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