COMM 907: Spread Spectrum Communications Lecture 10 - LTE (4G) -Technologies used in 4G and 5G
The Need for LTE Long Term Evolution (LTE) With the growth of mobile data and mobile users, it becomes essential to increase the system capacity. There are three main ways to increase the capacity of a mobile communication system: (1) Use of smaller cells. In a cellular network, the channel capacity is the maximum data rate that a single cell can handle. By building extra base stations and reducing the size of each cell, the capacity of a network can be increased.
Long Term Evolution (LTE) (2) Increase the bandwidth but there is only a finite amount of radio spectrum available. Therefore, there are limits as to how far this process can go. (3) Improve the communication technology that we are using. This brings several benefits, it lets us approach ever closer to the theoretical channel capacity and it lets us exploit the higher SNR and greater bandwidth that are made available by the other changes above. This progressive improvement in communication technology is the main reason for the introduction of LTE.
Long Term Evolution (LTE) LTE commonly marketed as 4G LTE. It is a standard for wireless communication of high-speed data for mobile phones and data terminals. The main requirements for the new access network are high spectral efficiency, high peak data rates, short round trip time as well as flexibility in frequency and bandwidth. In LTE, both Frequency-division duplex (FDD) and time-division duplex (TDD) transmission are supported. When using frequency division duplex (FDD), the base station and mobile transmit and receive at the same time, but using different carrier frequencies. Using time division duplex (TDD), they transmit and receive on the same carrier frequency but at different times as shown in Figure 1.
Long Term Evolution (LTE) FDD and TDD modes have different advantages. In FDD mode, the bandwidths of the uplink and downlink are fixed and are usually the same. This makes it suitable for voice communications, in which the uplink and downlink data rates are very similar. In TDD mode, the system can adjust how much time is allocated to the uplink and downlink. This makes it suitable for applications such as web browsing, in which the downlink data rate can be much greater than the rate on the uplink. Figure 1
Long Term Evolution (LTE) Orthogonal Frequency Division Multiple Access (OFDMA) is considered the most appropriate scheme for achieving high spectral efficiency for the LTE downlink. Since, OFDMA suffers from power distortion that may be particularly troublesome in uplink transmissions where excessive complexity in user terminal is an issue. Therefore, for the uplink, the LTE employs SC-FDMA due to its low PAPR properties compared to OFDMA. These multiple access solutions provide orthogonality between the users, reducing the interference and improving the network capacity. The uplink user specific allocation is continuous to enable single carrier transmission while the downlink can use resource blocks freely from different parts of the spectrum as shown in Figure 2.
Long Term Evolution (LTE) Figure 2
Long Term Evolution (LTE) The LTE enables spectrum flexibility where the transmission bandwidth can be selected between 1.4 MHz and 20 MHz depending on the available spectrum. It can also support adaptive modulation and allows the scheme to be changed according to the channel conditions. More capacity is a clear requirement for taking maximum advantage of the available spectrum and base station sites. The terminal power consumption must be minimized to enable more usage of the multimedia applications without recharging the battery.
Long Term Evolution (LTE) High level of mobility and security was used to a good level of security and mobility with the earlier systems starting from GSM, it was also a natural requirement that these should be sustained. Thus, the requirement for the LTE radio round trip time was set to be below 10 milliseconds and access delay below 300 milliseconds.
Downlink: OFDMA Figure 3. OFDMA system model for downlink.
Problems of OFDMA (1) Peak to average power ratio. When the independently modulated subcarriers are added coherently, the instantaneous power will be more than the average power. Fig. High peaks in OFDM signal generated by summing multiple sinusoids
Problems of OFDMA Such high peaks will produce signal that goes into nonlinear region of operation of the power amplifier (PA) at the transmitter, thereby leading to nonlinear distortions and spectral spreading. High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency.
