LTE Simulator Documentation: 理解与使用指南

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"LTE-SIM-doc 是一份关于LTE（长期演进）技术的详细说明文档，由Centre Tecnològic de Telecomunicacions de Catalunya (CTTC)于2012年9月发布，版本为ns-3.15。文档主要包含设计文档、用户文档、测试文档和性能分析文档四大部分，旨在全面介绍LTE模拟器的设计、使用、测试以及性能优化。” 在LTE技术中，SIM-doc 提供了以下关键知识点： 1. **总体架构(Overall Architecture)**：文档首先概述了LTE系统的整体架构，这包括了eNodeB（基站）、UE（用户设备）、EPC（演进分组核心网）等主要组件及其相互间的通信流程。 2. **LTE模型(LTE Model)**：这部分详细介绍了用于模拟的LTE模型，涵盖了物理层（PHY）和数据链路层（MAC）的关键功能，如OFDM（正交频分复用）调制、调度算法、信道编码和解码等。 3. **EPC模型(EPC Model)**：EPC模型涉及P-GW（分组数据网关）、S-GW（服务网关）、MME（移动管理实体）等网络节点的功能，以及它们在用户会话管理和移动性管理中的作用。 4. **协议元素的详细描述(Detailed description of protocol elements)**：这部分深入解释了LTE协议栈中的各个协议元素，如RRC（无线资源控制）、NAS（非接入层）等，以及它们在连接建立、资源分配、数据传输过程中的作用。 5. **信道和传播(Channel and Propagation)**：讨论了不同类型的无线信道，如下行链路与上行链路，以及多径传播、衰落模型等对信号传输的影响。 6. **辅助工具(Helpers)**：提供了辅助工具的介绍，这些工具可能用于配置模拟环境、数据处理或结果分析。 7. **用户文档(User Documentation)**：用户文档包含了使用LTE模拟器的基础知识，如背景信息、使用概览、基本模拟程序的运行、模型参数配置、模拟输出解析、衰落轨迹的使用、建筑物移动模型、物理层错误模型、MIMO（多输入多输出）模型、天线模型的应用、无线电环境地图、自适应调制和编码（AMC）模型及CQI（信道质量指示）计算、演进分组核心网的实现，以及示例程序和参考场景。 8. **测试文档(Testing Documentation)**：测试文档提供了测试套件的概述，用于验证模拟器的正确性和性能。 9. **性能分析文档(Profiling Documentation)**：这部分关注模拟器的性能分析，包括目标设定、框架描述以及实际测试结果。 10. **其他**：文档还包含了相关的参考文献和资料，为深入研究提供基础。这份LTE-SIM-doc是理解、开发和测试LTE系统的重要参考资料，无论是对研究人员、工程师还是学生，都能从中获取丰富的知识和实践指导。

LTE Simulator Documentation, Release ns-3.15

Resource Allocation Model

We now brieﬂy describe how resource allocation is handled in LTE, clarifying how it is implemented in the simula-

tor. The scheduler is in charge of generating speciﬁc structures calles Data Control Indication (DCI) which are then

transmitted by the PHY of the eNB to the connected UEs, in order to inform them of the resource allocation on a

per subframe basis. In doing this in the downlink direction, the scheduler has to ﬁll some speciﬁc ﬁelds of the DCI

structure with all the information, such as: the Modulation and Coding Scheme (MCS) to be used, the MAC Transport

Block (TB) size, and the allocation bitmap which identiﬁes which RBs will contain the data transmitted by the eNB to

each user.

For the mapping of resources to physical RBs, we adopt a localized mapping approach (see [Sesia2009], Section

9.2.2.1); hence in a given subframe each RB is always allocated to the same user in both slots. The allocation bitmap

can be coded in different formats; in this implementation, we considered the Allocation Type 0 deﬁned in [TS36213],

according to which the RBs are grouped in Resource Block Groups (RBG) of different size determined as a function

of the Transmission Bandwidth Conﬁguration in use.

For certain bandwidth values not all the RBs are usable, since the group size is not a common divisor of the group.

