LTE – Long Term Evolution
We have all used several wireless networks for multiple tasks, yet one thing they all lack is the concept of singularity in communication. For cellular we use HSDPA/HSUPA( our latest 3.5 G) or 2G networks in some countries, WIFI – IEEE 802.11a/b/g for our Internet access also some of the land lines are wireless differing in technologies. For each of this technology to exist different equipments and frequency bands are used. What LTE does is unify all this into a single entity for communication making the end-user save money and feel the comfort. WiMAX is a serious competitor to this technology but it loses to LTE one all grounds except for its deployment, which is complete in few countries.
LTE – Long Term Evolution is one of the latest communication technology that is currently being tested and deployed. LTE falls under 3G dubbed as 3.99G and LTE-Advanced is 4G compliant. 3GPP Release 8 defines the standards for LTE and Release 10 pertains to defining the standards for LTE-Advanced. LTE was formulated as a result of study on Evolved UTRA(Universal Terrestrial Radio Access) done by 3GPP. The System Architecture Evolution(SAE) – all IP based network is the core Network Architecture for establishing a LTE Network.
LTE Benefits :
- LTE can provide practical data rates of 100Mbps – downlink and 50 Mbps uplink (20 MHz
- LTE has very low latency period of <10ms.
- LTE has very low control plane latency <100ms.
- LTE has high spectral efficiency.
- Scalable bandwidth – 1.25, 1.6,2.5,5,10,15,20 MHz.
The reason behind how every benefit above is achieved is discussed below.
LTE uses Orthogonal Frequency Division Multiplexing – OFDM. For modulation it employs 64 QAM( Quadrature Amplitude Modulation Technique) which combines both ASK- amplitude shift keying and PSK- phase shift keying thereby enabling several bits to be transmitted per symbol. Every symbol used will now be a result of a particular amplitude and phase. Also the adjacent symbols are now wider hence bringing down the Bit Error Ratio. In LTE a 64 array QAM is used.
The data is multiplexed in several slow rate channels which sums up to more data. These sinusoidal signals as spaced close together orthogonally without interfering with each other. Hence the available spectrum is efficiently used i.e. more bits per seconds per Hz (Spectral Efficiency).Every sub-carrier is modulated by using QAM technique. OFDM signal is hence a composite of several low rate data streams which are flat-fading channels.
Time Domain – Orthogonality is ensured by limiting the integral number of cycles occurs for each
within a specific time window.
Frequency Domain – Ensured by placing the peak of any one of the carrier wave with the nulls of
The benefit from such spacing is that the demodulator can now easily and distinctly distinguish between the frequencies and hence the receiver extracts the correct frequencies. This is highly essential in terrestrial environment with several distortions.
Each Wave is spaced Orthogonally In the figure.
The binary source signal is demultiplexed into several parallel streams. These parallel streams of data are then modulated using QAM technique. These are then converted to time-domain samples by Inverse-FFT. The signal is converted to analog and then the carrier wave is added to it and transmitted.
OFDM Receiver :
The receiver system receives the analog signal is passed to the low pass filter to reject the 2fc and converted back to digital signal. A FFT converts them back to frequency domain. The output is a set of parallel streams which is passed though a symbol detector and converted to s series stream of the original signal.
Sub Carrier Spacing :
The Transmission bandwidth is usually from 1.25 – 20 MHz with a sub carrier spacing of 15 KHz. This concept of transmitting several narrow-band sub carriers is built into the circuits of the downlink radio station. OFDM used guard interval alias cyclic prefixes( Repeat of end signal at beginning) this eliminates the necessity of receiver side channel equalizer also reduces the inter- symbol interference aiding the high symbol rate modulation technique. This saves power and cost.
Multiple Antenna :
Since the distributive channel effects are eliminated in OFDM, MIMO – Multiple Input Multiple Output radio antennas can be used effectively also OFDM divides the channel into several sub- frequencies making this is an effective scheme to implement. This boosts the throughput as several independent data streams are transmitted in parallel via different antennas.
If the length of the guard interval is equal or less than the MIMO channel order then OFDM generates a set of parallel frequency flat MIMO channels. If the MIMO channel order is more than the guard interval length per tone equalization is used. While designing the interference is taken to our advantage by beamforming, this improves the throughput at the cell edge. NLOS- NON-LINE- OF_SIGHT is effectively achieved. Also the channel has a total of (number of transmission antennas)*(number of receiver antennas). This significantly improves the data rates. Say 6 base station antennas are set up with respect to 6 terminal side receive antennas hence one will be receiving 6 parallel data streams providing a theoretical multiplicating factor of 6 to the throughput. This is called Spatial multiplexing wherein the transmission rate boost is proportional to the number of transmission side antennas. MIMO only increases the spatial paths between the receiver and the transmitter. Hence the bandwidth is not increased to increase the throughput rather the spatial domain is exploited to increase the throughput making it unique and highly efficient.
