Tutorial 8: LTE Architecture and Operations

E‑UTRAN (Radio Access Network, comprising eNodeBs) and EPC (Evolved Packet Core, comprising MME, SGW, PGW, etc.).

The MME handles mobility management, authentication, paging, and bearer management (control‑plane functions).

S1‑U is the user‑plane interface between eNodeB and SGW; S1‑MME is the control‑plane interface between eNodeB and MME.

PDCP → RLC → MAC → PHY (from higher to lower).

Transparent (TM): no segmentation/ARQ; Unacknowledged (UM): segmentation but no ARQ; Acknowledged (AM): segmentation and ARQ for reliable delivery.

DL: OFDMA (Orthogonal Frequency Division Multiple Access). UL: SC‑FDMA (Single‑Carrier Frequency Division Multiple Access).

Hybrid ARQ combines error correction (FEC) with ARQ retransmissions to improve reliability and efficiency, especially in error‑prone wireless channels.

RRC_IDLE and RRC_CONNECTED.

A TA is a group of cells used to manage location of idle UEs. When a UE moves across TA boundaries, it performs a Tracking Area Update (TAU).

X2‑based handover uses the direct X2 interface between eNodeBs, avoiding core network involvement. S1‑based handover goes through the MME and involves more core network signalling; used when X2 is not available.

PDCP provides header compression (ROHC), ciphering (encryption), integrity protection, and in‑order delivery of packets.

UE and HSS share a secret key K. HSS generates authentication vectors (RAND, AUTN, XRES, KASME) and sends them to MME. MME sends RAND and AUTN to UE; UE verifies AUTN and sends RES; MME verifies RES against XRES to authenticate.

AES (Advanced Encryption Standard) with 128‑bit keys.

QCI is a scalar value (1‑9) that defines a specific QoS profile, including priority, packet delay budget, and acceptable packet loss rate.

GBR (Guaranteed Bit Rate) bearers provide a guaranteed minimum bit rate, used for real‑time services like VoIP. Non‑GBR bearers are best‑effort and share resources.

Carrier aggregation combines multiple component carriers to increase the total bandwidth (up to 100 MHz) and boost data rates.

Less than 10 ms (for the radio access network). The end‑to‑end latency is typically around 30‑50 ms.

The PGW (PDN Gateway) allocates the IP address to the UE as part of the bearer establishment.

The SGW routes and forwards user data packets, acts as a mobility anchor for intra‑LTE handovers, and buffers downlink data for idle UEs.

HSS (Home Subscriber Server) is the central database containing subscriber profiles, authentication credentials, and service subscription information.

RRC is the radio interface protocol that establishes the connection between UE and eNodeB. NAS signalling is between UE and MME (e.g., attach, TAU, authentication) and is carried over RRC in the air interface.

RACH is used for initial access, uplink synchronisation, and scheduling requests when the UE has no dedicated resources.

LTE supports inter‑RAT handover via S1 and the MME, which coordinates with the target network's core (e.g., 3G SGSN). The UE performs measurements and reports, and the network triggers the handover.

Up to 5 component carriers (in Release 10), each up to 20 MHz, totaling 100 MHz. Later releases support more.

The PCRF (Policy and Charging Rules Function) defines QoS and charging policies based on subscriber profiles and service requirements.

1. UE performs Random Access (RACH) to obtain synchronisation and a temporary C‑RNTI. 2. UE sends RRC Connection Request (with establishment cause). 3. eNodeB responds with RRC Connection Setup. 4. UE sends RRC Connection Setup Complete (including NAS Service Request). 5. NAS Service Request triggers MME to authenticate the UE (if needed) and set up bearers. 6. MME sends Initial Context Setup Request to eNodeB (with bearer QoS). 7. eNodeB establishes radio bearers (DRBs). 8. Data can now flow.

Data rate = (RB × subcarriers × symbols × bits/symbol × layers × (1/code rate?) Actually, bits per RE = 6 (64‑QAM) × 5/6 = 5 effective bits. Number of REs per subframe = 100 × 12 × 7 × 2 = 16800. Effective bits per subframe = 16800 × 5 × 2 (MIMO) = 168,000 bits. With 1000 subframes/sec (1 ms each), rate = 168 Mbps. With 10% overhead, about 151 Mbps. Standard says 300 Mbps with 4×4 MIMO.

