telecom study notes

Adaptive Modulation and Coding: modulation and code rates are changed adaptively, depending on channel conditions (typically, based on SNR reported by UEs)

• Automatic Repeat reQuest (ARQ): Errors are detected and requests for retransmission of erroneous data are sent
• Forward Error Correction (FEC): Data are encoded using an error-correcting code (ECC) prior to transmission. The additional information (redundancy) added by the code is used by the receiver to recover the original data at the expense of lower data rate (e.g., 1/2, 2/3, 5/6, etc).
• Hybrid automatic repeat-request (HARQ): ARQ and FEC are combined; packets with errors that cannot be corrected but only detected are retransmitted, otherwise only error correction is applied.

MIMO:

BF is used for low SNR

Spatial Multiplexing is used for high SNR

Open loop MIMO: no feedback from receiver, good for high speed of mobile device

Closed loop MIMO: feedback from receiver, can be used for specific UE. robust for low speed of mobile device

Fading:

• Slow fading (shadow): caused by major obstructions within the propagation environment (coherence time is larger than the delay of the channel)
• Fast fading (multipath; small-scale): small movements of a mobile or obstacle and media changes. (coherence time is smaller than the delay)

(FDD and TDD) for up link and down link

for multiple users (and CSMA collision detection)

compared with FDM, OFDM saves bandwidth

cyclic prefix: a repetition of data used to fix interference

for LTE: 

UE: mobile terminal (with sim card) and terminal equipment

• MT: radio interface and wireless network access and data transmission in general. Therefore, it supports the following features:

1. Transmission and reception of data and signalling over the radio interface.
2. Authentication and registration to the UMTS network.
3. Management (including creation, de-activation and modification) of PDP (Packet Data Protocol) contexts on request from a TE.
4. Session control.
5. Support of radio mobility functions, such as the handover.

• TE: the part the end-user has access to, as it supports all the functions related to user applications and interfaces. It contains the following features:

1. Control of application-related hardware functions, such as speaker, microphones, video cameras, displays, etc.
2. Support of user applications and services, such as email client, Web-browsing client, instant-messaging client, etc.
3. Support of application-related protocol and session-management functions (for IMS-based applications, this includes protocol stacks like SIP, SDP and RTP).