introduction to 3gpp lte technology
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Introduction to 3GPP LTE TechnologyTRANSCRIPT
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Introduction to 3GPP LTE Technology1
1. Introduction
Stream video applications and Internet connected consumer devices, along with
flat‐rate pricing strategy, contribute toward fast growing demand of mobile data
services for cellular industry. 3G HSPA operators may experience tremendous
increase of data traffic in 2007 and 2008, to push the demand of all‐IP cellular
technology. 3G long‐term evolution (LTE) has been developed to satisfy the market
need. LTE is an all‐IP wireless network based on orthogonal frequency division
multiplexing (OFDM), and over current 3G spectrum as well as new spectrum (2.6G
Hz band and digital dividend spectrum at 700M Hz) or refarmed GSM band with
bandwidth from 1.4M Hz to 20M Hz. To support multimedia broadcast and multicast
services (MBMS), LTE may transmits multicast/broadcast over a single frequency
network (MBSFN).
Although 3G technology is based on WCDMA, LTE adopts OFDM with advantages in
high spectral efficiency but sensitive to phase noise caused by channel fading etc.
The multiple access of LTE is OFDM with cyclic prefix in the downlink and
single‐carrier FDMA (SC‐FDMA) in the uplink. Both FDD and TDD modes are
supported. The modulations in the downlink (DL) and uplink (UL) are QPSK, 16QAM,
and 64 QAM.
The physical channels defined in the downlinks include
Physical Downlink Shared Channel (PDSCH)
Physical Multicast Channel (PMCH)
Physical Downlink Control Channel (PDCCH)
Physical Broadcast Channel (PBCCH)
Physical Control Format Indicator Channel (PCFICH)
Physical Hybrid ARQ Indicator Channel (PHICH)
The physical channels defined in the uplink include
1 This article summarized from references is prepared for the course of Mobile Communications at the National Taiwan University, and is authored by Dr. Kwang‐Cheng Chen, IEEE Fellow and Distinguished Professor, Graduate Institute of Communication Engineering and Department of Electrical Engineering, National Taiwan University. Email: [email protected]
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Physical Random Access Channel (PRACH)
Physical Uplink Shared Channel (PUSCH)
Physical Uplink Control Channel (PUCCH)
There are several physical layer procedures involved with LTE operation:
Cell search
Power control
Uplink synchronization and uplink timing control
Random access related procedures
HARQ related procedures
Radio characteristics are measured by UE and eNode‐B, and reported to the network,
such as measurements for intra‐ and inter‐frequency handover, inter RAT handover,
timing, and measurement for RRA.
Figure 1: Radio Interface Protocol Architecture [1]
2. Single‐Carrier FDMA
Although OFDM is sensitive to phase noise, its advantage for simple equalization
over frequency‐selective fading channels introduces great attraction to wireless
broadband communications requiring high spectral efficiency. The multiuser OFDM
or known as orthogonal frequency division multiple access (OFDMA) is therefore
widely considered and adopted in all‐IP broadband wireless systems. OFDM(A) type
systems suffer from peak‐to‐average‐ratio (PAPR) problem and phase noise caused by
fading and other issues. A slightly modified OFDMA has been proposed for the PHY
of LTE, known as single‐carrier frequency division (or domain) multiple access
(SC‐FDMA). For SC‐FDMA, the time‐domain data is transformed to frequency domain
by DFT prior to modulation, to result in another name as DFT‐spread OFDMA. Figure
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2 illustrates this slight difference between SC‐FDMA and OFDMA.
Figure 2: SC‐FDMA vs. OFDMA [6]
There are several ways to transmitted symbols to SC‐FDMA sub‐carriers, and two
categories are of most interests as Figure 3. In the distributed subcarrier mapping
mode, DFT outputs of the input data are allocated over the entire bandwidth with
zeros occupying the unused subcarriers resulting in a non‐continuous comb‐shaped
spectrum. Interleaved SC‐FDMA (IFDMA) is an important special case of distributed
SC‐FDMA. In contrast with IFDMA, consecutive subcarriers are occupied by the DFT
outputs of the input data in the localized subcarrier mapping mode, which results in
a continuous spectrum that occupies a fraction of the total available bandwidth.
