How QoS is managed in LTE/EPS system

eNB manage traffic QoS requirement on two aspectsOn the radio interfaceOn the transport (backhaul)

A. QoS mechanisms in the radio part
1) Radio bearer control

QoS functionality in the RBC is to dynamically monitors the quality of service status of ongoing RBs based on the interaction with the MAC Scheduler. When quality of service requirements for an ongoing RBs are not fulfilled, QoS invokes the RBC function to release or to reconfigure the corresponding radio bearer.

2) Scheduling

MAC scheduler plays an important role to achieve required QoS for each radio bearer.Scheduling priority is a continuous function of QCI, GBR, delay budget, etc. For example, below formula can be used when calculating scheduling metric:M = R * Mqos * Mqci / (T^Alpha)

R is the expected/wanted bit rate

Mqos is the QoS metric

Mcqi is the QCI metric

T is the actual bit rate in the last scheduling period

Alpha is a fairness factor, Alpha = 0 means the system doesn't care what has been achieved, scheduling metric is only determined by the demand, therefore it is unfair among users;

Alpha > 1 means the system gives more weight to each UE's the performance, finding fairness among users.

3) Admission control

4) Congestion control5） ICICBasically, like AC and CC, ICIC function module just provide constraints to scheduling function.

B. QoS mechanisms in the transport

1) DiffServ uses the 6-bit Differentiated Services Code Point (DSCP) field in the header of IP packets for packet classification purposes. eNB maps QCI against DSCP so that the IP packets are marked with corresponding QoS requirements.

2) Mapping DSCP with P-bit in VLAN. In case of VLAN is supported, QoS priority is preserved as P-bit (7 bit) within a VLAN ID.

Key technologies to improve LTE coverage and link quality

What are the key technologies being used in LTE to improve network coverage and link quality?
My tentative answer can be found below:

• Adaptive modulation and coding (AMC)

On the downlink, to optimize system capacity and coverage, eNodeB will try to match the information data rate for each user to the variations of its received signal quality. UE can be configured to report CQIs (Channel Quality Indicator), RI (Rank Indicator) and PMI to assist eNodeB in selecting an appropriate transmission parameters such as MCS index, number of MIMO ranks and pre-coding parameter for the downlink transmissions. On the uplink, eNodeB can dynamically choose MCS value to send in the DCI (Downlink control information), to specify uplink MCS to be used by the UE, according to CQI values that UE reports. ·

• Rank and MIMO scheme adaption

Rank adaption modifies the Tx layers based on UE feedback. MIMO scheme selection takes measurement statistics for PUCCH evaluated at eNB so that to select an appropriate MIMO scheme.

- Tx Diversity and 2x2 SU-MIMO transimission schemes, are dynamically updated per UE according to the channel conditions an UE experienced.

- Close loop MIMO is selected for low speed users, while for high speed users Tx diversity or OL MIMO is selected depend on UE feedback, e.g. CQIs.

• Frequency hopping

Type 1 (inter-TTI) or type 2 (Intra/Inter-TTI) frequency hopping in LR3.0. Frequency hopping is useful to combat frequency selective fading and minimize inter-cell interference.

• TTI bundling

For VoIP or other read time traffic, TTI bundling can be used to improve cell edge performance.

• SIC

A successive interference cancellation (SIC) receiver can detect and decode the CWs of the data streams in such a way that if the CW of one data stream is successfully decoded (indicated by a cyclic redundancy check (CRC) code), the decoded data is then reencoded, remodulated, etc., and cancelled from the originally received signal. Thus, interference is reduced for the remaining data streams.

network sharing

Driver:

The operators are facing unprecedented challenges in particular during these recession days.

- decreased in voice revenues

- maintaining profit

The solution would naturally be on two aspects:

- generating new revenue by introducing new services, such as mobile broadband, mobile TV, SDP (iTunes, Nokia OVI store), etc

- cost reduction

RAN sharing is one solution leading to cost reduction, other options such as complete network outsourcing,

network O&M outsourcing are also worth considering, depends on operators' specific situation.

Solution:

Site sharing installs two or more operators' network equipments (mostly RAN) on the same site/tower. This is the simplest way, no much requirement to eNB (as eNB are still seperate). Backhaul traffic aggregation plays the most important role in cost saving in this case.

Real RAN sharing shares the same eNB with two or more operators. This requires some specific features within eNB to enable the sharing.

- multi- PLMN-id broadcast. The eNB can be configured to broadcast more than one set of PLMN-ids and more than one set of SIBs - as if two networks co-exist.

- VLAN. eNB needs to support VLAN so that each operator use one or more VLANs

- multiple IP address. The eNB should be addressed by different IP address on different operator's network

- resource allocation/balancing. The eNB should support configuration that its resource (HW, radio, etc) can be divided between the operators so that the system is not overwhelmed by a single operator's traffic.