The above table shows the SNR for a Rx to Tx antennas. It can be inferred that a 41 % increase in SNR making such an implementation highly efficient. This will also enable the signal to propagate further as it has a high SNR. This will hence bring down the installation of number of standalone transmitters in an area bring down the cost greatly.
Interference to advantage :
As a result of multi-path propagation which may also result as a result of obstructions or various interferences there would be loss in packets. The data rate is always favoured to be constant for voice. Whereas in packet-data services it is not required at differential intervals i.e. falling short interval peaks of data rate will not be noticeable. LTE uses this phenomenon instead of suppressing the negative surges of data rate to efficiently uses the mobile resources.
Link sharing :
Link sharing aims at subdividing the available resources to the pertinent users. LTE systems employ dynamic link scheduling with a latency period of ~10ms. The network can adapt to the instantaneous load. The data rate is varied depending on the applications transfer rates required. If rich multimedia content then it switches to better rates. If just voice or chat the it can be switched to lower data rates. The switching between rates can be employed by changing the modulation order - In case of low data rates 16QAM is used if higher rates are required the switched to 64QAM. Since the users share a common channel the resources are dynamically allocated. The basic idea is to transmit at the fading peaks where the channel’s quality is high. This is because this architecture doesn’t use power control (Power varies to keep data rate constant) ,it uses Rate Control as shown. Now the battle is to determine how to allocate the users to channel at these instances. It is done in time, frequency and spatial domains to conserve the mobile resources. OFDM spectral efficiency aids the increase in time duration for each user in the peak mode.
Round-Robin : Round-Robin method can be used to allocate users with respect to time disregarding the channel quality. This is an unfair policy to the user.
Proportional Fair : This is a better method and the one employed in high performance LTE networks. Time Slot for every user I almost the same or atleast fair pit pushes the data when the channel’s condition improves. This is a fair policy to the user
Sub-Carrier Allocation :
The OFDMA orthogonal frequency division multiple access characterizes every user with a sub carrier. In a single channel spectrum several sub carriers are allocated to every used based on his requirement. This scheme is particularly useful in downlink when the number of users is high. If the data rate required is low then scheme is adapted as it consumes less resource and the delay is reduced effectively. The mobile users can be synchronised in time domain and frequency domain. This makes the uplink to be orthogonal and in sync. In LTE the data is sent in resource blocks with each of them encompassing 12 sub-carries in a single time slot. This resource block size is maintained constant.
Time And Freq Domain – user scheduling
The Channel’s quality can also be taken in this scenario wherein the user’s domain pertains to a particular time and frequency period with respect to the channel quality. Here the information on channel quality will have to be in transmission so that the base station can effectively slot the user in that that particular time and frequency domain. LTE reports the channel quality every 1 ms. At low mobility speeds frequency selective scheduling performs equally or even better than stationary as the channel quality varies rapidly hence there is more chance to transmit at good channel quality. The mobile device’s performance varies on different portions on the spectrum since the high frequency (20 MHz/0 causes frequency-selective fading. This best sub-frequencies can be determined by feedback information(Channel quality information) from the mobile device. There are two ways to select the better sub-carrier – Sequential scheduler and matrix based scheduler. The matrix based scheduler performs slightly better. (Inferred from Frequency Selective OFDMA
Scheduler with limited feedback – Wireless Communications and Networking Conference, 2008. WCNC 2008. IEEE published.)
Two Signals are primarily used to assign the resource elements to the UE.
Reference Signal : It is a product of an Orthogonal sequence and PRN sequence(pseudo-
random numerical). 3GPP estimates 510 unique reference signals. Each can be assigned to a cell to
identify it in the network.
Synchronisation Signals : There are two Synchronization signal Primary and Secondary.
