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Thursday, August 2, 2012

Telecom LTE Interview Questions

What speed LTE offers?
Ans:- LTE provides downlink peak rates of at least 100Mbit/s, 50 Mbit/s in the uplink and RAN (Radio Access Network) round-trip times of less than 10 ms.

What is LTE Advanced?

Ans:-LTE standards are in matured state now with release 8 frozen. While LTE Advanced is still under works. Often the LTE standard is seen as 4G standard which is not true. 3.9G is more acceptable for LTE. So why it is not 4G? Answer is quite simple - LTE does not fulfill all requirements of ITU 4G definition.

Brief History of LTE Advanced: The ITU has introduced the term IMT Advanced to identify mobile systems whose capabilities go beyond those of IMT 2000. The IMT Advanced systems shall provide best-in-class performance attributes such as peak and sustained data rates and corresponding spectral efficiencies, capacity, latency, overall network complexity and quality-of-service management. The new capabilities of these IMT-Advanced systems are envisaged to handle a wide range of supported data rates with target peak data rates of up to approximately 100 Mbit/s for high mobility and up to approximately 1 Gbit/s for low mobility.

What is LTE architecture?

Ans:- Figure shows LTE architecture

What is EUTRAN?

The E-UTRAN (Evolved UTRAN) consists of eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by means of the S1-U.

What are LTE Interfaces?

The following are LTE Interfaces : (Ref: TS 23.401 v 841)

S1-MME :- Reference point for the control plane protocol between E-UTRAN and MME.
S1-U:- Reference point between E-UTRAN and Serving GW for the per bearer user plane tunnelling and inter eNodeB path switching during handover.
S3:- It enables user and bearer information exchange for inter 3GPP access network mobility in idle and/or active state.
S4:- It provides related control and mobility support between GPRS Core and the 3GPP Anchor function of Serving GW. In addition, if Direct Tunnel is not established, it provides the user plane tunnelling.
S5:- It provides user plane tunnelling and tunnel management between Serving GW and PDN GW. It is used for Serving GW relocation due to UE mobility and if the Serving GW needs to connect to a non-collocated PDN GW for the required PDN connectivity.
S6a:- It enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between MME and HSS.
Gx:- It provides transfer of (QoS) policy and charging rules from PCRF to Policy and Charging Enforcement Function (PCEF) in the PDN GW.
S8:- Inter-PLMN reference point providing user and control plane between the Serving GW in the VPLMN and the PDN GW in the HPLMN. S8 is the inter PLMN variant of S5.
S9:- It provides transfer of (QoS) policy and charging control information between the Home PCRF and the Visited PCRF in order to support local breakout function.
S10:- Reference point between MMEs for MME relocation and MME to MME information transfer.
S11:- Reference point between MME and Serving GW.
S12:- Reference point between UTRAN and Serving GW for user plane tunnelling when Direct Tunnel is established. It is based on the Iu-u/Gn-u reference point using the GTP-U protocol as defined between SGSN and UTRAN or respectively between SGSN and GGSN. Usage of S12 is an operator configuration option.
S13:- It enables UE identity check procedure between MME and EIR.
SGi:- It is the reference point between the PDN GW and the packet data network. Packet data network may be an operator external public or private packet data network or an intra operator packet data network, e.g. for provision of IMS services. This reference point corresponds to Gi for 3GPP accesses.
Rx:- The Rx reference point resides between the AF and the PCRF in the TS 23.203.
SBc:- Reference point between CBC and MME for warning message delivery and control functions.

What are LTE Network elements?

1. eNB

eNB interfaces with the UE and hosts the PHYsical (PHY), Medium Access
Control (MAC), Radio Link Control (RLC), and Packet Data Control
Protocol (PDCP) layers. It also hosts Radio Resource Control (RRC)
functionality corresponding to the control plane. It performs many
functions including radio resource management, admission control,
scheduling, enforcement of negotiated UL QoS, cell information
broadcast, ciphering/deciphering of user and control plane data, and
compression/decompression of DL/UL user plane packet headers.

Mobility Management Entity
manages and stores UE context (for idle state: UE/user identities, UE mobility state, user security parameters). It generates temporary identities and allocates them to UEs. It checks the authorization whether the UE may camp on the TA or on the PLMN. It also authenticates the user.

2. Serving Gateway

The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW).

3. Packet Data Network Gateway

The PDN GW provides connectivity to the UE to external packet data networks by being the point of exit and entry of traffic for the UE. A UE may have simultaneous connectivity with more than one PDN GW for accessing multiple PDNs. The PDN GW performs policy enforcement, packet filtering for each user, charging support, lawful Interception
and packet screening.

What are LTE protocols & specifications?
  • Air Interface Physical Layer
  • GPRS Tunnelling Protocol User Plane (GTP-U)
  • GTP-U Transport
  • Medium Access Control (MAC)
  • Non-Access-Stratum (NAS) Protocol
  • Packet Data Convergence Protocol (PDCP)
  • Radio Link Control (RLC)
  • Radio Resource Control (RRC)
  • S1 Application Protocol (S1AP)
  • S1 layer 1
  • S1 Signalling Transport
  • X2 Application Protocol (X2AP)
  • X2 layer 1
  • X2 Signalling Transport

What is CS Fallback in LTE?

