capacity distribution of DL & UL, but DL:UL = 4:1 or 2:1 whatever we can make from AAA, but remember that we have to distribute user in different cell to reduce UL capacity cause any of TDD ratio UL will touch first. So, if one sector have too much high package, do not sell high package on that sector again, rather sell package other area or cell. Also provide outdoor CPE to fix that in one standalone free cell for safety.

Decision up to Management, what to do, we are ready to deliver as per requirement.

calculation & comparison of tdd ratio

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