Subcarrier mapping methods are further implemented as static and channel
dependent scheduling (CDS) methods. CDS assigns subcarriers to users according to
the individual channel frequency response of each user. For both scheduling
methods, distributed subcarrier mapping provides frequency diversity because the
transmitted signal is spread over the entire bandwidth. With distributed mapping,
CDS incrementally improves performance. On the other hand, CDS is of great benefit
with localized subcarrier mapping because its significant multi‐user diversity order.
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Figure 3: Sub‐carrier Mapping of SC‐FDMA [6]
For OFDMA, data are conveyed at each sub‐carrier, and equalization and data
detection are executed at each sub‐carrier, likely in frequency domain. A null in
spectrum can severely degrade the system performance, and channel coding with
power/rate control would be necessary. For SC‐FDMA, channel equalization is also
realized in the frequency domain but data detection is performed after the
frequency domain equalized data is converted back to time domain by IDFT. It is
therefore more robust to spectral nulls compared to OFDMA since the noise is
averaged out over the entire bandwidth. Additional disadvantages of OFDMA
compared to SC‐FDMA are the high sensitivity to phase noise caused by carrier
frequency offset and strong sensitivity to nonlinear distortion from the power
amplifier due to the high PAPR, while these are inherent properties of the
multicarrier nature for OFDMA.
The next issue plays an important role in OFDM type systems, that is,
peak‐to‐average‐power‐ratio (PAPR), which is particularly critical for a large number
of sub‐carriers (good for high mobility) and a high‐dimensional signaling. Given a 5M
Hz bandwidth, Figure 4 depicts PAPR for different realizations of SC‐FDMA and
OFDMA. SC‐FDMA signals indeed have lower PAPR than OFDMA signals. In the mean
time, LFDMA incurs higher PAPR compared to IFDMA but lower to OFDMA. Another
interesting observation is that pulse shaping significantly increases the PAPR of
IFDMA. A pulse shaping filter should be designed carefully in order to limit the PAPR
without degrading the system performance. Generally speaking, IFDMA is more
desirable than LFDMA in terms of PAPR and power efficiency. However, in terms of
system throughput, LFDMA is clearly superior when channel‐dependent scheduling
(CDS) is utilized [6]. CDS determines the allocation of time and frequency resources
fairly among users while achieving multi‐user diversity and frequency selective
diversity. It is common to introduce utility‐based scheduling where utility is an
economic concept representing level of satisfaction. The selection of a utility
measure influences the tradeoff between overall efficiency and fairness among users.
Two different utility functions are commonly considered: aggregate user throughput
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for maximizing system capacity and aggregate logarithmic user throughput for
maximizing proportional fairness. The objective is to find an optimum chunk
assignment for all users in order to maximize the sum of user utility at each transmit
time interval (TTI). In practice, the units of resource allocation are chunks, which are
disjoint sets of subcarriers. As a practical matter chunk‐based transmission is
desirable since the input data symbols are grouped into a block for DFT operation
before subcarrier mapping. The CDS completes through optimization, which usually
involves great complexity.
Figure 4: Complementary Cumulative Distribution Function of PAPR for IFDMA,
LFDMA, and OFDMA with M = 256 system subcarriers, N = 64 subcarriers per user,
and a = 0.5 rolloff factor; (a) QPSK; (b) 16‐QAM [6]
3. Physical Layer of 3G LTE
Physical layer of 3G LTE basically adopts OFDMA (or just OFDM) for the downlink and
SC‐FDMA for the uplink.
3.1 Frame Structure
LTE frames are of 10 msec duration as shown in Figure 5. Each frame is divided into
10 subframes, and each subframe has 1.0 msec duration. Each subframe is further
divided into two slots, each of 0.5 msec duration. Slots consist of either 6 or 7 ODFM
symbols, depending on whether the normal or extended cyclic prefix is employed.
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0 1 2 3 10 11 20… …
0 1 2 3 4 5 6
Cyclic Prefix
1 Frame (10 msec)
1 Sub‐Frame(1 msec) 1 slot
(0.5 msec)
7 OFDM Symbols with short cyclic prefix
Figure 5: LTE Genetic Frame Structure
Based on the frame structure, LTE operation relies on the concept of physical
resource block (PRB). A PRB is defined as consisting of 12 consecutive subcarriers for
one slot (0.5 msec) in duration. A PRB is the smallest element of resource allocation
assigned by the base station scheduler. In practical operation, the bandwidth and
PRBs have the following relationship as Table 1.
