LTE信号调试与优化权威指南：协议详解与故障诊断

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"LTE Signaling, Troubleshooting, and Optimization是由Ralf Kreher和Karsten Gaenger合著的一本详尽的4G网络技术参考书籍，于2011年首次出版。本书专注于第四代移动通信（4G）中的长期演进（Long Term Evolution, LTE）网络的呼叫流程、信令、故障诊断和优化。它全面涵盖了从基本性能测量计数器、无线质量到用户平面质量，以及不同接口的标准、架构、目标和功能等核心概念。书中首先概述了LTE/演进分组核心网（Evolved Packet Core, EPC）的网络架构、参考点、协议栈、信息元素和基本操作过程。随后的内容深入探讨了更高级的主题，包括LTE/EPC信令分析和无线质量评估，旨在帮助读者理解正常和异常网络行为的信号流程，并识别和解决常见故障的原因。该书不仅补充了3GPP标准提供的信息，还提供了一个通用的LTE/EPC协议序列，确保读者对涉及的问题有清晰的理解。对于监控UMTS网络的人员来说，它解释了在监测时需要分析的信令程序和参数。作者通过真实LTE/EPC跟踪数据提供了第一手资料，使读者能够掌握信令程序的基础知识，甚至对测试和测量专家来说，提供了详细级别的分析指南。此外，本书还包括了对LTE空口监视场景的描述，以及其他高级主题，如优化策略和方法。作为UMTS信令和性能测量系列的后续之作，LTE Signaling, Troubleshooting, and Optimization是研究4G网络性能、调试和提升的理想参考书籍，特别适合那些需要深入了解这一领域的专业人士。版权信息遵循英国《版权、设计和专利法》1988年的规定，所有权利受到保护，未经许可，不得任何形式复制、存储或传输。"

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6 LTE Signaling, Troubleshooting and Optimization

Figure 1.7 IP payload transmission using Release 99 bearers with UE in CELL_DCH state.

Since these early days two key parameters have driven the evolution of packet services further

toward LTE: higher data rates and shorter latency. EGPRS (or EDGE) focused mostly on higher

bit rates, but did not include any latency requirements or algorithms to guarantee a deﬁned Quality

of Service (QoS) in early standardization releases. Meanwhi le, i n parallel to the development of

UMTS standards, important enhancements to EDGE have been deﬁned that allow pre-emption of

radio resources for packet services and control of QoS. Due to its easy integration in existing GSM

networks, EDGE is widely deployed today in cellular networks and is expected to coexist with LTE

on the long haul.

Nevertheless, the ﬁrst standard that promised complete control of QoS was UMTS Release 99. In

contrast to the TBFs of (E)GPRS, the user is assigned dedicated radio resources for PS data that are

permanently available through a radio connection. These resources are called bearers.

In Release 99, when a PDP (Packet Data Protocol) context is act ivated the UE is ordered by the RNC

(Radio Network Controller) to enter the Radio Resource Control (RRC) CELL_DCH state. Dedicated

resources are assigned by the Serving Radio Network Controller (SRNC): these are the dedicated

physical channels established on the radio interface. Those channels are used for transmission of both

IP payload and RRC signaling – see Figure 1.7. RRC signaling includes the exchange of Non-Access

Stratum (NAS) messages between the UE and SGSN.

The spreading factor of the radio bearer (as the combination of several physical transport resources

on the Air and Iub interfaces is called) depends on the expected UL/DL IP t hroughput. The expected

data transfer rate can be found in the RANAP (Radio Access Network Application Part) part of

the Radio Access Bearer (RAB) assignment request message that i s used to establish the Iu bearer,

a GPRS Tunneling Protocol (GTP) tunnel for transmission of a IP payload on the IuPS interface

between SRNC and SGSN. While the spreading factor controls the bandwidth of the radio connection,

a sophisticated power control algorithm guarantees the necessary quality of the radio transmission.

For instance, this power control ensures that th e number of retransmitted frames does not exceed a

certain critical threshold.

