Wireless Communication fundamentals and 4G Technology

WC LTE

1. 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 IP based Flat network Architecture -fewer nodes and a less hierarchical structure

2. 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.

3. The LTE Network Architecture

4. 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)

5. 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.

6. 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)

7. The SNR loss is given by

8. 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.

9. 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

10. FDMA TDMA CDMA

11. 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 .

12. 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

13. 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.

14. 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

15. 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

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

17. SFBC and STBC