# Carrier Aggregation¶

## Introduction¶

The srsRAN 4G software suite supports 2-carrier aggregation in both srsENB and srsUE. To experiment with carrier aggregation using srsRAN 4G over-the-air, you will need an RF device that can tune different frequencies in different channels, for example the USRP X300 series from Ettus Research (NI). We’ve tested with UHD 3.15 LTS and UHD 4.0.

Alternatively, experiment with carrier aggregation without SDR hardware using our ZeroMQ-based RF layer emulation. See our ZeroMQ Application Note for more information about RF layer emulation.

## Carrier Aggregation using SDR Hardware¶

### eNodeB Configuration¶

To configure the eNodeB for carrier aggregation, we must first configure the RF front-end. We must then configure srsENB for multiple cells and define the primary/secondary relationships between them.

If you’re using a real RF device such as the USRP X300 it’s advisable to use an external clock reference, either using the 10 MHz/1 PPS input (clock=external) or the GPSDO (clock=gpsdo). For the X300, especially for newer UHD versions, it’s also required to specific the sample rate upon radio initialization. For example, if you’re planning to use 10 MHz cells (50 PRB) the sample rate of the radio will be 11.52 Msamples/s, hence a sampling_rate=11.52e6 shall be used. For 20 MHz cells (100 PRB) the sample rate will be 23.04 Msamples/s, hence sampling_rate=23.04e6 shall be used.

[rf]
device_name = uhd
device_args = type=x300,clock=external,sampling_rate=23.04e6


The second step is to configure srsENB with two cells. For this, one needs to modify rr.conf:

cell_list =
(
{
rf_port = 0;
cell_id = 0x01;
tac = 0x0007;
pci = 1;
root_seq_idx = 204;
dl_earfcn = 2850;

// CA cells
scell_list = (
{cell_id = 0x02; cross_carrier_scheduling = false; scheduling_cell_id = 0x01; ul_allowed = true}
)
},
{
rf_port = 1;
cell_id = 0x02;
tac = 0x0007;
pci = 4;
root_seq_idx = 268;
dl_earfcn = 3050;

// CA cells
scell_list = (
{cell_id = 0x01; cross_carrier_scheduling = false; scheduling_cell_id = 0x02; ul_allowed = true}
)
}
)


With these changes, simply run srsENB as usual.

### UE Configuration¶

In the UE, we must again set the RF configuration and configure the UE capabilities.

For the RF configuration, we need to set the list of EARFCNs according to the cells configured in the eNodeB and set the number of carriers to 2:

[rf]
dl_earfcn = 2850,3050
nof_carriers = 2


Adding more EARFCNs in the list makes the UE scan these frequencies and the number of carriers makes the UE use more RF channels.

For the UE capabilities, we need to report at least release 10 and category 7:

[rrc]
ue_category        = 7
ue_category_dl     = 10
release            = 10


With these changes, simply run srsUE as usual.

## Carrier Aggregation using ZeroMQ RF emulation¶

To experiment with carrier aggregation using the ZeroMQ RF emulation instead of SDR hardware, we simply need to configure srsENB and srsUE to use the zmq RF device.

### eNodeB Configuration¶

For srsENB, configure the zmq RF device as follows:

[rf]
device_name = zmq
device_args = fail_on_disconnect=true,id=enb,tx_port0=tcp://*:2000,tx_port1=tcp://*:2002,rx_port0=tcp://localhost:2001,rx_port1=tcp://localhost:2003


### UE Configuration¶

For srsUE, configure the zmq RF device as follows:

[rf]
device_name = zmq
device_args = tx_port0=tcp://*:2001,tx_port1=tcp://*:2003,rx_port0=tcp://localhost:2000,rx_port1=tcp://localhost:2002,id=ue,tx_freq0=2510e6,tx_freq1=2530e6,rx_freq0=2630e6,rx_freq1=2650e6


Since the ZMQ module is frequency agnostic, it is important that Tx and Rx frequencies are set in ZMQ config. This makes internal carrier switching possible.

## Known issues¶

• The eNodeB ignores UE’s band capabilities

• CPU hungry and real time errors for more than 10 MHz