- independent QoS management. However, the eNB should treat each operators traffic independently so that each operator has its own scheduling, RRM, QoS managment policy.

Architecture

Two types of RAN sharing architecture is defined.

- The first is called GWCN, where both GW and RAN are shared

- The second is called MOCN, where eNB connects to different MME/GW from each operator.

Most likely the second option will be more popular.

RSRP and RSRQ

The RSRP is comparable to the CPICH RSCP measurement in WCDMA. This

measurement of the signal strength of an LTE cell helps to rank between the different

cells as input for handover and cell reselection decisions. The RSRP is the average of

the power of all resource elements which carry cell-specific reference signals over the

entire bandwidth. It can therefore only be measured in the OFDM symbols carrying

reference symbols.

The RSRQ measurement provides additional information when RSRP is not sufficient

to make a reliable handover or cell reselection decision. RSRQ is the ratio between the

RSRP and the Received Signal Strength Indicator (RSSI), and depending on the

measurement bandwidth, means the number of resource blocks. RSSI is the total

received wideband power including all interference and thermal noise. As RSRQ

combines signal strength as well as interference level, this measurement value

provides additional help for mobility decisions.

How does CP removes ISI

We may all know that OFDM use CP to remove ISI. But how many of us know why and how? Below is a post on a BBS which is a typical question from who is new to OFDM/LTE.

"My question is assume the 2 maultipaths received at the receiver as below with CP prefixed to Symbols on both paths:

|--CP--|--symbol data--| ---> path 1
|-td-|--CP--|--symbol data--| ----> path 2 with delay td

|------------------| integration duration (Ts - symbol dur)
|-----| this is the part of the output still gets corrupted due to ISI, since CP is the part of the OFDM symbol it will be a non zero value.

So I am wondering how CP removes the ISI."

A very good answer can be found from book "LTE - The UMTS LONG TERM EVOLUTION, from theory to practice", chapter 5. Below is some excerpt with simplication to help easy understanding.

At a very high level, received signal through dispersive channel (frequency selective) can be represented below:
r(t) = x(t) * h(t) + z(t)
Assuming the channel delay is smaller than G, in a discrete way, the received signal is:
Rcp = A * h + Z

A =
xk[N-G] xk-1[N-1] xk-1[N-2] ... xk-1[N-G+1]
xk[N-G+1] xk[N-G] xk-1[N-1] ... xk-1[N-G+2]
. . . .
. . . .
. . . .
xk[N-2] xk[N-3] ... ... xk[N-G] xk-1[N-1]
xk[N-1] xk[N-2] ... ... xk[N-G+2] xk[N-G+1]
xk[0] xk[N-1] ... ... xk[N-G+2] xk[N-G+1]
. . . .
. . . .
. . . .
xk[N-1] xk[N-2] ... ... xk[N-G+1] xk[N-G]

Please note the red colored part is due to Inter-symbol interference. By discarding the first G samples of the received symbol, the inter-symbol interference can be easily discarded. And removed some part of intra-symbol interference as well (not very acturate here, but see below).
Only the blue part is not remained, we call it A'.

Now we extend A' to B. This doesn't change the output if adding a few zeros to the channel vertor h, which is now h'.

B =
xk[0] xk[N-1] ... ... xk[N-G+1] xk[N-G] ... xk[1]
xk[1] xk[0] ... ... xk[N-G+2] xk[N-G+1] ... xk[2]
. . . .
. . . .
. . . .
xk[N-1] xk[N-2] ... ... xk[N-G] xk[N-G-1] ... xk[0]

The beauty of matrix B (you must have realized!) is that it is circulant. It can be represented as B = FHXF

After Fourier transform, the output is
F(B*h') = F(FHXF*h) = F(FHXF) F(h) = X H

R = X H + Z

A convolution now transformed to multiply operation.

Getting back to the question posted at the begining of this article, the anwer to that question is:
- Inter symbol interference is easily discarded by insertion of CP at transmitter and removal of CP at receiver
- Intra symbol interference (which is actually the original question) is not removed BUT surpressed in a way where frequency selective channel is converted into a transmission over N parallel flat-fading channels in the frequency domain.

LTE Rel-10 : eICIC in detail

In my last post, some of the key techniques in LTE-A is summarized. Now I'd like to look into the detail of eICIC, which is currently a very active topic in 3GPP.

An index of latest eICIC studies within 3GPP can be found in R1-104238, which listed a set of references. In particular, I found the R1-103822 from NSN has a very good summary.

In general, there are two scenarios (except relay) need consideration:

A) Macro + Pico case

In this case, the UE is free to camp on a Macro or Pico cell which offers the best RSRP. It was concluded that co-channel deployment of macro + pico cases works without any explicit interference management mechanism. There is no need for introducing any new specifications (such as resources partitioning in time or frequency domain).