Activation of PDP context results also in the establishment of another GTP tunnel on the Gn

interface between SGSN and GGSN. In contrast to IuPS, where tunnel management is a task of

RANAP, on the Gn interface – as in (E)GPRS – the GPRS Tunneling Protocol – Control (GTP-C) is

responsible for context (or tunnel) activation, modiﬁcation, and deletion.

Standards, Protocols, and Functions 7

Figure 1.8 IP data transfer using HSDPA.

However, in Release 99 the maximum possible bit rate is still limited to 384 kbps for a single

connection and, more dramatically, the number of users per cell that can be served by this highest

possible bit rate is very limited (only four simultaneous 384 kbps connections per cell are possible on

the DL due to the shortness of DL spreading codes).

To increase the maximum possible bit rate per cell as well as f or the individual user, HSPA was

deﬁned in Releases 5 and 6 of 3GPP.

In High-Speed Downlink Packet Access (HSDPA) the High-Speed Downlink Shared Channel (HS-

DSCH) which bundles several High-Speed Physical Downlink Shared Channels (HS-PDSCHs) is used

by several UEs simultaneously – that is why it is called a shared channel .

A single UE using HSDPA works in the RRC CELL_DCH state. For DL payload transport the

HSDSCH is used, that is, mapped onto the HS-PDSCH. The UL IP payload is still transferred using a

dedicated physical data channel (and appropriate Iub transport bearer); in addition, the RRC signaling

is exchanged between the UE and RNC using the dedicated channels – see Figure 1.8.

All these channels have to be set up and (re)conﬁgured during the call. In all these cases both

parties of the radio connection, cell and UE, have to be informed about the required changes. While

communication between NodeB (cell) and CRNC (Controlling Radio NetworkController) uses NBAP

(Node B Application Part), the connection between the UE and SRNC (physically the same RNC unit,

but different protocol entity) uses the RRC protocol.

The b ig advantage of using a shared channel is higher efﬁciency in the usage of available radio

resources. There is no limitation due to the availability of codes and the individual data rate assigned

to a UE can be adjusted quicker to the real needs. The only limitation is the availability of processing

resources (represented by channel card elements) and buffer memory in the base station.

In 3G networks the beneﬁts of an Uplink Shared Channel ( UL-SCH) have not yet b een introduced

due to the need for UL power control, that is, a basic constraint of Wideband CDMA (WCDMA)

8 LTE Signaling, Troubleshooting and Optimization

networks. Hence, the UL channel used for High-Speed Uplink Packet Access (HSUPA) is an Enhanced

Dedicated Channel (E-DCH). The UL transmission data volume that can be transmitted by the UE on

the UL is controlled by the network using so-called “grants” to prevent buffer overﬂow in the base

station and RNC. The same “grant” mechanism will be found in LTE.

All in all, with HSPA in the UTRAN the data rates on the UL and DL have been signiﬁcantly

increased, but packet latency is still a critical factor. It takes quite a long time until the RRC con-

nection in the ﬁrst step and the radio bearer i n the second step are established. Then, due to limited

buffer memory and channel card resources in NodeB, an often quite progressive settings of user inac-

tivity timers leads to transport channel-type switching and RRC state change procedures that can be

summarized as intra-cell hard handovers. Hard handovers are characterized by the fact that the active

radio connection including the radio bearer is interrupted for a few hundred milliseconds. Similar

interruptions of the data transmission stream are observed during serving HSDPA cell change proce-

dures (often triggered by a previous soft handover) due to ﬂushing of buffered data in NodeB and

rescheduling of data to be transmitted by the RNC. That such interruptions (occurring in dense city

center areas with a periodicity of 10– 20 seconds) are a major threat for delay-sensitive services is

self-explanatory.

Hence, from the user plane QoS perspect ive the two major targets of LTE are:

• a further increase in the available bandwidth and maximum data rate per cell as well as for the

individual subscriber;

• reducing the delays and interruptions in user data transfer to a minimum.

