Testing Cellular Performance

Figure 1 — Power Profile of Power Saving Mode activities

The UE requests for a T3324 Active Timer value during Attach and TAU procedures. The MME allocates the T3324 value to the UE. The T3324 active timer determines the duration during which the device remains reachable for mobile terminated transaction on transition from connected to idle mode. The device starts the active timer when it moves from connected to idle mode and when the active timer expires, the device moves to Power Saving Mode. The MME takes the UE requested value and MME local configuration into account for determining the Active Timer value. The MME includes the T3324 value IE in the ATTACH ACCEPT/TAU ACCEPT message only if the T3324 value IE was included in the ATTACH REQUEST/TAU REQUEST message. A UE using PSM is available for mobile terminating services only for the period of an Active Time after a mobile originated event like data transfer or signalling, for example after a periodic TAU/RAU procedure. The MME allows a value of ‘0’ for the T3324 timer. In this case the UE enters the Power Saving Mode immediately. Table 1 summarizes the units (increment) for the Active Timer T3324. The value is 5 bits in length and must be multiplied by the increment to get its real value.

Table 1 — T3324 Active Timer Unit and Value

The T3412 timer is also referred to as the periodic Tracking Area Update (TAU) timer. Periodic tracking area updating is used to periodically notify the availability of the UE to the network. The procedure is controlled in the UE by the periodic tracking area update timer (timer T3412). The value of timer T3412 is sent by the network to the UE in the ATTACH ACCEPT message and can be sent in the TRACKING AREA UPDATE ACCEPT message. The UE shall apply this value in all tracking areas of the list of tracking areas assigned to the UE, until a new value is received. A longer periodic TAU timer is possible using T3412 extended timer.

When the UE includes the T3324 value IE and the UE indicates support for extended periodic timer value in the MS network feature support IE, it may also include the T3412 extended value IE. Apart from the value requested by the UE, the MME verifies the local configuration into account while selecting a value for the T3412 extended timer. When the MME includes the T3412 extended value IE in the ATTACH ACCEPT message or TRACKING AREA UPDATE ACCEPT message, the MME uses timer T3412 extended value IE as the value of timer T3412. Table 2 summarizes the units (increment) for the Extended Timer T3412. The value is 5 bits length and must be multiplied by the increment to get its real value.

Table 2 — Automation Test of Power Saving Mode

To measure the power profile of a device in normal and PSM modes, one can use a Digital Power Meter to measure current consumption. This digital power meter will basically power the device, measure the dropped current, and then compute the power consumption and draw the power profile. A typical setup would monitor the device from an external computer, the power meter and the PicoLTE network emulator is depicted in “Error! Reference source not found.” This setup is not part of the current release, but will be provided in future releases as a reference design.

In a procedure containing several steps, the operations are numbered (1, 2, 3…). The diamond icon (♦) is used to indicate a procedure containing only one step, or secondary steps. Lowercase letters (a, b, c…) can also be used to indicate secondary steps in a complex procedure.

The abbreviation NC is used to indicate no connection.

Capital letters (A,B,C…) are used to identify any term marked as is on an instrument, such as the names of connectors, buttons, indicator lights, etc. Capital letters are also used to identify key names of the computer keyboard.

All terms used in software, such as the names of menus, commands, dialog boxes, text boxes, and options, are presented in bold font style.

The abbreviation N/A is used to indicate something that is not applicable or not available at the time of press.

Note:

The screen captures in this document are taken from the software version available at the time of publication. For this reason, they may differ slightly from what appears on your screen, depending on the software version that you are using. Furthermore, the screen captures may differ from what appears on your screen if you use different appearance settings.

For demonstration purpose, the PicoLTE is running Cat.M1 cell at 1970 MHz (LTE band 20) as depicted in Figure 2. The configuration files are provided in the directory /opt/nutaq/picolte/examples/R1902/eps/ciot/emtc/enb.

The main configuration file is enb.cfg located in the “./enb” directory. It includes the following configuration files (see Table 3):

Table 3 — eNodeB’s configuration files

The eNodeB is connected to the EPC and supports different PDNs including the Non-IP PDN to handle Non-IP connectivity and NIDD both for MO and MT cases.

As the DL and UL central frequencies in LTE are related, one can use the DL EARFCN to setup the operating carrier frequencies both for the Uplink and Down Link. The DL EARFCN for standard LTE bands can be configured in the enb.cfg configuration file located in the folder ./enb. The parameter’s name is dl_earfcn (see Table 4).