B) Macro + CSG Femto case

In this case, it is observed that so called macro-cell coverage holes can be experienced by macro-UEs being close to CSG HeNBs. This is because although the HeNB offers better signal quality, but UEs are not allowed to connect due to HeNB's close-access mode. This issue is primarily observed in the downlink. There are a few candidate solutions:

1. Apply power control for the HeNBs (reducing the Tx power for some HeNB)
2. Introducing resource partitioning between Macro eNBs and CSG HeNBs
3. Relaxing the CSG contraint, so Macro-UEs get temporily access to HeNB.

There are many variants for the 1st option. One simple example is to have HeNB measure its RSRP towards the strongest co-channel deployed macro-eNB, and reduce its Tx power accordingly. Others are more complex, may require signalling between different network elements.

For the 2nd option, i.e. resource partitioning, there are mainly two types. One is to partition the resources on frequency domain, e.g. certain carriers/sub-carriers are only reserved to Macro-eNB, the other is to partition the resources on time domain, for e.g. shift the HeNB's downlink frames by at least 3 OFDM symbols to avoid HeNB's CCH overlapping with MeNB's CCH. The time domain solution requires synchronization between Macro eNB and HeNB, which could be a challenge.

I am not sure if the 3rd option had been discussed in 3GPP (as I am not a 3GPP delegate).

by July 2010, the RAN WG1 #61bis meeting has concluded (which nothing concluded actually) on Macro-Femto case:

· Consider power control and time domain solution as baseline solutions

· Frequency domain solution is not precluded

LTA-Advanced related technologies

3GPP release 10 is to introduce some LTE-A features. LTE-A can archieve >1Gbps data rate by 4-by-4 MIMO, on ~70MHz system bandwidth.

The spectrum efficiency for LTE Rel-8 satisfies IMT-Advanced requirements in the downlink, however, in the uplink, SE needs to double to satisfy IMT-Advanced requirements.

LTE-A related Rel-10 work items:

• Carrier Aggregation (CA)
• Enhanced downlink multiple antenna transmission
• Uplink multiple antenna transmission
• Relay
• Enhanced ICIC for non-CA deployments of Heterogeneous networks (eICIC)

1) Carrier Aggregation (CA)
Entire system BW up to 100MHz, composed of component carriers
Each component carrier can be configured in a backward compatible way
Support both contiguous and non-contiguous spectrum


2) Enhanced downlink multiple antenna transmission
Up to 8 layers transmission, increased from 4 layers in Rel-8/9
Additional RS signal specified
MU-MIMO (max 4 layers, 2 per user)
Dynamic switching between SU and MU-MIMO


3) Uplink multiple antenna transmission
UL transmit diversity for PUCCH
SU-MIMO up to 4-streams

4) Relay
For places where wired backhaul is not avialable or very expensive
"type 1" - inband relay, RN (Relay Node) appears to be a Rel-8/9 eNB.
"type 1a" - outband relay, backhaul use a different frequency from access link

5) eICIC
For Micro-Pico scenario,
DL - No Problem for control channel, reuse Rel8/9 ICIC for data channel
UL - Reuse ICIC for data channel, reuse power control in Rel8/9

For Micro-Femto scenario,
Consider power control and time domain solution as baseline solutions.
At least one common TDD and FDD solution whenever possible

HeNB mobility - cont.

My last post on 8th september did not give a clear picture of HeNB mobilties and its enhancement. Therefore I am here posting another update hopefully gives a better touch on this topic.

Background:

Based on below table (from R3-101932, NTT Docomo), a large number of inbound/outbound handover occurs. Hence, LTE network should be able to support the high frequency of mobility as shown in this table.

Table1: The number of handover between Pico NB* and macro NBs in UMTS network

Deployment place		Peak of congestion time	Outbound handover [/hour]	Inbound handover [/hour]
1	Subway	6PM	5505	4549
2	High-rise building	5PM	1810	881
3	Office building	0PM (noon)	2753	507

In UMTS, RNC takes charge of handover procedures but on LTE (Rel-9), S1 handover is used for inbound/outbound mobility therefore MME has to be invovled in every single handover. The load to CN will be considerable high. In order to minimize the load to CN, optimized HeNB mobility seems to be very desirable.

Back in September 2009, 3GPP Rel-9 introduced in-bound mobility to HeNB either from another HeNB or from a macro eNB (via S1 handover). For Rel-10, HeNB mobilty enhancement is proposed to support:

- Enhanced HeNB to HeNB mobility (mainly for enterprise environments)

- Enhanced eNB to HeNB mobility (and vice versa).

The solutions

Initially two solutions as HeNB mobility optimization has been proposed.

Solution 1) HeNB to support X2 interface

Advantages:

- May work for legacy eNBs

– potentially only need an OAM configuration change to indicate that the HeNB is eligible for X2.

- Does not require a HeNB GW to be deployed to enable the enhancements.

