To be become a LTE Expert?: FAQ

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Wednesday, 8 October 2014

07:50 -

Why a new control channel in LTE rel 11?

One of the major enhancements in 3GPP Release 11 is the introduction of a new downlink control channel, the Enhanced Physical Downlink Control Channel (EPDCCH). The standardization of the E-PDCCH was necessary to support new features like CoMP, downlink MIMO and the considered introduction of a new carrier type with 3GPP Release 12 all with the intention to support the following goals:

Support of increased control channel capacity.
Support of frequency-domain ICIC.
Achieve improved spatial reuse of control channel resources.
Support beamforming and/or diversity.
Operate on a new carrier type and in MBSFN subframes.
Coexist on the same carrier as legacy Rel-8 and Rel-10 devices.

06:22 -

FAQ

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What is an antenna port and their mapping?

The LTE standard defines what are known as antenna ports. These antenna ports do not correspond to physical antennas, but rather are logical entities distinguished by their reference signal sequences. Multiple antenna port signals can be transmitted on a single transmit antenna (C-RS port 0 and UE-RS port 5, for example). Correspondingly, a single antenna port can be spread across multiple transmit antennas (UE-RS port 5, for example).The LTE standard defines what are known as antenna ports. These antenna ports do not correspond to physical antennas, but rather are logical entities distinguished by their reference signal sequences. Multiple antenna port signals can be transmitted on a single transmit antenna (C-RS port 0 and UE-RS port 5, for example). Correspondingly, a single antenna port can be spread across multiple transmit antennas (UE-RS port 5, for example).

The 3GPP TS 36.211 LTE standard defines antenna ports for the downlink. An antenna port is generally used as a generic term for signal transmission under identical channel conditions. For each LTE operating mode in the downlink direction for which an independent channel is assumed (e.g. SISO vs. MIMO), a separate logical antenna port is defined. LTE symbols that are transmitted via identical antenna ports are subject to the same channel conditions. In order to determine the characteristic channel for an antenna port, a UE must carry out a separate channel estimation for each antenna port. Separate reference signals (pilot signals) that are suitable for estimating the respective channel are defined in the LTE standard for each antenna port. FIG 1 shows the antenna ports defined in the LTE standard in Releases 8,9 and 10.

The way in which these logical antenna ports are assigned to the physical transmit antennas of a base station is up to the base station, and can vary between base stations of the same type (because of different operating conditions) and also between base stations from different manufacturers. The base station does not explicitly notify the UE of the mapping that has been carried out, rather the UE must take this into account automatically during demodulation (FIG 2). As far asThe way in which these logical antenna ports are assigned to the physical transmit antennas of a base station is up to the base station, and can vary between base stations of the same type (because of different operating conditions) and also between base stations from different manufacturers. The base station does not explicitly notify the UE of the mapping that has been carried out, rather the UE must take this into account automatically during demodulation (FIG 2).

Let us consider antenna ports used for PDSCH allocations since they probably have the most variations. Initially, the 89600 VSA's LTE demodulator supported only analysis of PDSCH transmitted on Antenna Ports 0, (0 and 1), (0, 1, 2), or (0, 1, 2, 3). These ports are considered C-RS antenna ports, and each port has a different arrangement of C-RS resource elements. Various configurations are defined that use these C-RS antenna ports, including 2- or 4-port Tx Diversity and 2-, 3-, or 4-port Spatial Multiplexing.

Then beamforming support was added and single-layer PDSCH allocations transmitted on Port 5 could be analyzed. The LTE demodulator has since been enhanced to support the LTE Release 9 which added Transmission Mode 8--Dual-Layer Beamforming (i.e. beamforming + spatial multiplexing)--where PDSCH is transmitted on Antenna Ports 7 and 8 (note that single-layer beamforming in Rel 9 can also use port 7 or port 8 in addition to port 5). In Rel 10 of the standard, the new transmission mode 9 (TM9) added up to 8-layer transmissions using Ports 7-14. TM9 is supported by the LTE-Advanced demodulator.

As Ports 0-3 are indicated by the existence of C-RS, so Ports 5 and 7-14 are indicated by the UE-specific Reference Signal (UE-RS). The following is a table that summarizes the various PDSCH mappings that can be used along with the corresponding reference signal and antenna ports.