X2 interface connects eNodeBs for direct signalling, enabling fast handover. During X2 handover, the source eNodeB sends a Handover Request to the target eNodeB including UE context, QoS, and security info. Target eNodeB performs admission control and replies with an ACK. Then the source eNodeB forwards user data (buffered) to target. The UE then performs random access to target, and the target signals completion. A Path Switch Request is sent to MME to update SGW.

UE sends a Bearer Resource Allocation Request (NAS) to MME. MME interacts with PGW/PCRF to request a dedicated bearer. MME sends E‑RAB Setup Request to eNodeB (with QCI 1, GBR). eNodeB sets up a radio bearer (DRB) with appropriate QoS. The eNodeB replies with E‑RAB Setup Response. The bearer is established, and VoIP traffic can flow.

1. UE sends Attach Request (with IMSI or GUTI) to MME. 2. MME sends Authentication Data Request to HSS. 3. HSS derives authentication vectors (RAND, AUTN, XRES, KASME) and sends to MME. 4. MME sends Authentication Request (RAND, AUTN) to UE. 5. UE verifies AUTN, computes RES, and sends Authentication Response (RES). 6. MME verifies RES against XRES. 7. MME derives KASME from K using KDF. 8. MME sends Security Mode Command (with ciphering/integrity algorithms) to eNodeB, which forwards to UE. 9. UE derives CK and IK and responds with Security Mode Complete. 10. Now encryption and integrity are active for subsequent messages.

RLC UM (Unacknowledged Mode) provides segmentation and reassembly but no ARQ. It is used for delay‑sensitive services (e.g., VoIP) where retransmissions would add too much delay. RLC AM (Acknowledged Mode) provides ARQ with retransmissions for reliable delivery; used for data services (e.g., TCP) that require low error rates.

Carrier aggregation combines multiple component carriers (CCs) to increase the total bandwidth. Intra‑band aggregation uses carriers within the same band (contiguous or non‑contiguous). Inter‑band aggregation uses carriers from different bands. The UE can receive/transmit on multiple CCs simultaneously, increasing peak data rates.

In RRC_IDLE, UE performs cell reselection based on measurements and informs the network of its location via Tracking Area Updates (TAUs) when crossing TA boundaries. For incoming data/voice, the MME sends a paging message to all eNodeBs within the UE's current TA; each eNodeB broadcasts a Paging message over the air. The UE responds with a Service Request to initiate connection setup.

During handover, PDCP ensures in‑order delivery of packets. It can perform retransmission (if RLC AM) and reordering after handover. It also handles ciphering and integrity; the keys are updated (via key derivation) during handover.

Downlink scheduling: eNodeB allocates resource blocks (RBs) in the downlink to UEs based on channel quality (CQI), buffer status, and QoS. Uses dynamic scheduling with grants sent on PDCCH. Uplink scheduling: eNodeB grants uplink resources (UL grant) to UEs based on SR (Scheduling Request) and Buffer Status Reports (BSR). The UE transmits on allocated RBs using SC‑FDMA.

S1‑U is the user‑plane interface between eNodeB and SGW, carrying GTP‑U tunnels for user data. S1‑MME is the control‑plane interface between eNodeB and MME, carrying S1‑AP messages for mobility and bearer management. They are separate to allow independent scaling and security.

1. UE attaches to LTE and establishes a default EPS bearer. 2. UE performs IMS registration (SIP REGISTER) via the PGW to IMS core. 3. For a call, UE sends SIP INVITE to IMS. 4. IMS triggers PCRF to request a dedicated bearer for voice (QCI 1). 5. PCRF pushes policy to PGW, which initiates a dedicated bearer (via MME/eNodeB). 6. A radio bearer (DRB) is set up with GBR and low latency. 7. Call proceeds with RTP/RTCP streams.

Rate = (N_RB × 12 × 14 × bits_per_subcarrier × coding_rate × layers × (1 - overhead) × 1000) / 10^6 Mbps. With N_RB=100, 64‑QAM (6 bits), coding 5/6, 2 layers, overhead 10%, rate ≈ 100 × 12 × 14 × 6 × 0.833 × 2 × 0.9 × 1000 / 1e6 = 100 × 12 × 14 = 16800 REs; 16800 × 6 × 0.833 = 84000 bits per subframe; × 2 = 168000; × 0.9 = 151200 bits/ms = 151.2 Mbps. With carrier aggregation (5 carriers) → 756 Mbps.