Table 4 — Setting DL EARFCN

Figure 2 — Spectrum of an LTE 10MHz supporting Cat.M1 at 1960 MHz (LTE Band 2 – Downlink)

There are many web tools that can be used to find the suitable EARFCN depending on your setup. The following one proposes the computation of both carrier frequency and ARFCNs: http://niviuk.free.fr/lte_band.php.

The output Power can be configured in the rf_driver.cfg configuration file located in the folder ./enb. The parameter’s name is power_output (see Table 5).

Table 5 — Setting RF Output Power

The receiving gain can be configured in the rf_driver.cfg configuration file located in the folder ./enb. The parameter’s name is rx_fssssront_end_gain (see Table 6).

Table 6 — Setting Receiving Gain

The RF port can be configured in the rf_driver.cfg configuration file located in the folder ./enb. The parameters names are downlink_rf_port_x and upink_rf_port_x, where x is the cell index (see Table 7). The following table specifies the suitable RF ports for one cell scenario. In the case of multiple cells please refer to PicoLTE’s User Guide for more details about multiple cell configuration.

Table 7 — Setting RF ports

To tune any other parameter please read the description of this parameter in the configuration parameters User Guide and contact [email protected] for any help.

For demonstration purpose, the PicoLTE is running an EPC configuration allowing the management of Cat.M1 devices. The configuration files are provided in the/opt/nutaq/picolte/examples/R1902/eps/ciot/emtc/epc.

One can use one of those examples to test Power Saving Mode. The main configuration file is mme.cfg located in the “./epc” directory. It might include the UE Data Base if it is not already defined there using the JSON List of Objects: ue_db. Every object of this list defines a subscriber which is defined by the USIM information (see Table 8).

Table 8 — EPC’s configuration files

One or multiple PDNs can be defined in the mme.cfg configuration file using the pdn_list. It is a JSON list of one or more PDNs objects. The following table summarize the key parameters of a PDN object.

Table 9 — Public Data Network parameters

The EPC can handle:

• Non-IP PDN connection establishment
• MO Non-IP data
• MT Non-IP data
• Non-IP Events

The EPC provides an API that can be used by an external service to register to Non-IP events and forward Non-IP Data both on the Uplink and Downlink sides between the remote application server and the EPC. The EPC handles Non-IP traffic to and from the NB-IoT devices.

A Non-IP PDN must be configured to enable NonIP Data Delivery. We just need to set the pdn_type parameter of a PDN object to “non-ip”.

The UE Data base can be defined in the mme.cfg configuration file or it can be defined in a separate file and included in the mme.cfg configuration file using the directive include (include “ue_db-ims.cfg“,). The parameter ue_db is a JSON array of objects that must be used to configure the user database (see Table 10). Each element of this array is an entry for one user. The following properties are available:

Table 10 — Setting the UE Database

This section describes how to configure the Power Saving Mode feature on the MME. The MME provides a set of parameters listed in Table 11 to enable and set PSM Active and Extended Timers. Those parameters, if used, must be defined and set in the MME configuration file available in NB-IoT examples.

Table 11 — MME’s Power Saving Mode Parameters

The following test cases can be configured and tested with the PicoLTE and C030 device (see Table 12).

Table 12 — Power Saving Mode Test Cases

The Skyworks SKY66430 application board is an out-of-the-box rapid prototyping solution for IoT applications supporting CIoT such as NB-IoT and Cat.M1 modes.

The following figure shows a dashboard of the different terminals used to monitor the LTE eNB and MME,  the modem console as well as the UI capture the spectrum used by the PicoLTE cell and the Power profile of the modem.

This dashboard will be used along this document to capture the modem activities for the different test cases.

Figure 3 — Dashboard of terminals

The following figure shows the selection of Cat.M1 mode and band 2 (AT+SQNBANDSE=0,”standard”,”2”). When the device is turned on (AT+CFUN=1) it performs a cell search at band 2. The modem detects the PicoLTE cell and send an Attach Request (PRACH trace on then eNB CLI) to connect to the network. As the device is using a test SIM card registered in the HSS, the network accepts the attach request and registers the device. We can see in the MME CLI the device is attached to the PicoLTE on the PDN having the APN “internet”. The device gets assigned the IP address “192.168.3.1”

Figure 4 — Selection of Cat.M1 mode and band 2

The PicoLTE comes with a web-based GUI to help understanding what is going on the LTE network (eNB and MME) in a user-friendly manner. The web GUI uses web socket to capture the messages of the layers and interfaces enabled in configuration files.