- Works for both HeNB to HeNB mobility enhancements and eNB to HeNB mobility enhancements

- Most optimal routing and improvement of HO latency for enterprise HeNBs as the signalling remain in the enterprise network and does not need to be routed through operator nodes

Disadvantages:

- Requires X2 support for HeNBs, i.e., does not work for legacy HeNBs.

- Large IOT effort due to many HeNB vendors

Solution 2) To terminate the S1 handover signaling at the HeNB GW

Advantages:

- Works for legacy HeNBs since only the HeNB GW needs to change

Disadvantages:

- Does not support enhanced eNB to HeNB mobility and vice versa

- For enterprise, the packets still need to be routed through the HeNB GW located near the core network so this will offload the HO signalling from the MME but not necessarily improve mobility performance from the UE perspective since the latency will be about the same

- Requires the HeNB GW to be deployed and both HeNBs to be using the same HeNB GW

X2 Gateway

A variant to above solutions is the X2 gateway based solution, in which a handover between HeNB and (H)eNBs will involve the HeN

B

GW but instead of doing a S1 handover terminating at HeNB GW, X2 interface between HeNB GWs is supported, therefore it is a X2 handover but without direct X2 connection between the two handover nodes.

The advantage of this solution to solution 1):

- no need for IOT on X2 between various HeNBs

- eNB doesn't need to support many X2 connections

The advantage of this solution to solution 2):

- support enhanced

eNB to HeNB mobility and vice versa

- the handover HeNBs are not necessarily connect to the same HeNB GW

However, this solutions still need a gateway, the latency issue (if HeNB is close the ePC) and the architecture issue (a HeGW node is not mandatory in Rel-9) still exists.

Ref: 3GPP R3-101932, R3-101559, R3-102179, R3-102113

PDCCH - how UEs are scheduled

The book "LTE - from Theory to Practice" provided a very good summary of scheduling process from Control Channel point of view. Here is some of the excerpt:

"A typical seqeuence of steps carried out by eNodeB could be envisaged as follows:

1. Determine which UEs should be granted resources in the uplink, based on information such as channel quality measurements, scheduling requests and buffer status reports. Also decide on which resources should be granted.

2. Determine which UEs should be scheduled for packet transmission in the downlink, based on information such as channel quality indicator reports, and in the case of MIMO, rank indication and perferred precoding matrix

3. Indentify common control channel messages which are requred (e.g. power control commands using DCI format 3)

4. For each message decide on the PDCCH format (i.e. 1,2,4,8 CCEs), and any power offset to be apllied, in order to reach the intended UE(s) which sufficient reliability, while minimizing PDCCH overhead.

5. Determine how much PDCCH resource (in terms of CCEs) will be required, how many OFDM symbols would be needed for these PDCCHs and therefore what should be signalled on PCFICH.

6. Map each PDCCH to a CCE location within the appropriate search space.

7. If any PDCCHs cannot be mapped to a CCE location because all locations in the relevant search space have allready been assigned, either:
- continue to next step (step 9) accepting that one or more PDCCHs will not be transmitted, and now all DL-SCH and/or UL-SCH resources will be used, will a likely loss in throughput.
- allocate one more OFDM symbol to support the required PDCCHs and possibly revisit step 1 and/or 2 and change UE selection and resource allocation (e.g. to fully use uplink and downlink resources)

8) allocation the necessary resources to PCFICH and PHICH

9) Allocate resources to PDCCHs

10) Check that total power level per OFDM symbol does not exceed maximum allowed, and ajust if necessary

11) Transmit downlink control channels"


A few notes to help understanding the above description:

- CCE denotes Control Channel Elements, each CCE is nine REGs, where each REG is four physical resource elements

- LTE has four PDCCH formats (0-3), which has 1,2,4,8 CCEs respectively. The total number of bits can be transmitted over PDCCH for each format is 72, 144, 288, 576 respectively.
note: 1 CCE = 9 REGs = 4REs/REG * 9 REG = 36REs * 2bit/symbol = 72 bits

- the PDCCH can occupy the first 1, 2 or 3 OFDM symbols in a subframe, extending over the entire system bandwidth.

- PCFICH carriers control format indicator (CFI) which indicates the number of OFDM symbols (1,2 or 3) used for control channels. In principle the UE could deduce the value of CFI without PCFICH, for example by multiple attemps to decode the control channels assuming each possible number of symbols, but this would result in significant processing load.

- PHICH carriers HARQ ACK/NACK, it is modulated by BPSK, can be mapped onto 1, 2 or 3 OFDM symbols (and CFI needs to be set correspondingly).

LTE HeNB mobility

3GPP is actively working for solutions on HeNB related mobility.There are many proposals to enhance the HeNBs or HeNB-eNB mobilities. X2 based mobility between eNBs/HeNBs and HeNBs is being defined in the following cases:Between an eNB and an open access cell HeNB Between two open access cell HeNBs Between two closed HeNBs only if they have the same CSG IDBelow is a selection of some of the proposals, with my personal comments.