In a MIMO or Tx Diversity configuration, each C-RS antenna port must be transmitted on a separate physical antenna to create spatial diversity between the paths. Single-layer beamforming, on the other hand, is accomplished by sending the same signal to each antenna but changing the phase of the each antenna's signal relative to the others. Since the same UE-RS sequence is sent from each antenna, the 89600 VSA can compare the received UE-RS sequence with the reference sequence and calculate the weights that were applied to the antennas to accomplish the beamforming.

Multi-layer beamforming adds some complexity to beamforming by transmitting as many UE-RS sequences as there are layers to allow demodulation of each layer's PDSCH data. The UE-RS sequence for each antenna port is orthogonal to the others, either in time/frequency domain or in the code domain. This can be thought of as beamforming of each layer independently. N-layer beamforming is an extension of dual-layer beamforming and supports up to 8 data layers with the ability to beamform each layer separately.

Friday, 3 October 2014

12:43 -

FAQ

4 comments

How Multiplexing and Logical Channel Prioritization happen in LTE?

The multiplexing and logical channel prioritization is left to the eNodeB implementation, for the uplink the process by which a UE creates a MAC PDU to transmit using the allocated radio resources is fully standardized; this is designed to ensure that the UE satisfies the QoS of each configured radio bearer in a way which is optimal and consistent between different UE implementations. Based on the uplink transmission resource grant message signalled on the PDCCH, the UE has to decide on the amount of data for each logical channel to be included in the newMAC PDU, and, if necessary, also to allocate space for a MAC Control Element.

The Logical Channel Prioritization procedure is applied when a new transmission is performed.

One simple way to meet this purpose is to serve radio bearers in order of their priority. Following this principle, the data from the logical channel of the highest priority is the first to be included into the MAC PDU, followed by data from the logical channel of the next highest priority, continuing until the MAC PDU size allocated by the eNodeB is completely filled or there is no more data to transmit.

RRC controls the scheduling of uplink data by signalling for each logical channel: priority where an increasing priority value indicates a lower priority level, prioritisedBitRate which sets the Prioritized Bit Rate (PBR), bucketSizeDuration which sets the Bucket Size Duration (BSD).

The UE shall maintain a variable Bj for each logical channel j. Bj shall be initialized to zero when the related logical channel is established, and incremented by the product PBR × TTI duration for each TTI, where PBR is Prioritized Bit Rate of logical channel j. However, the value of Bj can never exceed the bucket size and if the value of Bj is larger than the bucket size of logical channel j, it shall be set to the bucket size. The bucket size of a logical channel is equal to PBR × BSD, where PBR and BSD are configured by upper layers.

Although this kind of priority-based multiplexing is simple and favours the highest priorities, it sometimes leads to starvation of low-priority bearers. Starvation occurs when the logical channels of the lower priority cannot transmit any data because the data from higher priority logical channels always takes up all the allocated radio resources.

To avoid starvation, while still serving the logical channels according to their priorities, in LTE a Prioritized Bit Rate (PBR) is configured by the eNodeB for each logical channel. The PBR is the data rate provided to one logical channel before allocating any resource to a lower-priority logical channel.

In order to take into account both the PBR and the priority, each logical channel is served in decreasing order of priority, but the amount of data from each logical channel included into the MAC PDU is initially limited to the amount corresponding to the configured PBR. Only when all logical channels have been served up to their PBR, then if there is still room left in the MAC PDU each logical channel is served again in decreasing order of priority.

In this second round, each logical channel is served only if all logical channels of higher priority have no more data for transmission.

In most cases, a MAC Control Element has higher priority than any other logical channel because it controls the operation of a MAC entity. Thus, when a MAC PDU is composed and there is a MAC Control Element to send, the MAC Control Element is included first and the remaining space is used to include data from logical channels. One exception to this rule occurs when a UE transmits the first RRC message to a target cell during a handover procedure – in this case, a MAC Control Element such as a BSR has lower priority than SRBs. This is because it is more important to complete the handover procedure as soon as possible than to inform the eNodeB of the UE’s buffer status; otherwise, the data transfer interruption time would be longer and the probability of handover failure would increase due to the delayed signalling.

Example:

LTE MAC multiplexing

LTE MAC multiplexing by way of example. First, channel 1 is served up to its PBR, channel 2 up to its PBR and then channel 3 with as much data as is available (since in this example the amount of data available is less than would be permitted by the PBR configured for that channel). After that, the remaining space in the MAC PDU is filled with data from the channel 1 which is of the highest priority until there is no further room in theMAC PDU or there is no further data from channel 1. If there is still a room after serving the channel 1, channel 2 is served in a similar way.