This is for instance the case when the bandwidth is equal to 25 RBs, which results in a RBG size of 2 RBs, and

therefore 1 RB will result not addressable. In uplink the format of the DCIs is different, since only adjacent RBs can

be used because of the SC-FDMA modulation. As a consequence, all RBs can be allocated by the eNB regardless of

the bandwidth conﬁguration.

Adaptive Modulation and Coding

The simulator provides two Adaptive Modulation and Coding (AMC) models: one based on the GSoC model

[Piro2011] and one based on the physical error model (described in the following sections).

The former model is a modiﬁed version of the model described in [Piro2011], which in turn is inspired from [Seo2004].

Our version is described in the following. Let i denote the generic user, and let γ

be its SINR. We get the spectral

efﬁciency η

of user i using the following equations:

BER = 0.00005

Γ =

−ln (5 ∗ BER)

1.5

= log



1 +



The procedure described in [R1-081483] is used to get the corresponding MCS scheme. The spectral efﬁciency is

quantized based on the channel quality indicator (CQI), rounding to the lowest value, and is mapped to the corre-

sponding MCS scheme.

Finally, we note that there are some discrepancies between the MCS index in [R1-081483] and that indicated by the

standard: [TS36213] Table 7.1.7.1-1 says that the MCS index goes from 0 to 31, and 0 appears to be a valid MCS

10 Chapter 1. Design Documentation

LTE Simulator Documentation, Release ns-3.15

scheme (TB size is not 0) but in [R1-081483] the ﬁrst useful MCS index is 1. Hence to get the value as intended by

the standard we need to subtract 1 from the index reported in [R1-081483].

The alternative model is based on the physical error model developed for this simulator and explained in the following

subsections. This scheme is able to adapt the MCS selection to the actual PHY layer performance according to

the speciﬁc CQI report. According to their deﬁnition, a CQI index is assigned when a single PDSCH TB with the

modulation coding scheme and code rate correspondent to that CQI index in table 7.2.3-1 of [TS36213] can be received

with an error probability less than 0.1. In case of wideband CQIs, the reference TB includes all the RBGs available in

order to have a reference based on the whole available resources; while, for subband CQIs, the reference TB is sized

as the RBGs.

Round Robin (RR) Scheduler

The Round Robin (RR) scheduler is probably the simplest scheduler found in the literature. It works by dividing

the available resources among the active ﬂows, i.e., those logical channels which have a non-empty RLC queue. If

the number of RBGs is greater than the number of active ﬂows, all the ﬂows can be allocated in the same subframe.

Otherwise, if the number of active ﬂows is greater than the number of RBGs, not all the ﬂows can be scheduled in a

given subframe; then, in the next subframe the allocation will start from the last ﬂow that was not allocated. The MCS

to be adopted for each user is done according to the received wideband CQIs.

Proportional Fair (PF) Scheduler

The Proportional Fair (PF) scheduler [Sesia2009] works by scheduling a user when its instantaneous channel quality

is high relative to its own average channel condition over time. Let i, j denote generic users; let t be the subframe

index, and k be the resource block index; let M

i,k

(t) be MCS usable by user i on resource block k according to what

reported by the AMC model (see Adaptive Modulation and Coding); ﬁnally, let S(M, B) be the TB size in bits as

deﬁned in [TS36213] for the case where a number B of resource blocks is used. The achievable rate R

(k, t) in bit/s

for user i on resource block group k at subframe t is deﬁned as

(k, t) =

S (M

i,k

(t), 1)

where τ is the TTI duration. At the start of each subframe t, each RBG is assigned to a certain user. In detail, the

index

(t) to which RBG k is assigned at time t is determined as

(t) = argmax

j=1,...,N



(k, t)

(t)



where T

(t) is the past througput performance perceived by the user j. According to the above scheduling algorithm,

a user can be allocated to different RBGs, which can be either adjacent or not, depending on the current condition of

the channel and the past throughput performance T

(t). The latter is determined at the end of the subframe t using the

following exponential moving average approach:

(t) = (1 −

(t − 1) +

(t)

where α is the time constant (in number of subframes) of the exponential moving average, and

(t) is the actual

throughput achieved by the user i in the subframe t.