Bandwidth (M Hz) 1.25 2.5 5 10 15 20
Sub‐carrier Bandwidth (kHz) 15
Physical Resource Block
(PRB) Bandwidth (k Hz)
180
Number of Available PRBs 6 12 25 50 75 100
Table 1: Available (Downlink) Bandwidth v.s. Physical Resource Blocks
3.2 Slot structure and physical resources in uplink
3.2.1 Resource grid
The transmitted signal in each slot is described by a resource grid of UL RBRB scN N
subcarriers and ULsymbN SC‐FDMA symbols. The resource grid is illustrated in Figure 6.
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The quantity ULRBN depends on the uplink transmission bandwidth configured in the
cell and shall fulfill
min, max,UL UL ULRB RB RBN N N
where min, 6ULRBN and max, 110UL
RBN is the smallest and largest uplink bandwidth ,
respectively, supported by the current version of this specification. The set of
allowed values for ULRBN is given by [6]. The number of SC‐FDMA symbols in a slot
depends on the cyclic prefix length configured by higher layers and is given in Table 2.
subc
arri
ers
UL
RB
RB
scN
N
subc
arri
ers
RB
scN
One uplink slot slotT
SC-FDMA symbolsULsymbN
1UL RBRB sck N N
Resource block resource elementsUL RBsymb scN N
Resource element ( , )k l
0l 1ULsymbl N
0k
:
:
Figure 6: Uplink Resource Grid
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3.2.2 Resource elements
Each element in the resource grid is called a resource element and is uniquely
defined by the index pair ( , )k l in a slot where
0,..., 1 and 0,..., 1UL RB ULRB sc symbk N N l N are the indices in the frequency and time
domain, respectively. Resource element ( , )k l corresponds to the complex value
,k la . Quantities ,k la corresponding to resource elements not used for transmission
of a physical channel or a physical signal in a slot shall be set to zero.
3.2.3 Resource blocks
A physical resource block is defined as ULsymbN consecutive SC‐FDMA symbols in the
time domain and RBscN consecutive subcarriers in he frequency domain, where
ULsymbN and RB
scN are given by Table 2. A physical resource block in the uplink thus
consists of UL RBsymb scN N resource elements, corresponding to one cost in the time
domain and 180 kHz in the frequency domain.
Table 2: Resource block parameters.
Configuration RBscN
ULsymbN
Normal cyclic prefix 12 7
Extended cyclic prefix 12 6
The relation between the physical resource block number PRBn in the frequency
domain and resource element ( , )k l in a slot is given by
PRB RBsc
kn
N
3.3 Slot structure and physical resource elements in downlink
3.3.1 Resource grid
The transmitted signal in each slot is described by a resource grid of DL RBRB scN N
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subcarriers and DLsymbN OFDM symbols. The resource gird structure is illustrated in
Figure 7. The quantity DLRBN depends on the downlink transmission bandwidth
configured in the cell and shall fufil
min, max,DL DL DLRB RB RBN N N
where min, 6DLRBN and max, 110DL
RBN are the smallest and largest downlink
bandwidth ,respectively, supported by the current version of this specification.
The set of allowed values for DLRBN is given by [6]. The number of OFDM symbols in
a slot depends on the cyclic prefix length and subcarrier spacing configured and is
given in Table 3.
In case of multi‐antenna transmission, there is on resource gird defined per antenna
port. An antenna port is defined by its associated reference signal. The set of
antenna ports supported depends on the reference signal configuration in the cell:
- Cell‐specific reference signals, associated with non‐MBSFN transmission,
support a configuration of one, two, or four antenna ports and the antenna
port number p shall fufil 0, {0,1}, and {0,1,2,3}p p p ,respectively.
- MBSFN reference signals, associated with MBSFN transmission are
transmitted on antenna port 4p
- UE‐specific reference signals are transmitted on antenna port 5p
3.3.2 Resource elements
Each elements in the resource gird for antenna port p is called a resource element
and is uniquely identified by the index pair ( , )k l in a slot where
0,..., 1DL RBRB sck N N and 0,..., 1DL
symbl N are the indices in the frequency and time
domain, respectively. Resource element ( , )k l on antenna port p corresponds to
the complex value ( ),p
k la . When there is no risk for confusion, or no particular antenna
port is specified, the index p may be dropped.