Logical Channel Prioritization

The UE shall perform the following Logical Channel Prioritization procedure when a new transmission is performed:

- The UE shall allocate resources to the logical channels in the following steps:

- Step 1: All the logical channels with Bj > 0 are allocated resources in a decreasing priority order. If the PBR of a radio bearer is set to “infinity”, the UE shall allocate resources for all the data that is available for transmission on the radio bearer before meeting the PBR of the lower priority radio bearer(s);

- Step 2: the UE shall decrement Bj by the total size of MAC SDUs served to logical channel j in Step 1

NOTE: The value of Bj can be negative.

- Step 3: if any resources remain, all the logical channels are served in a strict decreasing priority order (regardless of the value of Bj) until either the data for that logical channel or the UL grant is exhausted, whichever comes first. Logical channels configured with equal priority should be served equally.

- The UE shall also follow the rules below during the scheduling procedures above:

- the UE should not segment an RLC SDU (or partially transmitted SDU or retransmitted RLC PDU) if the whole SDU (or partially transmitted SDU or retransmitted RLC PDU) fits into the remaining resources;

- if the UE segments an RLC SDU from the logical channel, it shall maximize the size of the segment to fill the grant as much as possible;

- UE should maximise the transmission of data.

The UE shall not transmit data for a logical channel corresponding to a radio bearer that is suspended (the conditions for when a radio bearer is considered suspended are defined in [8]).

For the Logical Channel Prioritization procedure, the UE shall take into account the following relative priority in decreasing order:

- MAC control element for C-RNTI or data from UL-CCCH;

- MAC control element for BSR, with exception of BSR included for padding;

- MAC control element for PHR;

- data from any Logical Channel, except data from UL-CCCH;

- MAC control element for BSR included for padding.

Query: The significance of Bucket size duration(BSD) in logical channel prioritization.
Ans: The BSD is basically used to set the maximum amount of pending data allowed for a logical channel, with respect to the prioritization checks. The more data a logical channel has, the higher its priority tends to be, but it can't exceed the value set by Prioritized Bit Rate x Bucket Size Duration. (e.g. 100 kbps x 1 second = 100 kbits). This keeps a logical channel experiencing a very high burst of data from taking over the transmission and blocking out a lower rate channel. See 36.321, section 5.4.3.1.

The bucket size duration indicates how much time for transmitting uplink data of a logical channel by using the prioritized bit rate until the bucket size is reached, value in milliseconds.

For a LCID the bucket size = BSD x PBR (both are in MAC config struct) that is the maximum UL data a LCID can buffer. The first time scheduling will happen based on Token bucket model for all LCID's with different priorities. Here every LCID can send PBR amount of data.

Each LCID accumulates BSDxPBR amount of data. The rate of accumulation is PBR * tti upto BSDxPBR.

There is another counter Bj in MAC that is used to make sure that there is no starvation of lower priority LCID's.

The second time on,

If Bj > 0 then higher priority LCID can send RLC data size and reduce Bj accordingly. Other LCID's may or may not be scheduled. If Bj is negative for higher priority LCID then other LCID with lower priority are scheduled.

lRRC IEs LogicalChannelConfig

- LogicalChannelConfig

l Priority (1 to 16)

l PrioritisedBitRate (0kbps to 2048kbps)

l BucketSizeDuration (ms50 to ms1000)

l LogicalChannelGroup (0 to 3)

Monday, 29 September 2014

23:22 -

FAQ

1 comment

How DL/UL stuff works in LTE?

DL Link Adaptation

1.UE reports CQI, PMI, RI in PUCCH (or PUSCH)
2.Scheduler at eNB dynamically allocated DL resources to the UE (PDCCH)
3.eNB sends user data in PDSCH
4.UE attempts to decode the received packet and sends ACK/NACK using PUCCH (or PUSCH)

UL Scheduling – w/o resource

1.If UE does not have UL-SCH resources, UE sends SR on PUCCH (In absence of PUCCH resources, UE must complete a RACH procedure to request UL-SCH resources.)
2.Scheduler at eNB allocates resources (PRBs and MCS to be used) to UE through “uplink grant” on PDCCH
3.UE sends user data on PUSCH
4.If eNB decodes the uplink data successfully, it sends ACK on PHICH

UL Scheduling – modifying resource

1.UE sends BSR (Buffer Status Report) & PHR (Power Headroom Report) to network on PUSCH
2.Scheduler at eNB dynamically adjusts resources assigned to UE - Grant on PDCCH is adjusted
3.Based on the adjusted grant, UE sends user data on PUSCH
4.If eNB decodes the uplink data successfully, it toggles NDI (New Data Indicator) on PDCCH, and sends ACK on PHICH

23:05 -

FAQ

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How Scheduler Design in LTE?