X2 interface between HeNB GW
It is proposed that X2 interface between HeNB GW should be supported, in order to support handover between two different areas (for e.g. shopping mall A to B).
My comment: handover between two different HeNB GW will be a very rare case, don't see why there is a need to support such interface. This is probably drived by HNB/HeNB GW manufactures.

eNB to HeNB handover

Since macro eNBs are not designed to handle many SCTP connections, therefore allowing direct connection between the eNB and HeNB will require upgrade the macro nodes in order to support large number of SCTP connections. The second issue relates to the NRTO table management by the macro eNBs. Also, require large IOT effort due to the existence of many HeNB vendors. Therefore, it is proposed that X2-Gateway based indirect interface is suggested to RAN3 for the eNB to HeNB mobility enhancement.

My comment: Don't understand why a handout from eNB would increase eNB's SCTP connections?

HeNB to HeNB mobility enhancement

It is proposed Direct X2 interface is acceptable for the HeNB to HeNB mobility enhancement.
My comment: This seems to be the correct direction.

The evolution of wireless technology - an analogy to human society

For anyone looking for anything serious, please walk away.

In David Tse's famous book, it often says the best communication system exploits the dimension of freedom the most. Inspired by this, I came across a seemingly interesting idea, which concludes there is much synergy between the evolution of wireless technology and the evolution of human society.

GSM is kinda like Socialism. Less freedom, less efficient, but highly practical. Each user get a constant quota (a timeslot, 200kHz), as if people get quota on food and other living essentials in China 30 years ago.

CDMA (if without power control) is like Capitalism. Resources are shared but contended. Whoever has the highest "power" will get better service and may block others. With proper power control (Government intervention or Law), it can be more efficient than GSM. But essentially, each user is still an interference to the others, such as in capitalism people/companies are born to compete each other within a set of rules(law).

OFDMA, you may have already known the answer, is no doubt Communism. People are no longer interfering each other (orthogonal). It is also much more efficient than GSM. Resources (RBs) are assigned to the person who can make best use of them (frequency selective scheduling). And, there are plenty bandwidth (20MHz and more :-) )

Have a nice weekend (if it is yet finished for you) and a lovely start of the coming week.

CSN.1 and ASN.1

CSN.2and ASN.1 are commonly used in wireless specifications. Although their acronyms are similar, CSN.1 is substantially different from ASN.1.

ASN.1 defines an abstract data structure: in other words, an ASN.1 specification tells you, for example, that we have a structure with some fields in it, or a union, an array and so on. In ASN.1 we describe our data structure as we do when we are declaring C structs.

When we need to actually send the data over the network, we follow the encoding rules we have chosen. These encoding rules, like BER o PER, tell us that a structure is always coded in a given way, an integer is always coded in another way and so on. Therefore any ASN.1 tools can easily generate a C data structure and some C code able to take a BER or PER coded binary stream and decode it into that C structure.

CSN.1, instead, defines at bit-level the encoded stream. A CSN.1 definition says, for example, that we expect a 1 followed by 8 more bits or a 0 followed by 4 bits and another 0:

<> ::= 1 | 0 0;

Most of the CSN.1 tools read the CSN.1 specification file and produce a CSN.1 parser; this parser, once compiled, reads the encoded binary stream and when it recognize some valid elements in it invokes a user defined callback function.

In other words, it generates code that will say to you: hey, I found the 1 followed by the 8 bits you were expecting: the 8 bits are 11001001; do whatever you like.

Reference:http://csn1.info

Why soft handover no longer exist in LTE?

This morning I came across this question in my mind while driving to the office. The reason seems to be obvious, since there is no longer a RNC in E-UTRAN, data streams from two active sets can't be combined unless ePC plays a part (or one eNB serves as SRNC), which is clearly undesirable. However, this topic worth to be looked into a little detail.

The root cause of having soft-handover in CDMA systems is that, in CDMA, each channel is an interference to other channels, therefore transmit power control is key to maximize system capacity. Especially at cell edge, a high transmission power could block other UEs from accessing the network. Here is where soft-handover can play its part. It allows transmission on different links therefore adds Rx diversity gain. During soft-handover, two NodeBs transmit/receive the same date stream on different physical channels. The data from different active set will be combined at UE (downlink) or at RNC (uplink). As such, the bit stream can be decoded much more reliably than if only one NodeB were transmitting/receiving. In other words, without soft-handover, to achieve the same SINR, higher power is required from a NodeB or from UE, which is undesirable.

In LTE, due to the orthogonal on both downlink and uplink within a cell, power control is not as important as in 3G. Therefore, soft-handover can be dropped from the system design without having much penalty to the system performance. This reduces network architecture complexity.