They are used in the cell search by mobile device. They are transmitter in the 0 and 10th slot in the
Uplink Modulation & Sharing:
The mobile device has limited power. The OFDM and OFDMA have high peak to average power ratio which makes it energy consuming hence is not favourable to a mobile device. So in the uplinks a Single-Carrier DFT spread OFDM is used with better Peak to average Power Ratio. This is more efficient in using the power at optimum coverage saving the resources of the mobile device. 3GPP has standardised on using the Single Carrier – FDMA (SC-FDMA). LTE Uplink is Orthogonal in regards with a single cell to which they are connected. A Distributed mapping is chosen for the SC-FDMA to exploit the orthogonality. This reduces the overall peak to average power ratio. The users in a cell can communicate without any interference between them also the null sub-carriers are used.
Uplink Channel Sharing
The mobile device might have frequency domain in channel with a band of sub-carriers in communication with the cell it is connected to. A distant cell will have another device using the same sub-carriers. This could cause interference. Also if the neighbouring cell is closer it causes strong interference. Hence during designing of the network structure the cells are placed at an optimum rage from each other limiting the interference it a low value. With respect to the mobile device its identification can be easily done with respect to its proximity to each cell and removing the interference. The proximity detection also aid in cell switching when the user is in motion.
Error Handling :
The Data sent is in packets hence there might be a loss of packets as a result of noise interference or fading as in any wireless network. Hence LTE implements Two Layered Retransmission schemes to preserve the data. A Fast Hybrid Automatic Repeat request(HARQ) protocol is implemented over a Selective repeat ARQ protocol. i.e. Forward Error Correction bits are appended over the traditional CRCs.
The ruptured data blocks are not discarded and are stored by the receiver. The retransmitted block is combined with it and then decoded. The joining between the blocks is done by incremental redundancy. Every retransmitted block is punctured and transmitted. The puncture patterns can be deducted from the retransmission number.
LTE Network Architecture :
CORE Architecture :
System Architecture Evolution is the Core-Network Architecture. The important factor of this network architecture is that it is heterogeneous. It enables other wireless networks to co-exist (2.5G, 3G) and LTE is merely an extension. SAE has been evolved from the GPRS Core Network. This makes the implementation on the LTE networks cheap and use the existing network infrastructure. This makes LTE a much better choice in comparison with the other wireless technologies. Also the upgrade to the LTE Advanced (4G)will be a much smoother process. The SAE Architecture is an all IP based network. Every service in Network is in PS ( Packet Service) Domain and not CS (Circuit Switched) Domain. The Architecture also inadvertently supports mobility to other systems like the WiMAX, 2G and 3G( Although latter many not be necessary the former will be essential).
In addition to the GPRS core of the network, the Evolved Packet Core embedded to facilitate LTE. This will provide IP-Based voice and data service simultaneously. Also the scalability of the services application and users will increase. There will also be minimal upgrades once it is embedded into the network as the services become IP based. The Control Plane functions can now be made simpler and easier to implement.
Radio Access Architecture:
The 3GPP has set the standards for the Evolved UMTS(Universal Mobile Telecommunication System) Terrestrial Radio Access Network – E-UTRAN. The Mobile devices connect to these radio networks. The communication is IP packet based making the Entire system unified in the IP network. The packet data based network enables the switch between the speeds for voice-data packets and rich multimedia packets offering high data rates.
Each Frame consists of 10 sub-frames containing 14 OFDM Symbols. The Frame is of 10 ms duration and each sub frame is 1 ms in duration. Now dividing the sub-frame.
The LTE downlink physical channels consist of ,
Physical Downlink Shared Channel (PDSCH) – consumes 3 ODFM symbols per sub-frame. This is 21% of the
Symbols in the sub-frame, hence 14% over- head for control.
Physical Downlink Control Channel (PDCCH) – consumes 3 ODFM symbols. It states the mobile device specific information to optimise the communication.
Common Control Physical Channel (CCPCH) - It states the cell-wide control information. It is transmitted near to the centre frequency. It is the CCFI – Control Channel Format Indicator. It is transmitted in the Physical Control Format Indicator Channel(PCFICH). The Cyclic prefixes used can be seen in the diagram, They also have pilots as a reference for the MIMO. Every Resource Block can be divided into 12(sub carrier frequency range) x 14 Symbols and the mapping is bade on scheduling – localized or Distributed( Distributed preferred ).
LTE Channel Architecture :
The RLC (Radio Link Control) Services the higher layers. It passes the data to the MAC Layer as Logical channel. The MAC Layer constructs the frame and passes the frame to the Physical Layer. The Packet Data Convergence Protocol ( PDPC) employs ROHC( Robust Header Compression ) to compress the TCP or IP or UDP headers. The ARQ does the error correction and the HARQ brings downs the number of retransmission.