In the following figure we can see the NAS messages on the MME. It shows the call flow between the device (UE) and the MME.

We can see all the procedure from the “Attach Request” up to the “Attach Accept” and “Attach complete”.

Figure 5 — NAS messages on the MME

The next figure shows the registered UEs (IMSI/IMEI) and their status.

Figure 6 — Registered UEs

To test the detach function, we use issue AT+CGATT=0 command to the modem and we can check its status on the MME CLI on the REG column which is reporting N in this case (the device is not registered).

Figure 7 — Checking status

The web GUI also shows the device’s status change from “true” to “false” which means that the device is no longer registered on the network.

Figure 8 — Displayed Status

The NAS messages show the detach procedure triggered by the device as illustrated in the following figure (“Detach Request” and “Detach Accept”). The following traces shows also the NAS and RRC messages of the detach procedure on the eNB side.

Figure 9 — NAS and RRC messages

To measure the Throughput on the DL/UL, we use “Iperf” and send UDP traffic to the device having the IP address “192.168.3.2”.

iperf -c 192.168.3.2 -u -b 1M -t 3000 -d -i 1

The eNB CLI reportes the measured Data rate on the DL and UL as shown in the columns brate (276 kbps on the Dl and 19.6 kbp on the UL).

Figure 10 — Data Rate on DL and UL

The PicoLTE’s Web GUI captures also the measured throughput by the eNB both on the DL and UL as illustrated in the following bitrate tab.

Figure 11 — Measured Throughput on Bitrate Tab

A UE is PSM enabled only when:

• It sends T3324 timer in ATTACH/TAU.
• Power Saving Mode is enabled in configuration by providing T3324 active and T3412 extended timers or by configuring “UE requested” timer values.
• T3412-Extended is expressed as a string of a binary number encoded on 8 bits: 3 bits for the unit, 5 bits for the value.
• T3324 encoding is expressed as a string of a binary number encoded on 8 bits: 3 bits for the unit, 5 bits for the value.

Activate PSM with AT+CPSMS=1,,,”10000110″,”00001000″

• T3324:16s, (8 x 2 seconds)
• T3412-Extended: 3min (6 x 30 seconds)
• Here T3412 is set to very small value to facilitate testing.

Please Note: In the PSM and eDRX tests the UE is registered on a PDN having the IP address “192.168.7.1”. The IP address of the device is “192.168.7.2”.

The following figure shows the PSM configuration on the modem console on the top right corner and the devices connection established on the PicoLTE network as shown in the eNB and MME CLIs as well as the current curve on the power monitor.

Figure 12 — PSM configuration and Device Connection

To check the PSM timers request by the devices we can display the NAS “Attach Request” message as depicted in the following figure. At the bottom right corner, we can see the T3324 and T3412 as well as their values requested by the devices.

Figure 13 — NAS “Attach Request” Message

At the bottom right corner, we can see the T3324 and T3412 as well as their values accepted by the network (MME) in the “Attach Accept” NAS message.

Figure 14 — “Attach Accept” NAS Message

The Power Profile of the following figure shows the inactivity of the device based on the requested T3324 and T3412 timers and confirms the expected behavior of the device.

Figure 15 — Power Profile

The following figures show the NAS Tracking Area Update messages (TAU request and TAU Accept) when the device wake up from deep sleep mode for many cycles. The measured power profile confirm the expected behavior of the device.

Figure 16, 17, 18, 19, 20, and 21 — NAS Tracking Area Update Messages

In the following figure, the device initiates a ping request (AT+PING=”www.sequans.com”) which immediately wake up the radio and send the traffic, even when the device is in the deep sleep window.

Figure 22, 23 and 24 — Ping Request

Activate eDRX with the parameters

• AT+SQNEDRX=2,4,”0101″,”0000″
  • eDRX cycle length is 81.92s
  • PTW is 1.28s

Enable CEREG URC

• AT+CEREG=2

Attach to the networks

• AT+CFUN=1

In the figures below, the modem console on the top right corner shows the setting of eDRX parameters and the connectivity trigger (AT+CFUN=1). The device is then registered on the network using the requested eDRX parameters as shown in the MME and eNB CLIs as well as the NAS messages (“Attach Request” and “Attach Accept”). The measured power profile depicts the device activity while using the requested Paging Time Window (PTW) and eDRX cycle.