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subc
arri
ers
DL
RB
RB
scN
N su
bcar
rier
sR
Bsc
N
One downlink slot slotT
OFDM symbolsDLsymbN
1DL RBRB sck N N
Resource block resource elementsDL RBsymb scN N
Resource element ( , )k l
0l 1DLsymbl N
0k
:
:
Figure 7: Downlink Resource Grid
3.3.3 Resource blocks
Resource blocks are used to describe the mapping of certain physical channels to
resource elements. Physical and virtual resource blocks are defined
A physical resource block is defined as DLsymbN consecutive OFDM symbols in the time
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domain and RBscN consecutive subcarriers in the frequency domain, where DL
symbN
and RBscN are given by Table 3. A physical resource block thus consists of
DL RBsymb scN N resource elements, corresponding to one slot in the time domain and
180 kHz in the frequency domain.
Physical resource blocks are numbered from 0 to 1DLRBN in the frequency domain.
The relation between the physical resource block number PRBn in the frequency
domain and resource elements ( , )k l in a slot is given by
PRB RBsc
kn
N
Table 3: Physical resource blocks parameters.
Configuration RBscN
DLsymbN
Normal cyclic prefix 15kHzf 12
7
Extended cyclic prefix 15kHzf 6
Extended cyclic prefix 7.5kHzf 24 3
A virtual resource block is of the same size as a physical resource block. Virtual
resource blocks are numbered from 0 to 1DLRBN . Two types of virtual resource
blocks are defined:
Virtual resource blocks of localized type
Virtual resource blocks of distributed type
Virtual resource blocks of localized type are mapped directly to physical resource
blocks such that virtual resource block PRB VRBn n .
Virtual resource blocks of disturbed type are mapped to physical resource blocks
such that virtual resource block VRBn corresponds to physical resource block
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( , )PRB VRB sn f n n ,where sn is the slot number within a radio frame. The
virtual‐to‐physical resource block mapping is different in the two slots of a subframe.
3.3.4 Resource‐elements groups
Resource‐element groups are sued for defining the mapping of control channels to
resource elements. A resource‐elements group is represented by the index pair
' '( , )k l of the resource elements with the lowest index k in the group with all
resource elements in the group having the same value of l . The set of resource
elements ( , )k l in a resource‐element group depends on the number of cell‐specific
reference singles configured as described below with 0 ,0RB DLPRB sc PRB RBk n N n N .
In the first OFDM symbol of the firs slot in a subframe the two
resource –element groups in physical resource block PRBn consist of
resource elements ( , 0)k l with 0 0 00, 1,..., 5k k k k and
0 0 06, 7,..., 11k k k k , respectively
In the second OFDM symbol of the first slot in a subframe in case one or
two cell‐specific reference signals configured , the three resource‐element
groups in physical resource block ( , 1)k l with 0 0 00, 1,..., 3k k k k ,
0 0 04, 5,..., 7k k k k and 0 0 08, 9,..., 11k k k k ,respectively.
In the second OFDM symbol of the first slot in a subframe in case of four
cell‐specific reference signals configured, the two resource element groups
in physical resource block PRBn consist of resource elements ( , 1)k l
with 0 0 00, 1,..., 5k k k k and 0 0 06, 7,..., 11k k k k ,
respectively
In the third OFDM symbol of the first slot in a subframe, the three
resource‐element groups in physical resource block PRBn consist of
resource elements ( , 2)k l with 0 0 00, 1,..., 3k k k k ,
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0 0 04, 5,..., 7k k k k and 0 0 08, 9,..., 11k k k k ,respectively.
Mapping of a symbol‐quadruplet ( ), ( 1), ( 2), ( 3)z i z i z i z i onto a
resource‐element group represented by resource‐element ' '( , )k l is defined such
that elements ( )z i are mapped to resource elements ( , )k l of the
resource‐element group not used for cell‐specific reference signals in increasing
order of and i k . In case a single cell‐specific reference signal is configured,
cell‐specific reference signal shall be assumed to be present on antenna port 0 and 1
for the purpose of mapping a symbol‐quadruplet to a resource‐element group,
otherwise the number of cell‐specific reference signals shall be assumed equal to the
actual number of antenna ports used for cell‐specific reference signals.
For half‐duplex FDD operation, a guard period is created by the UE by not receiving
the last part of a downlink subframe immediately preceding an uplink subframe from
the same UE. For frame structure type2, the GP field serves as a guard period.
3.4 PHY Operation
The following figure summarizes the physical channel processing for the downlink as
an example of PHY operation. The scrambling sequence generator is initiated at the
start of each sub‐frame depending on the transport channel type. The modulation
can be QPSK, 16QAM, and 64QAM. Layer mapping can be trivial for single antenna.