(t) is measured according to the following procedure. First we

determine the MCS

(t) actually used by user j:

(t) = min

(t)=j

j,k

(t)

then we determine the total number

(t) of RBGs allocated to user j:

(t) =



{k :

(t) = j}



1.4. Detailed description of protocol elements 11

LTE Simulator Documentation, Release ns-3.15

where | · | indicates the cardinality of the set; ﬁnally,

(t) =



(t),

(t)



Transport Blocks

The implementation of the MAC Transport Blocks (TBs) is simpliﬁed with respect to the 3GPP speciﬁcations. In

particular, a simulator-speciﬁc class (PacketBurst) is used to aggregate MAC SDUs in order to achieve the simulator’s

equivalent of a TB, without the corresponding implementation complexity. The multiplexing of different logical

channels to and from the RLC layer is performed using a dedicated packet tag (LteRadioBearerTag), which performs

a functionality which is partially equivalent to that of the MAC headers speciﬁed by 3GPP.

1.4.2 RLC and PDCP

Overview

The RLC entity is speciﬁed in the 3GPP technical speciﬁcation [TS36322], and comprises three different types of

RLC: Transparent Mode (TM), Unacknowledge Mode (UM) and Acknowledged Mode (AM). We implement only the

UM and the AM RLC entities.

The RLC entities provide the RLC service interface to the upper PDCP layer and the MAC service interface to the

lower MAC layer. The RLC entities use the PDCP service interface from the upper PDCP layer and the MAC service

interface from the lower MAC layer.

Figure Implementation Model of PDCP, RLC and MAC entities and SAPs shows the implementation model of the RLC

entities and its relationship with all the other entities and services in the protocol stack.

Service Interfaces

PDCP Service Interface

The PDCP service interface is divided into two parts:

• the PdcpSapProvider part is provided by the PDCP layer and used by the upper layer and

• the PdcpSapUser part is provided by the upper layer and used by the PDCP layer.

PDCP Service Primitives The following list speciﬁes which service primitives are provided by the PDCP service

interfaces:

• PdcpSapProvider::TransmitRrcPdu

– The RRC entity uses this primitive to send an RRC PDU to the lower PDCP entity in the transmitter peer

• PdcpSapUser::ReceiveRrcPdu

– The PDCP entity uses this primitive to send an RRC PDU to the upper RRC entity in the receiver peer

RLC Service Interface

The RLC service interface is divided into two parts:

• the RlcSapProvider part is provided by the RLC layer and used by the upper PDCP layer and

12 Chapter 1. Design Documentation

LTE Simulator Documentation, Release ns-3.15

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Figure 1.8: Implementation Model of PDCP, RLC and MAC entities and SAPs

• the RlcSapUser part is provided by the upper PDCP layer and used by the RLC layer.

Both the UM and the AM RLC entities provide the same RLC service interface to the upper PDCP layer.

RLC Service Primitives The following list speciﬁes which service primitives are provided by the RLC service

interfaces:

• RlcSapProvider::TransmitPdcpPdu

– The PDCP entity uses this primitive to send a PDCP PDU to the lower RLC entity in the transmitter peer

• RlcSapUser::ReceivePdcpPdu

– The RLC entity uses this primitive to send a PDCP PDU to the upper PDCP entity in the receiver peer

MAC Service Interface

The MAC service interface is divided into two parts:

• the MacSapProvider part is provided by the MAC layer and used by the upper RLC layer and

• the MacSapUser part is provided by the upper RLC layer and used by the MAC layer.

MAC Service Primitives The following list speciﬁes which service primitives are provided by the MAC service

interfaces:

• MacSapProvider::TransmitPdu

– The RLC entity uses this primitive to send a RLC PDU to the lower MAC entity in the transmitter peer

1.4. Detailed description of protocol elements 13

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