To support soft-handover, Iur is mandatory in case of different NodeBs involved in the soft handover are not controlled by the same RNC. In this case, the SRNC will be responsible to combine the data flow and DRNC only controls the NodeB involved in the soft-handover. The cost of having an Iur interface is significant especially if operators have multi-vendors RNC. 3GPP removed RNC node from E-UTRAN therefore simplified the architecture. There is however a X2 interface, which is similar to Iur but not the same. X2 can be used to forward packets in case of a handover simply for lossless mobility. But unlike an Iur interface, which have to support much more complex signalling, the X2 is a lot simpler.

I shall compare X2 with Iur with detail sometime later.

Interference rejection combining - Rx diversity series (2）

As mentioned earlier, while using Rx diversity, multiple received signals need to be combined in order to maximize the Rx signal's quality.

The combination is just a matter of choosing different weight to each receiving taps.

Selective combining (SC) assigns weight 1 to the strongest signal, assign weight 0 to all the others;

Equal gain combining (EGC) assigns all signal the same weight, i.e. the decoded signal is the sum of all signals after phase adjustion;

Maximum ratio combining (MRC) assigns weight according to each Rx signal's SINR, i.e. the signals are equalized based on each channel before being summed up. The output SNR is, therefore, the sum of the SNR at each element.

The MRC technique assumed that the fading signal at each element is independent of the signal at other elements. Interference rejection combining (IRC) on the other hand, not only estimate each channel independently, but also calculates the Covariance matrix. After knowing the coherence between each channel, interference from other channels can be removed.

one good website on this topic:

http://www.dsplog.com/

TTI Bundling

TTI bundling is a feature mainly used for extending VoIP (or other real-time traffic) uplink cell edge coverage. If a UE is at cell edge, due to its limited available transmit power, it can't transmit on many RBs, otherwise the power per RB would be too low to achieve a successful tranmission. In this case, TTI bundling can be activated (via RRC signalling). The benefits of TTI bundling are:

- uplink resources over multiple TTIs can be granted via a single grant;

- There is only one HARQ feedback message per bundle

- Less RLC/MAC overhead because RLC SDUs are not segmented

TTI boundling take the RLC SDU, make several redundancy versions corresponding to the entire RLC SDU and then transmit them over several (2,3 or 4) consecutive TTIs. Only when the last redundancy version of the transport block is received by eNB, the HARQ feedback is sent. This is similar to IR in HARQ, but transmit before receiving the HARQ NACK.

Unlike TTI boundling, HARQ re-transmission could also improve the probability of successful reception. However, re-tranmission means overhead (HARQ ACK/NACK) and more importantly, a significant delay (8ms on per re-transmission). Therefore, TTI boundling is more desirable especially for real-time applications.

RLC layer could also segment the RLC SDU into smaller RLC PDUs, which can be transmitted via lower MCS, therefore also help to achieve successful transmission on the uplink. However, RLC segmentation has a few drawbacks, including more RLC/MAC header overhead, more control signalling (resource grant and HARQ), and possibly a VoIP packet loss due to HARQ feedback error (note RLC is UM here).

For details please refer to:

"LTE coverage improvement by TTI bundling" Riikka Susitaival, Michael Meyer,

Ericsson Research

LIPA and SIPTO

I guess LIPA and SIPTO are hot topics nowadays. But what exactly are they referring to? How do these solutions provide value to customer/mobile operator?

LIPA is the abbrievation of "LOCAL IP ACCESS"

SIPTO is the abbrievation of "Selected IP Traffic Offload"

Both LIPA and SIPTO divert certain IP traffic (local residential/enterprise access for LIPA, some selected IP traffic for SIPTO) from operator's EPC P-GW. Control plane is not affected, MM, SM messages still terminates at EPC.

Solution 1)

To support LIPA/SIPTO, below network elements changes are required:

A new element called "Local PDN Gateway", i.e. L-GW is introduced. The L-GW is part of the access network, and it is responsible for:

- Filtering packets based on predefined policy

- UE IP address assignment;

- Direct tunnelling between L-GW and RAN in connected mode;

When a UE is camped on a cell, it can be signalled by NAS signalling or by AS SIBs that LIPA/SIPTO can be supported, or the UE could detect the cell belongs to a pre-defined list of cells. if LIPA/SIPTO is supported, the UE could potentially initiate another LIPA/SIPTO PDN connection.

The LIPA connection establishment is no much different with traditional connection establishment, apart from that a pre-defined APN is used (or received from NAS signalling), then at the PDP activation, S-GW will establish a tunnel with L-GW instead of P-GW.

Solution 2)

A OPM (Offload Processing Module) is collocated with eNB, which has NAT and routing functions. The UE establishes a LIPA enabled connection by using standard signalling procedures. The same PDN connection is used for both traffic to public network and traffic to local home/enterprise network. The OPM decides if an uplink traffic should be routed to S-GW or to the collcated NAT function. This is transparent to UE.