•Most scheduling strategies need information about:
–Channel condition
–Buffer status and priorities of the different data flows
–Interference situation in neighboring cells

Remember few major things while designing:
•Throughput
•Efficiency
•QoS Support
–Different types and levels of QoS, respective for different service applications
–Attributes such as bandwidth, delays, error rate and jitter
–Need to serve each subscriber at a certain minimum QoS based on his/her Service Level Agreement (SLA)
•Fairness
–Is a measure of customer satisfaction.
–Neglecting subscribers unfairly in order to increase throughput may lead to high churn rates

22:38 -

FAQ

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How is the UE getting information that it is scheduled?

By reading the PDCCH (this is valid for both UL scheduling grants and DL scheduling assignments).

PDCCH contains DCI(DL control information), which indicate 3 different messages:-

1. Uplink scheduling grants for PUSCH
2. Downlink scheduling assignment for PDSCH
3. TPC command for PUSCH and PUCCH

22:10 -

FAQ

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What is BSR?

The Buffer Status reporting procedure is used to provide the serving eNB with information about the amount of data available for transmission in the UL buffers of the UE.

At what scenario UE triggers BSR?

UL data, for a logical channel which belongs to a LCG, becomes available for transmission in the RLC entity or in the PDCP entity and either the data belongs to a logical channel with higher priority than the priorities of the logical channels which belong to any LCG and for which data is already available for transmission, or there is no data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR";
UL resources are allocated and number of padding bits is equal to or larger than the size of the Buffer Status Report MAC control element plus its subheader, in which case the BSR is referred below to as "Padding BSR"
retxBSR-Timer expires and the UE has data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR"
periodicBSR-Timer expires, in which case the BSR is referred below to as "Periodic BSR".

When different types of BSR are Triggered?

For Regular and Periodic BSR:

if more than one LCG has data available for transmission in the TTI where the BSR is transmitted

report Long BSR

else,

report Short BSR.

For Padding BSR:

if the number of padding bits is equal to or larger than the size of the Short BSR plus its subheader but smaller than the size of the Long BSR plus its subheader:

if more than one LCG has data available for transmission in the TTI where the BSR is transmitted: report Truncated BSR of the LCG with the highest priority logical channel with data available for transmission;

else

report Short BSR.

else if the number of padding bits is equal to or larger than the size of the Long BSR plus its subheader,

report Long BSR.

Saturday, 27 September 2014

06:05 -

FAQ

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UE Identifiers in LTE

The IMSI (International Mobile Subscriber Identity) and IMEI (International Mobile Equipment Identity) are permanent identifiers assigned to the USIM card and the Mobile Equipment, respectively. They are permanently associated with the subscriber and stored in a permanent provider database like the HSS (Home Subscriber Server) and will be used by other nodes in the network to identify the user. Similar to 2G and 3G technologies, for reasons of security, efficiency and practicality - the LTE network minimizes the exchange of these two identifiers with the UE.

1. IMSI - International Mobile Subscriber Identity:

The IMSI is a permanent identity assigned by the Service Provider
It is valid as long as the Service is Active with the Service Provider
It is stored on the USIM card and on the HSS (Home Subscriber Server)
It globally and uniquely identifies a user on any 3GPP PLMN (Public Land Mobile Network)

2. IMEI - International Mobile Equipment Identity

The IMEI is a permanent identity assigned by the Device Manufacturer
Valid as long as the Device is in Use
Stored on the Device hardware and on the HSS (Home Subscriber Server)

During the Initial Attach procedure between the UE and the LTE Network the UE is assigned three additional dynamic identifiers by different LTE Network nodes that have varying scopes of use.