LTE technology supports packet based services only, however 3GPP does specifies fallback for circuit switched services as well. To achieve this LTE architecture and network nodes require additional functionality, this blog is an attempt to provide overview for same.

In LTE architecture, the circuit switched (CS) fallback in EPS enables the provisioning of voice and traditional CS-domain services (e.g. CS UDI video/ SMS/ LCS/ USSD). To provide these services LTE reuses CS infrastructure when the UE is served by E UTRAN.

How does measurements work in LTE?

In LTE E-UTRAN measurements to be performed by a UE for mobility are classified as below
  • Intra-frequency E-UTRAN measurements
  • Inter-frequency E-UTRAN measurements
  • Inter-RAT measurements for UTRAN and GERAN
  • Inter-RAT measurements of CDMA2000 HRPD or 1xRTT frequencies

How does Intra E-UTRAN Handover is performed?

Intra E-UTRAN Handover is used to hand over a UE from a source eNodeB to a target eNodeB using X2 when the MME is unchanged. In the scenario described here Serving GW is also unchanged. The presence of IP connectivity between the Serving GW and the source eNodeB, as well as between the Serving GW and the target eNodeB is assumed.

The intra E-UTRAN HO in RRC_CONNECTED state is UE assisted NW controlled HO, with HO preparation signalling in E-UTRAN.

What is SON & how does it work in LTE?

Self-configuring, self-optimizing wireless networks is not a new concept but as the mobile networks are evolving towards 4G LTE networks, introduction of self configuring and self optimizing mechanisms is needed to minimize operational efforts. A self optimizing function would increase network performance and quality reacting to dynamic processes in the network.

This would minimize the life cycle cost of running a network by eliminating manual configuration of equipment at the time of deployment, right through to dynamically optimizing radio network performance during operation. Ultimately it will reduce the unit cost and retail price of wireless data services.

How does Network Sharing works in LTE?

3GPP network sharing architecture allows different core network operators to connect to a shared radio access network. The operators do not only share the radio network elements, but may also share the radio resources themselves.

What is carrier aggregation in LTE-Advanced?

To meet LTE-Advanced requirements, support of wider transmission bandwidths is required than the 20 MHz bandwidth specified in 3GPP Release 8/9. The preferred solution to this is carrier aggregation.

It is of the most distinct features of 4G LTE-Advanced. Carrier aggregation allows expansion of effective bandwidth delivered to a user terminal through concurrent utilization of radio resources across multiple carriers. Multiple component carriers are aggregated to form a larger overall transmission bandwidth.
How to calculate LTE peak capacity?      
     This is the maximum possible capacity which in reality can only be achieved in lab conditions. To understand the calculations below, one needs to be familiar with the technology (I will provide references at the end). But for now, let’s assume a 2×5 MHz LTE system. We first calculate the number of resource elements (RE) in a subframe (a subframe is 1 msec):

12 Subcarriers x 7 OFDMA Symbols x 25 Resource Blocks x 2 slots = 4,200 REs

Then we calculate the data rate assuming 64 QAM with no coding (64QAM is the highest modulation for downlink LTE):

6 bits per 64QAM symbol x 4,200 Res / 1 msec = 25.2 Mbps

The MIMO data rate is then 2 x 25.2 = 50.4 Mbps. We now have to subtract the overhead related to control signaling such as PDCCH and PBCH channels, reference & synchronization signals, and coding. These are estimated as follows:

1. PDCCH channel can take 1 to 3 symbols out of 14 in a subframe. Assuming that on average it is 2.5 symbols, the amount of overhead due to PDCCH becomes 2.5/14 = 17.86 %.

2. Downlink RS signal uses 4 symbols in every third subcarrier resulting in 16/336 = 4.76% overhead for 2×2 MIMO configuration
3. The other channels (PSS, SSS, PBCH, PCFICH, PHICH) added together amount to ~2.6% of overhead.

The total approximate overhead for the 5 MHz channel is 17.86% + 4.76% + 2.6% = 25.22%.

The peak data rate is then 0.75 x 50.4 Mbps = 37.8 Mbps.

Note that the uplink would have lower throughput because the modulation scheme for most device classes is 16QAM in SISO mode only.
There is another technique to calculate the peak capacity which I include here as well for a 2×20 MHz LTE system with 4×4 MIMO configuration and 64QAM code rate 1:

Downlink data rate:
  • Pilot overhead (4 Tx antennas) = 14.29%
  • Common channel overhead (adequate to serve 1 UE/subframe) = 10%
  • CP overhead = 6.66%
  • Guard band overhead = 10%
Downlink data rate = 4 x 6 bps/Hz x 20 MHz x (1-14.29%) x (1-10%) x (1-6.66%) x (1-10%) = 298 Mbps.

Uplink data rate:

1 Tx antenna (no MIMO), 64 QAM code rate 1 (Note that typical UEs can support only 16QAM)
  • Pilot overhead = 14.3%
  • Random access overhead = 0.625%
  • CP overhead = 6.66%
  • Guard band overhead = 10%
Uplink data rate = 1 * 6 bps/Hz x 20 MHz x (1-14.29%) x (1-0.625%) x (1-6.66%) x (1-10%) = 82 Mbps.

To conclude, the LTE capacity depends on the following:
  • Channel bandwidth
  • Network loading: number of subscribers in a cell which impacts the overhead
  • The configuration & capability of the system: whether it’s 2×2 MIMO, SISO, and the MCS scheme.


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