Transport Channels : The logical channel data is framed and sent as transport channel to the physical layer from the MAC layer.
Broadcast Control Channel (BCH) : Broadcast in the network with system information. Downlink Shared Channel : Implements HARQ and discontinuous receive to save energy also aids in resource allocation.
Paging Control Channel (PCH) : Broadcast in the network. It continuously states information on dynamically allocated resources so change can be monitored. Also enables discontinuous reception hence saving power in the device.
Multicast Channel (MCH): Transmission to multiple devices.
Logical Channels Broadcast Control Channel (BCCH): Label broadcast in the network with system information.
Paging Control Channel (PCCH) : Broadcast in the network. It continuously states informationon dynamically allocated resources so change can be monitored.
Common Control Channel (CCCH): transmits control information of the network to the device.
Multicast Control Channel (MCCH): Groups within the network get common information.
System Architecture Evolution SAE :
The E-UTRAN can be integrated only with SAE. The Evolved Packet Core(EPC) consists of two components.3GPP Anchor and the SAE Anchor. The MME connects the radio network to the EPC. The Mobility Management Entity corresponds to the user equipment control plane context by authorising the mobile device and the Public Land Mobile network is identified and registered. Every PLMN is interconnected and is unique to a subscriber. The MME also implements NON- Access Stratum security and idle state mobility handling. The MME support tracking and handover. The 3GPP anchor provides mobility between the GRPS core and the LTE i.e. 3GPP and non 3GPP systems. The SAE Anchor Provides mobility between the LTE and WLAN technologies( Inter- mobility within 3GPP systems).
The Packet network gateway of the EPC registers the mobile device with an IP address. IP based packet filtering can also be done at this stage. Here the bandwidth allocation or limitations can be set to every restricted user which in turn can be used in billing factors to determine the usage. The control over the application can also be done as here the IP level control is present. Further Firewall and content filtering can be implemented here.
Since the mobile devices are under constant motion it is always better and efficient to preserve the IP address rather than give a new one every time it switches between cells. The Network Based mobility management ensures that the homing of IP to the same mobile device is ensured. When the UE switches to another cell. The Network entity providing the IP address to the UE ensures that its location in the MME’s routing table is updated. So that the path is changed based on the cell to which the device is associated. The following are the components involved in the mobility architecture.
PDN Gateway(Packet Data Network Gateway) : This entity assign the IP address to the mobile device it can be IPv4 or IPv6. The PDN Gateways are employed by the PDN in the EPC. The Gateway’s identity is to redirect the device during the handover and authentication. The PDN is usually a Operator network form where the user is connected within the network of the operator to allow faster communication between he mobile devices that are registered to the same operator in the city of state. this brings downs the cost to the service provided as not external factor in involved. This makes the call rates really affordable by the users within the same network. It is also connected to the Internet wherein PDN acts as a gateway. The connection to Internet is set in when online contents are viewed or communication with other subscribers. Also connection between subscribers can be made to save cost by elimination the usage of Internet for voice call within the state.
Servicing Gateway (S-GW) : This aids in the mobility between different 3GPP radio networks E-UTRAN UTRAN and acts at layer 2.
Access Gateway (A-GW) : This facilitates the mobility between 3GPP and NON-3GPP radio networks.
Evolved Packet Data gateway : In case of untrusted non- 3GPP networks the security is established by using IPSec over a validated session from the user to the core. 3GPP has standardised IMSI- International mobile subscriber identity to be given at all the subscribed mobile device this uniquely identifies the mobile device across the different networks this makes the device connectable between Non-3GPP and 3GPP with an identity.
SGSN (Serving GPRS Support Node ) : The SGSN which has inbuilt mobility management forwards the incoming and outgoing packets to the mobile Station the existing 2G and 3G networks data traffic is also passed thorough it.
LTE providing end to end IP service will improve the mobile services and will bring in the concept of singularity to the mobile devices. Wherein all mobile devices ranging from pagers to laptops can be serviced by the same network. The Architecture is built over the existing framework making it easier for the companies to transfer to. Already Companies such as Qualcomm, Motorola, Alcatel-Lucent etc.. have tested and demonstrated the capabilities of LTE network and will soon become the future network overtaking technologies such as WiMAX. Aslo the upgrade to 4G – LTE Advanced will be a lot easier. The most important benefit that I observe is cost effectivness to the Mobile Network operators. When every data that traverses is just packets then the control becomes easier and scalability increases. The operators can provide more benefits at very low cost making high returns.
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