Because downlink packet is now addressing to OPM rather than directly addressing to UE. when the OPM (eNB) receives downlink packet, it needs to send MME a paging indication if the UE is in idle model, then the UE can be paged and downlink packet can be transferred after UE has established connection with network.

Solution 3)

There are many other candidate solutions, for details please refer to 3GPP TR 23.829.

The HS-DSCH downlink channel is shared between users using channel-dependent scheduling to make the best use of available radio conditions. Each user device periodically transmits an indication of the downlink signal quality, as often as 500 times per second. Using this information from all devices, the base station decides which users will be sent data on the next 2 ms frame and how much data should be sent for each user. More data can be sent to users which report high downlink signal quality.

The amount of the channelisation code tree, and thus network bandwidth, allocated to HSDPA users is determined by the network. The allocation is "semi-static" in that it can be modified while the network is operating, but not on a frame-by-frame basis. This allocation represents a trade-off between bandwidth allocated for HSDPA users, versus that for voice and non-HSDPA data users. When the base station decides which users will receive data on the next frame, it also decides which channelisation codes will be used for each user. This information is sent to the user devices over one or more HSDPA "scheduling channels"; these channels are not part of the HSDPA allocation previously mentioned, but are allocated separately. Thus, for a given 2 ms frame, data may be sent to a number of users simultaneously, using different channelisation codes. The maximum number of users to receive data on a given 2 ms frame is determined by the number of allocated channelisation codes. By contrast, in CDMA2000 1xEV-DO, data is sent to only one user at a time.

36.213 series (2) - Random Access

RACH is the interface between non-synchronized UEs and eNB.

Difference between LTE and WCDMA on RACH:

- RACH can't carry any user data, just pre-ambles. One exception is the scheduling request (SR), if the UE doesn't have any uplink resource block to send SR, it is allowed to send SR via RACH

- RACH is orthogonal with other resources such as PUSCH/PUCCH

When to RACH?

- A UE in RRC_CONNECTED state, but not uplink synchronized, need to send uplink data or control message (e.g. meas. report, ACK/NACK for downlink data)

- A UE in RRC_CONNECTED statebeing handover from one cell to another

- A transition from RRC_IDLE to RRC_CONNECTED (e.g. initial access or TAU)

- Recovering from radio link failure

Contention based Random Access

step 1) UE randomly choose RACH preamble signature. One UE says : "hello, I am Zhang-san", but the other can also say the same. A collision could happen and a collision resolution is required. The random access message is sent according to PRACH configuration (acquired by UE via system information or handover command). Initial transmission power is based on open loop estimation with full path-loss compensation.

step 2) eNodeB sends RAR (random access response) on PDSCH, and addressed with an ID, the Random Access Radio Network Temporary Identifier (RA-RNTI). RA-RNTI identifies the Time-frequency slot in which the preamble was detected. If multiple UEs had collided by selecting the same signature at the same time, they would each receive the RAR.

The RAR gives UE a C-RNTI (temporately), timing information and an initial uplink resource grant.

step 3) UE(s) sends the uplink message (For e.g. a ACK/NACK, measurement report, RRC connect request, etc) on the uplink resource granted by RAR. If a collision happened during step 1), they are going to collide again at this step. Each UE shall sent its own UE identity or C-RNTI (if its has one) in this uplink message, so that proper contention resolution can be done in the next step. Note in this step, the C-RNTI or UE identity is now unique for each UE.

Step 4) Possibly, the uplink message from none of the UE can be decoded, then all UE will have to re-RACH (triggered by guard timer expiry). Or, if one UE is succesfully decoded, it will receive a contention resolution message address to it (by C-RNTI or UE identity, which echos the message from UE in step 3). Other UE will also hear this message but the message is not addressing to them! Other UEs will now realize there was a collision and quit current RACH procedure to start a new one.

Contention free Random Access

UE use pre-allocated preamble signature. eNB can reserve a set of preambles signatures to be used for contention free Random Access (total number of available preamble signatures in LTE is 64).

RSCP and Eb/No in WCDMA

RSCP is the CPICH chip power at receiver - it tells the path loss
EC/No is the Chip power to Noise ratio at receiver - it tells not only path loss, but also interference

On WCDMA - uplink and downlink has some essential difference:

Uplink:

For the W-CDMA up link, the interference is received at the Node B, and is thus the same for all links in that cell. Ideally then, the same received power at the Node B would be required for all links with the same SIR target. The received interference level will vary between the cells, but since the number of UEs can be expected to be large compared to the number of cells, and due to the similar placements of Node B antennas, this variation is relatively small. At low to medium loads, the noise rise at the Node B is typically between 0-5 dB, to be compared with the at least 50-60 dB range of the path loss. This means that the UE output power mainly will be determined by the path loss to the Node B.

Since uplink coverage is limited by the UE output power, the uplink quality will be strongly dependent on the path loss and hence on the CPICH RSCP.

downlink:

The W-CDMA downlink differs from the uplink in many ways.