For spatial multiplexing and transmit diversity, layer mapping can be employed by
appropriate definition of number of layers and code words. Pre‐coder takes a block
of vectors from layer mapping to generate a block of vectors mapped onto resources
on each antenna port.
Figure 8: Physical Channel Processing (Downlink)
To ensure successful signal reception of high mobility, data‐aided channel estimation
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is needed with reference signals.
Figure 9: LTE Reference Signal Arrangement [5]
3G LTE allows MIMO transmission. The receiver therefore has to execute further
channel estimation to compute channel response, through sequentially transmitting
known reference signals from each antenna as the following figure.
Figure 10: Reference Signals Transmitted Sequentially to Compute Channel
Responses for MIMO Operation
4. 3G LTE Radio Access
Figure 11 depicts the basic protocol structure of LTE. LTE uses channel dependent
scheduling (CDS).
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Figure 11: Simplified LTE Protocol Structure [9]
The uplink and downlink of LTE are orthogonal. The LTE system performance (such as
spectrum efficiency or data rates) is more limited by interference, especially near the
cell edge. Uplink power control is one of the mechanisms to reduce/control the
inter‐cell interference. LTE uplink power control supports fractional path‐loss
compensation, which implies users near the cell border use relatively less transmit
power. Inter‐cell interference coordination (ICIC), as an advanced interference control
scheme, is essentially a scheduling strategy to limit the inter‐cell interference [9]. A
simple method to improve cell‐edge data rates is to restrict the usage of parts of the
bandwidth statically, for example, through a reuse larger than one. Such schemes
improve the signal‐to‐interference ratios of the used frequencies. However, the loss
due to reduced bandwidth availability is typically larger than the corresponding gain
due to higher signal to interference ratio (SIR), leading to an overall loss of efficiency.
Therefore, the LTE system supports dynamic inter‐cell interference coordination of
the scheduling in neighboring cells such that cell‐edge users in different cells are
preferably scheduled on complementary parts of the spectrum when required.
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Please note that a major difference from static reuse schemes is that LTE still allows
for the entire available spectrum to be used in all cells. Bandwidth restrictions are
applied only when the traffic and radio conditions are concerned. Interference
coordination can be applied to both uplink and downlink, although with some
fundamental differences between these two links. In the uplink, the interference
originates from several geographically separated terminals, and thus, the overall
interference varies over time with the scheduling decisions. On the other hand, in
the downlink, the interference originates from the stationary base stations. Hence,
the observed interference depends more heavily on the scheduling decision in the
uplink case, compared to the downlink case, and inter‐cell interference coordination
can be more suitable to the uplink. In addition, as the LTE interference coordination
mechanism is based on scheduling restrictions in the frequency domain, it is mainly
for relatively narrowband services not requiring the full system bandwidth. As the
uplink transmission power generally is significantly smaller than the downlink
transmission power, uplink transmissions are likely narrowband in nature than
downlink transmissions. This further indicates that inter‐cell interference
coordination is primarily used in the uplink. To aid uplink inter‐cell coordination, LTE
defines two indicators exchanged among base stations: the high‐interference
indicator and the overload indicator. The high‐interference indicator provides
information to neighboring cells about the part of the cell bandwidth upon which the
cell intends to schedule its cell‐edge users. Because cell‐edge users are susceptible to
inter‐cell interference, upon receiving the high‐interference indicator, a cell might
want to avoid scheduling certain subsets of its own users on this part of the
bandwidth. This subset includes users close to the cell that issues the
high‐interference indicator. The overload indicator provides information on the
uplink interference level experienced in each part of the cell bandwidth. A cell
receiving the overload indicator may reduce the interference generated on some of
these resource blocks by adjusting its scheduling strategy, for example, by using a
different set of resources, and in this way, improve the interference situation for the
neighbor cell that issues the overload indicator.
In the downlink, inter‐cell coordination implies restrictions of the transmission
power in some parts of the transmission bandwidth. In principle, this parameter
could be configured on a static basis; however, as mentioned above, this is not very
efficient. Instead dynamic, downlink coordination is supported through the
definition of a relative narrowband transmission‐power indicator. A cell can provide
this information to neighboring cells, indicating the part of the bandwidth where it
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intends to limit the transmission power. A base station receiving the indication can
schedule its downlink transmissions within this band, reducing the output power or
completely releasing the resources on complementary parts of the spectrum. A
crucial part of the supported inter‐cell‐interference coordination scheme in LTE is
that full‐frequency reuse in neighboring cells is possible. Both uplink and downlink
inter‐cell interference coordination strategies benefit from knowledge about the
radio‐wise position of a terminal relative to neighbor cells.