LTE FDD uses separate frequency bands for DL and UL, with 10 ms frames (10 subframes). Each subframe is 1 ms. TDD uses the same frequency but splits time between DL and UL, with configurable subframe allocations (e.g., 1:3, 2:2). TDD allows more flexible spectrum use but introduces latency due to switching and may require guard periods. FDD provides consistent latency.

S1‑AP is the control‑plane protocol between eNodeB and MME. It handles procedures: Initial UE Message (attach), Initial Context Setup (bearer establishment), Handover Preparation (S1 handover), UE Context Release, Paging, and Tracking Area Update. It uses non‑transparent signalling.

HARQ in LTE uses synchronous UL HARQ (with 8 processes). The RTT is 8 ms (for FDD). Each retransmission adds 8 ms latency. For low‑latency services, this can be significant. Techniques like TTI shortening (in 5G) address this. Also, HARQ with incremental redundancy improves efficiency but adds complexity.

1. UE sends Attach Request (with IMSI/GUTI) + RRC Connection Setup. 2. eNodeB forwards Attach Request (S1‑AP Initial UE Message) to MME. 3. MME performs authentication with HSS (AKA). 4. MME sends Update Location Request to HSS; HSS replies with subscription data. 5. MME sends Create Session Request to SGW/PGW to establish default bearer. 6. PGW sends Create Session Response with IP address. 7. MME sends Initial Context Setup Request to eNodeB (with bearer QoS). 8. eNodeB sets up radio bearers and sends RRC Connection Reconfiguration to UE. 9. UE responds. 10. eNodeB sends Initial Context Setup Response to MME. 11. MME sends Attach Accept to UE (NAS). 12. Attach Complete.

EPS bearer is a logical connection with QoS attributes, similar to PDP context but more streamlined. In 3G, PDP context is packet‑switched; in LTE, the EPS bearer is the core QoS entity. Advantages: simpler (flat architecture), better QoS granularity with QCI, and integration with policy control (PCRF).

SON automates network planning, configuration, optimization, and healing. It includes: self‑configuration (e.g., Automatic Neighbour Relation – ANR), self‑optimization (e.g., mobility robustness optimization, load balancing), and self‑healing (automatic detection and compensation for failures). This reduces manual intervention and operational costs.

CoMP coordinates transmission/reception among multiple cells (eNodeBs). Joint transmission (JT) sends data from multiple cells to a UE, increasing the received signal. Joint reception (JR) combines signals from multiple cells for UL. This improves SINR, especially for edge users. Challenges: high backhaul requirements, tight synchronisation, and increased signalling overhead.

Key hierarchy: K (permanent) → KASME (master key derived after AKA) → KeNB (used for RRC/UP ciphering). During handover, the target eNodeB derives a new KeNB* from the current KeNB using a key derivation function (KDF) with the target cell ID and frequency. This provides forward secrecy.

LTE (EPC) uses fixed functional entities and point‑to‑point interfaces, which are less flexible. 5GC uses service‑based architecture (SBA) with network functions (e.g., AMF, SMF) communicating via HTTP/2, enabling modularity, cloud‑native deployment, network slicing, and easier evolution. 5GC is more scalable and supports edge computing, whereas LTE is more monolithic.

LTE introduced MIMO with 2×2 (Release 8). Release 10 added 8×8 DL MIMO and 4×4 UL MIMO. Release 12 added 3D‑MIMO (FD‑MIMO) with active antenna systems. MU‑MIMO (Release 11) allows multiple UEs on the same time‑frequency resources. These improvements increased spectral efficiency and peak rates.

Use 1800 MHz or 2.6 GHz. Cell radius ~500 m (urban). Use 3‑sector sites. Link budget: Tx power 40 dBm, antenna gain 15 dBi, receiver sensitivity -100 dBm, path loss with Okumura‑Hata: ~125 dB at 500m. Fade margin 10 dB. Capacity: assume 50 Mbps per sector, 3 sectors = 150 Mbps per site. With 500 users per km², each generating 0.05 Erlangs, total traffic ≈ 25 Erlangs/km². Number of sites determined by required capacity.