These are the reasons why LTE has an alw ays-on concept in which the radio bearer is set up imme-

diately when a subscriber is attached to the network. And all radio resources provided to subscribers

by the E-UTRAN are shared resources, as shown in Figure 1.9. Here it is illustrated that the IP payload

as well as RRC and NAS signaling are transmitted on the radio interfaces using unidirectional shared

channels, the UL-SCH and the Downlink Shared Channel (DL-SCH). The payload part of this radio

connection is called the radio bearer. The radio bearer is the bidirectional point-to-point connection

for the user plane between the UE and eNodeB (eNB). The RAB is the user plane connection between

Figure 1.9 Packet data transfer in E-UTRAN/EPC.

Standards, Protocols, and Functions 9

the UE and the Serving Gateway (S-GW) and the S5 bearer is the user plane connection between the

S-GW and public data network gateway (PDN-GW).

Note that a more detailed explanation of the LTE/EPC bearer concept is given in Section 1.6 of

this chapter.

The end-to-end connection between the UE and PDN-GW, that is, the gateway to the IP world

outside the operator’s network, is called a PDN connection in the E-UTRAN standard documents and

a session in the core network standards. Regardless, the main characteristic of this PDN connection

is that the IP payload is transparently tunneled through the core and the radio access network.

To control the tunnels and radio resources a set of control p lane connections runs in parallel with

the payload transport. On the radio interface RRC and NAS signaling messages are transmitted using

the same shared channels and t he same RLC transport layer that is used to transport the IP payload.

RRC signaling terminates in the eNB ( different from 3G UTRAN where RRC was transparently

routed by NodeB to the RNC). The NAS signaling information is – as in 3G UTRAN – simply

forwarded to the Mobility Management Entity (MME) and/or UE by the eNB.

For registration and authentication the MME exchanges signaling messages with the central main

subscriber databases of the networ k, the Home Subscriber Server (HSS).

To open, close, and modify the GTP/IP tunnel between the eNB and S-GW, the MME exchanges

GTP signaling messages with the S-GW and the S-GW has the same kind of signaling connection

with the PDN-GW to establish, release, and maintain the GTP/IP tunnel called the S5 bearer.

Between the MME and eNB, together with the E-RAB, a UE context is established to store

connection-relevant parameters like the context information for ciphering and integrity protection.

This UE context can be stored in multiple eNBs, all of them belonging to the list of registered

tracking areas for a single subscriber. Using this tracking area list and UE contexts, the inter-eNB

handover delay can be reduced to a minimum.

The two most basic LTE standard documents are 3GPP 23.401 “GPRS Enhancements for E-UTRAN

Access” and 3GPP 36.300 “Overall Description Evolved Universal Terrestrial Radio Access (E-UTRA)

and E-UTRAN.” These two specs explain in a comprehensive way the major improvements in LTE

that are pushed by an increasing demand for higher bandwidth and shorter l atency of PS user plane

services. The basic network functions and signaling procedures are explained as well as the network

architecture, interfaces, and protocol stacks.

Although this book will not become simply a copy of what is already described in the standard

documents, it i s necessary to give a summary of the facts and parameters that are required to understand

the signaling procedures and key performance indicators of the network and services. Additional

explanations will be given to highlight facts that cannot be found in the specs.

1.2 LTE Radio Access Network Architecture

The E-UTRAN comes with a simple architecture that is illustrated in Figure 1.10. The base stations of

the network are called eNodeB and each eNB is connected to one or multiple MMEs. These MMEs in

turn are connected to a S-GW that may also be co-located (comprising the same physical hardware)

with the MME. The interface between the eNB and MME is the called the S1 in terface. In case the

MME and S-GW are not found in the same physical entity, the S1 control plane interface (S1-MME)

will connect the eNB and MME while the S1 user plane interface (here S1-U) will connect the eNB

with the S-GW.

In case one eNB is connected to multiple MMEs, these MMEs form a so-called MME pool and the

appropriate network functionality is called S1 ﬂex. The initial signaling procedure used to connect an

eNB with a MME is the S1 setup procedure of the S1 Application Part (S1AP).

The X2 interface is used to connect eNBs with each other. The main purpose of this connectivity is

intra-E-UTRAN handover. In the real world it will not be possible for all eNBs of the network to be

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