The eNodeB (Evolved Node B) assigns the UE a C-RNTI (Cell Radio Network Temporary Identifier) to identify the UE during exchange of all information over the air. The C-RNTI is assigned during the setup of the RRC Connection (Idle Mode à Connected Mode transition) between a UE and an eNodeB and is valid only for that RRC Connection. Once the UE leaves the coverage area of an eNodeB the RRC Connection must be moved (Inter-eNodeB Handover) and the "new" eNodeB will assign a "new" C-RNTI to the UE. The C-RNTI is an E-UTRAN (Evolved Universal Terrestrial Radio Access Network) specific identifier and the EPC (Evolved Packet Core) Network has no visibility to it.

The MME (Mobility Management Entity) assigns the UE a GUTI (Globally Unique Temporary Identifier) to identify the UE during all message exchanges and procedures with the EPC. The GUTI is assigned during the Attach procedure (Deregistered State à Registered State transition) between the UE and the MME and is valid only as long as the UE is attached to the MME that assigned the GUTI. Once the UE leaves the Tracking Area(s) of an MME the "Attachment" has to be moved (Inter-MME handover) and the "new" MME will assign a "new" GUTI to the UE. Embedded within the GUTI are the PLMN ID of the service provider and the MME Identity. Thus, the GUTI uniquely and globally identifies a UE attached to a specific MME in a specific Service Providers LTE Network in a specific Country. The MME may choose to periodically re-assign a "fresh" GUTI to a UE that is attached to it.

The PGW (Packet Data Network Gateway) assigns the UE an IP address to facilitate data connectivity between the UE and any internal or external PDN (Packet Data Network). This could be an IPv4, IPv6 or Dual Stack IP address and the PGW could use a variety of IP address allocation schemes associated with the type of IP Address. The UE may set up PDN Connections with more than one PGW and may be assigned more than one IP address. The first IP address is assigned to the UE during the Initial Attach procedure and it stays with the UE as long as the UE is attached to the LTE Network. Unlike the other temporary identifiers the IP address is more "persistent" or "sticky" and does not change as long as the UE is attached - thus uninterrupted IP connectivity is provided to the UE. For all practical purposes, the UE is assigned an IP address when it powers on and loses its IP connectivity when it powers off. It is important to recognize that the eNodeB, MME and the SGW do not have any use for this UE IP address for connectivity purposes. It is used for IP forwarding decisions by the PGW and all nodes "north" (between the PGW and the PDN) of the PGW.

3. C-RNTI - Cell Radio Network Temporary Identity

Dynamic Identity assigned by the eNodeB
Valid as long as the UE is Connected to the eNodeB that assigned the C-RNTI
Stored in the UE and the eNodeB

4. GUTI - Globally Unique Temporary Identity

Dynamic Identity assigned by the MME (Mobility Management Entity)
Valid as long as the UE is Registered with the EPC (Evolved Packet Core) and Attached to the MME that assigned the GUTI
Stored on the UE and the MME

5. IP Address

Dynamic Identity assigned by the PGW
Valid as long as the UE is Registered with the EPC (Evolved Packet Core)
Stored in the UE and the PGW and any other node "north" of the PGW

05:52 -

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How Buffer Status Reports and Uplink Scheduling works in LTE?

The eNodeB is responsible for UL QoS management. In order to fulfill this responsibility eNB needs ongoing information from the UE. The UE needs a way to report to the eNB which radio bearers (RBs) need UL resources and how much resource they need. The eNB can then schedule the UE based on the QoS characteristic of the corresponding radio bearers and the reported buffer status.

If a UE is connected to a number of PDNs, say IMS, Internet and a VPN, it may have quite a few radio bearers configured in addition to the RRC signaling RBs. Keeping the eNB informed of the status of a large number of radio bearers will require considerable signaling overhead. Consequently the LTE standards include the concept of a Logical Channel Group (LCG). This signaling reduction mechanism allocates radio bearers to one of four groups. The mapping of a radio bearer (or logical channel) to a Logical Channel Group is done at radio bearer setup time by the eNB based on the corresponding QoS attributes of the radio bearers such as QoS Class Identifier (QCI).

The introduction of the LCG has an impact on the UE buffer status reports which still need to keep the eNB informed as much as possible. The UE reports an aggregate buffer status for the combination of radio bearers in a logical channel group. The eNB knows the radio bearers contained in the group and their priorities. Although the eNB may not have status on an individual radio bearer, provided that the QoS requirements of the bearers in an LCG are similar it can schedule the UE in a fair and appropriate fashion.

