WC LTE

2. Key enabling technologies and
features of LTE
1. OFDM
2. SC-FDE and SC-FDMA
3. Channel dependent multi user resource
scheduling
4. Multi antenna Techniques
- Transmit diversity vs Beamforming vs
Spatial Multiplexing vs Multi User MIMO
5. IP based Flat network Architecture
-fewer nodes and a less hierarchical
structure

3. The LTE network Architecture
• Core network of LTE IS called Evolved Packet
Core(EPC)
• EPC is designed to provide a high capacity, all IP,
reduced latency, flat architecture
• Also provides internetworking with 2G and 3G
networks.
• Functions of EPC include access control, packet
routing and transfer, mobility management,
security, radio resource management, network
management.

4. The LTE Network Architecture

5. • EPC includes four new elements
• Serving Gateway(SGW) , which terminates the
interface towards the 3GPP radio access
networks
• Packet Data Network Gateway(PGW) which
controls IP data services, does routing, allocates
IP addresses, enforces policy and provides access
for non 3GPP access networks
• Mobility Management Entity (MME) which
supports user equipment context and identity, as
well as authenticate and authorize users.
• Policy and Charging Rules Function(PCRF)

7. Data Transmission Using Multiple Carriers
The simplest form of multicarrier modulation divides the data
stream into multiple substreams to be transmitted over different orthogonal
subchannels centered at different subcarrier frequencies.
The number of substreams is chosen to make the symbol time on
each substream much greater than the delay spread of the channel or,
equivalently, to make the substream bandwidth less than the channel
coherence bandwidth. This ensures that the substreams will not experience
significant ISI.

27. • In order to demodulate an OFDM signal, there are two important
synchronization tasks that need to be performed by the receiver.
First, the timing offset of the symbol and the optimal timing instants
need to be determined (timing synchronization)
Second, the receiver must align its carrier frequency as closely as
possible with the transmitted carrier frequency. (frequency synchronization)

30. The SNR loss is given by

46. SC-FDE maintains OFDM’s three most important benefits
• (1) Low complexity even for severe multipath channels.
• (2) Excellent BER performance, close to theoretical bounds
• (3) A decoupling of ISI from other types of interference,
notably spatial interference, which is very useful when
using multiple antenna transmission.
By utilizing single-carrier transmission, the PAR is also
reduced significantly (by several dB) relative to
multicarrier modulation.

48. Noise enhancement issues of FDE in OFDM vs SC FDE
Since the performance difference between SC-FDE and OFDM is not that
significant, other considerations are more important in determining which is the
appropriate method to use for a given application. An obvious difference is that
SC-FDE has a lower-complexity transmitter but a higher-complexity receiver,
compared to OFDM. Since the receiver was already considerably more complex
than the transmitter in a typical OFDM system due to channel estimation,
synchronization, and the error correction decoder, this further skews the
asymmetry

49. FDMA
TDMA
CDMA

57. It should be noted that uplink OFDMA is considerably more
challenging than downlink OFDMA since the uplink is naturally
asynchronous, that is the users’ signals arrive at the receiver offset slightly
in time (and frequency) from each other.
This is not the case in the downlink since the transmitter is
common for all users. These time and frequency offsets can result in
considerable self-interference if they become large.
Particularly in the distributed subcarrier mode, sufficiently large
frequency offsets can severely degrade the orthogonality across all
subcarriers.

62. Distributed Subcarrier Allocation: Takes advantage of frequency diversity by spreading the
resource block hop across the entire channel bandwidth.
Adjacent Subcarrier allocation: The approach relies on channel aware allocation of resources,
so that each user can be allocated a resource block where they have a strong channel

65. Overhead signalling is done on a logical control channel , the PDCCH
(Physical Downlink Control Channel) which specifies
Once a user is able to decode the PDCCH, it knows precisely where to receive
(downlink) or to transmit (uplink), and how. The PDCCIT is sent over the first 2—3
OFDM symbols of each subframe across all the subcarriers.
To aid the base station in uplink scheduling, LTE units utilize buffer status reporting
(BSR), wherein each user can notify the BS about its queue length, and channel
quality information (CQI) feedback in downlink to specify AMC.

66. • Power control is about addressing two types of interference
-- Intercell interference , especially for cell edge users.
As they doubly suffer from lower desired power and high
interference power .
--Self interference in uplink , related to imperfect time-
frequency-power synchronization between different users
with different power levels , dynamic range during ADC.
• Fractional power control , is a solution where channels are
inverted such that transmit power is proportional to h-s
where s is a fractional vale between 0 and 1

67. Allocation based on SINR of adjacent cells as High power transmission at cell edges
,affect low power users in adjacent cell.
Approaches can be
-- Using unique frequency hopping pattern for each user base station to randomize
the other cell interference.
-- Having master scheduler for all base stations for multi cell resource allocation

75. In additive noise, the bit error probability (BEP) can be written for virtually any
modulation scheme as:

82. SFBC and STBC