Problems of OFDMA (2) Carrier frequency offset. The ability of OFDMA systems to achieve higher data rates and facilitate bandwidth friendly communication is impaired by the necessity of accurate frequency synchronization between the receiver and the transmitter. With frequency deviation, the sub-carriers will no longer be orthogonal causing inter-carrier interference.
UpLink: SC-FDMA System Figure 4. SC-FDMA system Model for UPlink
Technologies used in 4G and 5G
Cooperative Communication with Relays
Relay communication is a technique to overcome the effects of fading in wireless networks. In the simplest model, it involves the use of an extra radio, which is wirelessly connected to a transmitter and is called the relay, to forward the message to a receiver, as illustrated in Fig. 1.1. Thus, in addition to the (single-hop) direct channel between the transmitter and the receiver, the message travels through the two-hop channel from the transmitter via relay to receiver.
Cooperative Communication with Relays
Fig. 1.2 illustrates the benefits of relay communication in extending radio range and combating shadowing in cellular systems. Two relays RS1 and RS2 assist a base station (BS) to communicate with two mobile stations MS1 and MS2, respectively. Note that for reliable detection and decoding, the desired signal received at a mobile station must have a power value sufficiently above local thermal noise power level. Unfortunately, signals travelling directly from BS to the mobile stations attenuate significantly because of either high path loss (as MS1 is located outside the boundary of BS's cell area) or shadowing (as there is a building between BS and MS2).
Signals traveling via the relays experience less power attenuation on BS-relay hop and relay-ms hop, thus improving the received signal strength at MS1 and MS2. Of course, in terms of coverage extension, relays are not needed when mobile stations, e.g., MS3, could receive strong enough signals directly from BS.
Relay Transmission Strategies Decode and Forward Amplify and Forward Compress and forward Most popular are: Decode and Forward (DF) and Amplify and Forward (AF)
Decode and Forward (DF) During the first interval, the transmitter sends the signal to the relay: The relay decode and re-encode the received signal and during the second interval it forwards it to the destination:
Amplify During the first interval, the transmitter sends the signal to the relay: and Forward (AF) relay multiplies its received signal by a coefficient (Gain) and during the second interval it forwards it to the destination.
Comparison DF relays have higher computational complexity due to the requirement of decoding their received signals. They are only helpful if they can successfully decode the signals. DF has better BER performance than Amplify and forward since AF amplifies the noise. AF relays have lower complexity and faster signal processing. AF relays provide higher end-to end throughput (e.g., when the transmitter-relay channel is weak), than DF relays in singleantenna half-duplex relay. Moreover, DF relays act as conventional users while receiving data from base stations. They also act as conventional base stations while transmitting to users. Thus, DF relays can be integrated into current cellular systems with less required changes than AF relays.
Compress and Forward (CF) The relay transmits a quantized and compressed version of the received signal message. The quantization and compression process is a process of source coding.
Introduction to MIMO
MIMO System MIMO System can be classified into: - Spatial Multiplexing (SM) - Space Time Block Coding (STBC) - Hybrid SM/STBC
MIMO System (Spatial Multiplexing)
MIMO System (Spatial Multiplexing) Spatial Multiplexing (SM) aims to increasing the data rate but not the robustness of the transmission. The serial symbol stream is converted to parallel sub-streams. These sub-streams are transmitted simultaneously from nt antennas and received by nr antennas.
Space Time Block Code In STBC, the modulated symbols and their replicas are transmitted from multiple antennas at different time slots (to achieve diversity). The main difference between STBC and SM is that SM just de-multiplexes the modulated symbols over nt transmit antennas and sends them at one time slot. Consequently, SM offers higher capacity (in multiplexing), STBC offers higher reliability (in diversity)
Space Time Block Code
Hybrid SM/STBC
Hybrid SM/STBC SM, STBC, and Hybrid SM/STBC can be defined as: increasing data rate, enhances reliability, and compromise between the two extremes respectively. This imposes a trade of between capacity and reliability where both are traded off with complexity