To help the eNB, the UE sends Buffer Status Reports (BSRs) for the LCGs. BSRs are triggered under the following conditions:

o New data arrives in previously empty buffers: Assuming we are at the “beginning” of UL data transmission when all data buffers are empty, if data becomes available for transmission in the UE for any radio bearer a BSR is triggered.
o Higher Priority data arrives: If the UE has already sent a BSR and is waiting for a grant but then higher priority data becomes available for transmission, the eNB needs to know this and therefore a new BSR is triggered. Note that this happens even when the triggering RB is in the same LCG for which there is an outstanding BSR.
o To update the eNB about the current status of buffers: If, for example, a UE is uploading a file, the data is arriving in the UE transmission buffer asynchronously with respect to the grants it receives from eNB. Consequently there is an ongoing need to keep the eNB updated as to the amount of data still to be transmitted. For this purpose the UE keeps a timer. When the periodicBSR-Timer expires, a BSR is triggered. The timer, configured by RRC, ranges from 5ms up to 2.56 seconds. It can be disabled by setting it to infinity, which is also the default.
o To provide BSR robustness: The LTE standard provides a mechanism to improve the robustness of buffer status reporting. We want to avoid deadlock situations which may occur when the UE sends a BSR but never receives a grant. A BSR retransmission mechanism is built into the UE implementation. The UE keeps a retxBSR-Timer which is started when a BSR is sent and stopped when a grant is received. If the timer expires, and the UE has still has data available for transmission, a new BSR is triggered. The retransmission timer, configured by RRC, ranges from 320ms up to 10.24 seconds. Unlike the periodic timer it cannot be disabled. The default is 2.56 seconds.

Relationship between BSR and Grant processing:

Interestingly, there is no direct relationship between the BSRs sent by the UE and how it processes a grant from eNB. Resource grants are allocated by the UE to radio bearers on a logical channel priority basis. Membership in a particular LCG is not relevant. For example, let’s say a UE requests resources for LCG 2 in order to send a HTTP request. Before the grant was received an RRC message becomes ready to send. Then when the grant is received the RRC message gets priority and uses up as much of the resource as it needs. The HTTP request will get the leftovers, if any. Note that RRC messages are sent on SRBs which are assigned to LCG 0 by default.

Padding BSR

o When a UE does not have enough data to completely fill a resource allocation from the eNB the unused space is referred to as “padding”. If this padding space is large enough to accommodate a BSR then the UE is expected to send a BSR, even when there is no pressing reason for doing so. Hence this type of BSR is called a “Padding BSR”. Depending on the amount of padding space available it could be a short or long BSR, and if short, the UE sends info related to the LCG containing the highest priority logical channel that has data available for transmission. The idea is that the eNB scheduler benefits from getting more up to date info.

o Note that if either a Regular or Periodic BSR is triggered it will be sent at the next opportunity along with data if there is data and there is room for both the data and the BSR. These BSRs have higher priority than the data. In contrast, a Padding BSR has lower priority than data so is only sent when there is available space and no more data.

o It is also possible that a Padding BSR and a Regular/Periodic BSR are triggered for the same TTI. In this case the longer of the two choices is sent.

05:43 -

FAQ

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How to Troubleshoot Downlink Throughput?

Step 1: Identify cell with low DL (downlink) throughput

a) The first thing is to identify those cells with low throughput. This threshold is defined by your network policies and practices (it also depends on your design parameters). Reports should be run for a significant number of days so that data is statistically valid.

Step 2: Identify Downlink interference

a) Cells with downlink interference are those whose CQI values are low (an exception to this rule is when most traffic is at the cell edge –bad cell location-). Analyze the CQI values reported by the UE for

Transmit Diversity
MIMO one layer
MIMO two layers

Typical values for transmit diversity oscillate between 7 and 8.

Typical values for MIMO one and two layers oscillate between 10 and 12.

b) If low CQI values are found after a CQI report is obtained, then downlink interference might be the cause of low throughput.

c) Common sources of interference in the 700 MHz band (LTE deployment in the USA) are: inter-modulation interference, cell jammers and wireless microphones

Step 3: BLER Values

a) Run a report for BLER in the cells identified. The BLER should be smaller or equal than 10%. If the value is larger, then, there is an indication of bad RF environment.

b) Typical causes of bad BLER are downlink interference, bad coverage (holes in the network, etc.)