5. Core Network
The evolved 3GPP system is hybrid mobile network architecture to support multiple
radio access technologies and multiple mobility mechanisms, as Figure 12 showing.
Figure 12: Simplified Evolved 3GPP Network Architecture [10]
Evolved packet system (EPS) provides access network operators and service
operators with a set of tools to enable service and subscriber differentiation [11],
which is important as operators are moving from a single to a multi‐service offering
at the same time as both the number of mobile broadband subscribers and the
traffic volume per subscriber is rapidly increasing. The bearer plays a central role in
the EPS QoS concept and provides the level of granularity for bearer‐level QoS
control (that is, all packet flows mapped to the same bearer receive the same
packet‐forwarding treatment). The network‐initiated QoS control paradigm specified
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in EPS is facilitated by a set of signaling procedures for managing bearers and
controlling their QoS assigned by the network. Such EPS QoS concept is class‐based,
where each bearer is assigned one and only one QoS class identifier (QCI) by the
network. The QCI is a scalar used within the access network as a reference to
node‐specific parameters that control packet forwarding operations. This class‐based
approach, together with the network‐initiated QoS control paradigm, gives network
operators full control over the QoS provided for its offered services for each of its
subscriber groups.
Figure 13: Bearer and Associated QoS Parameters [11]
6. LTE‐Advanced
As of June 2009, LTE‐Advanced as a new evolution of LTE is still in early stage. The
technology of interests includes
Carrier aggregation to form a larger overall bandwidth up to 100M Hz
Relay to improve coverage and to reduce deployment cost
Extending multi‐antenna transmission by increasing the number of downlink
transmission layers to 8 and the number of uplink transmission layer to 4, to
support higher data rates
Coordinated multipoint (CoMP) transmission/reception, where transmission and
reception could be performed jointly across multi‐cell, as an extension of ICIC.
References:
[1] 3GPP TS 36.201, V8.1.0, LTE Physical Layer – General Description (Release 8),
Nov. 2007.
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[2] 3GPP TS 36.211, V8.2.0, Physical Channels and Modulation (Release 8), Mar.
2008.
[3] 3GPP TS 36.212, V8.3.0, Multiplexing and Channel Coding (Release 8), May
2008.
[4] 3GPP TS 36.213, V8.2.0, Physical layer Procedure (Release 8), Mar. 2008.
[5] J. Zyren, “Overview of the 3GPP Long Term Evolution Physical Layer”, Freescale
White Paper, July 2007.
[6] H.G. Myung, J. Lim, D.J. Goodman, “Single Carrier FDMA for Uplink Wireless
Transmission”, IEEE Vehicular Technology Magazine, vol. 1, no. 3, pp. 30‐38, Sep.
2006.
[7] D. Falconer, S.L. Ariyavisitakul, A. Benyamin‐Seeyar, and B. Eidson, “Frequency
Domain Equalization for Single‐Carrier Broadband Wireless Systems,” IEEE
Commun. Mag., vol. 40, no. 4, pp. 58–66, Apr. 2002.
[8] H. Sari, G. Karam, and I. Jeanclaude, “Transmission Techniques for Digital
Terrestrial TV Broadcasting,” IEEE Commun. Mag., vol. 33, no. 2, pp. 100–109,
Feb. 1995.
[9] D. Astely, E. Dahlman, A. Furuskar, Y. Jading, M. Lindstrom, S. Parkvall, “LTE: The
Evolution of Mobile Broadband”, IEEE Communications Mag., vol. 47, no. 5, pp.
44‐51, April 2009.
[10] A.K. Salkintzis, M. Hammer, I. Tanaka, C. Wong, “Voice Call Handover
Mechanisms in Next Generation 3GPP Systems”, IEEE Communications Mag.,
vol. 47, no. 2, pp. 46‐56, February 2009.
[11] H. Ekstrom, “QoS Control in the 3GPP Evolved Packet System”, IEEE
Communications Mag., vol. 47, no. 2, pp. 76‐83, February 2009.