• In the down link, the interference is received at the terminal, and is thus different for different locations within the network. The interference level is significantly higher at the cell border than at the cell centre, due to other cell interference. At the cell border, the other cell interference may be as much as three times larger than the interference from the home cell.
• Another aspect is that all the downlink power transmitted in the serving cell, except the own signal, will be recieved as interference. This interference will be subject to exactly the same coupling loss at every instant as the signal. The effect of this own cell interference will be reduced by the OVSF codes, which ideally cancel all own cell interference (except from P-SCH and S-SCH). Under realistic conditions the OVSF codes cancel about 50% of the interfering own cell power.
• Further, there is a floor of interference caused by the non-power controlled control channels, such as CPICH, CCPCH, P-SCH, S-SCH and so on. This means that even at low load there is a substantial amount of interference, many, many times larger than the receiver noise power.
• The total available transmission power is normally at least 100 times larger at the Node B than in the terminal.

As a consequence of these facts, the link power for the down link will not be proportional to the path loss between serving cell and terminal. Instead, the downlink power always has to match the interference level, which due to own cell interference is large even close to the cell centre.

Conclusion:

RSCP is suitable for Uplink quality evaluation, but EC/No is suitable for downlink quality.

TDMA, FDMA and CDMA

A novice to digital communication system may wonder what are the meanings of these terminologies.

Let's start from the "M" in the middle, Multiplexing. In telecom system, multiplexing is a term used to refer to a process where multiple signals are combined into one signal over a shared medium. The reason of multiplexing is to share a medium (or channel in terms of information theory), for example, a wire connection, or a radio frequency, which is often limited resource.

It should now easy to understand the initials: "T" - time, "F" - frequency or "C" - code are just different ways of multiplexing.

TDM - channel is divided into sub-channels by time slots. A typical example is the congestion reduction strategy in Beijing, today is my turn, tomorrow is yours :-)
FDM - channle is divided into sub-channels by frequencies - by modulating baseband signals onto different frequency carriers. A typical example is dividing the road into lanes, cars can drive along without interfering each other on diff. lanes (ideally :-)
CDM - channel is divided into sub-channels by scrambling codes. Difficult to understand? Not at all. An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can communicate.

What's the "A" then? - Relation to multiple access:

A multiplexing technique may be further extended into a multiple access method or channel access method, for example TDM into Time-division multiple access (TDMA) and statistical multiplexing into carrier sense multiple access (CSMA). A multiple access method makes it possible for several transmitters connected to the same physical medium to share its capacity. Multiplexing is provided by the physical layer, while multiple access also involves a media access control protocol, which is part of the data link layer.

36.213 series (1) - Power Control in LTE

Power control in LTE is not as critical as in WCDMA. This is due to two factors:

- In LTE uplink are orthogonal, therefore no intracell interference (at least in theory)

- LTE has the freedom to do frequency selective scheduling, which can be used to further reduce the interference from other cells.

However, power control still plays an important part in LTE because a proper power control can maximize system capacity, minimize inter-cell interference, also look after the fairness between cell centre and cell edge UEs.

UL Power Control

Different formulas are specified for UL PUSCH, PUCCH, SRS. Fundamentally the formulas can be considered as a sum of two main parts:

1. a basic open-loop operating point derived from static or semi-static parameters signaled by eNB
   
2. a dynamic offset updated from sub-frame to sub-frame.
   

Furthermore, the 1st part, basic open loop operating point is composed of two parts: A) a semi-static P0, which is a cell specific power level and a UE specific offset (to compensate for example UE's PL estimation error), and B) a open loop path-loss compensation.

Basic operating point = P0 + α ● PL

α is a factor to trade off the fairness of uplink scheduling against total cell capacity. Full path-loss compensation (alpha = 1) maximize fairness for cell edge UE, but causing more interference to neighboring cells. For PUCCH, full PL compensation is always used.

The 2^nd part, the dynamic offset is also composed of two parts, one part is MCS dependent power offset ∆_TF (TF is transport format), the other part the TPC command related power. The TPC can command UE to use a relative power offset comparing to its previous Tx power, or command UE to use a absolute power, regardless its previous Tx power.

Downlink Power Allocation

In the downlink, cell specific RS EPRE (Energy per RE) is semi-static, it only change when the eNB signals UE so. PDSCH EPRE is proportional to RS EPRE's, depends on PDSCH RE's position. If the PDSCH RE is on the same symbol where there is a RS (index 0, 4), ρ_B is used, otherwise ρ_A is used. The value of ρ_{A /} ρ_B is determined by P_A and P_B which are specified by eNB via MAC/RRC signaling, they are UE specific. For details, see 36.213 section 5.2. Below diagram is an example when P_A=0.8 (ρ_A = 0.8) and P_B=2 (ρ_{B /} ρ_A = 0.6)