Step 4: MIMO Parameters

a) Identify the transmission mode of your network. There are seven transmission modes defined.

b) Adjust the SINR thresholds for transition of transmission modes as recommended by the OEM. Request the Link Level simulations they used to set these thresholds and see if the conditions under which the values were calculated apply to your network. Otherwise, update them if the parameters are settable and not restricted.

Step 5: Low Demand

a) Run a report using the counters provided by the OEM to find

Maximum number of RRC connections supported per cell (parameter or feature)
Maximum number of RRC connections active per cell
Average number of RRC connections active per cell
Maximum number of users per TTI supported per cell (parameter or feature)
Maximum number of users scheduled per TTI in the cell(s) of interest
Average number users scheduled per TTI in the cell(s) of interest

b) If the maximum number of RRC connections active per cell is close or equal to the maximum number of RRC connections supported, then. The cause for low throughput is load.

c) A high number of scheduled users per TTI does not necessarily mean that demand is the cause for low throughput.

Step 6: Scheduler Type

a) Find the scheduler types your OEM supports

b) Select the one that is more convenient for the type of cell you are investigating. Examples of schedulers are: round robin, proportional fairness, maximum C/I, equal opportunity, etc. OEMs allow you to switch the scheduler in your network but recommend one in particular.

c) The wrong scheduler may be the reason for bad throughput.

Step 7: CQI reporting parameters

a) Check if your network is using periodic or aperiodic CQI reporting (or both).

b) Verify the frequency in which the CQI reporting is carried out for periodic reporting as well as the maximum number of users supported per second.

c) If the value is too small compared with the maximum number of RRC active connections, then, increase the values of the parameters CQIConfigIndex as well as RIConfigIndex (deal with in future blog).

d) If your network is not using aperiodic CQI reporting, then enable it.

e) Slow frequencies of CQI reporting might yield bad channel estimations that prevent the eNodeB from scheduling the right amount of data and Modulation and Coding Schemes to UE.

Step 7: Other

a) Run a VSWR report or ask your OEM to run it for you.

b) High values of VSWR result in low throughput due to losses.

c) Check your backhaul capacity. Often times, the backhaul links are shared among multiple RATs. Make sure your backhaul is properly dimensioned.

At the end of this methodology, you will be able to determine if the reasons for low throughput in your cells is one of the following or a combination, thereof:

- BLER (bad coverage)

- Downlink Interference (Bad CQI)

- MIMO Parameters

- Scheduling algorithm

- Low Demand

- CQI reporting frequency

- Other (VSWR, Backhaul capacity)

05:30 -

FAQ

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How to calculate peak data rate in LTE?

You may hear it many times that the peak data rate of LTE is about 300Mbps? How is the number calculated? What are the assumptions behind? Let's estimate it in a simple way. Assume 20 MHz channel bandwidth, normal CP, 4x4 MIMO.

First, calculate the number of resource elements (RE) in a subframe with 20 MHz channel bandwidth: 12 subcarriers x 7 OFDMA symbols x 100 resource blocks x 2 slots= 16800 REs per subframe. Each RE can carry a modulation symbol.
Second, assume 64 QAM modulation and no coding, one modulation symbol will carry 6 bits. The total bits in a subframe (1ms) over 20 MHz channel is 16800 modulation symbols x 6 bits / modulation symbol = 100800 bits. So the data rate is 100800 bits / 1 ms = 100.8 Mbps.
Third, with 4x4 MIMO, the peak data rate goes up to 100.8 Mbps x 4 = 403 Mbps.
Fourth, estimate about 25% overhead such as PDCCH, reference signal, sync signals, PBCH, and some coding. We get 403 Mbps x 0.75 = 302 Mbps.

Ok, it is done through estimation. Is there a way to calculate it more accurately? If this is what you look for, you need to check the 3GPP specs 36.213, table 7.1.7.1-1 and table 7.1.7.2.1-1. Table 7.1.7.1-1 shows the mapping between MCS (Modulation and Coding Scheme) index and TBS (Transport Block Size) index. Let's pick the highest MCS index 28 (64 QAM with the least coding), which is mapping to TBS index of 26. Table 7.1.7.2.1-1 shows the transport block size. It indicates the number of bits that can be transmitted in a subframe/TTI (Transmit Time Interval). For example, with 100 RBs and TBS index of 26, the TBS is 75376. Assume 4x4 MIMO, the peak data rate will be 75376 x 4 = 301.5 Mbps.