LTE fast facts

LTE targets

• LTE targets more complex spectrum utilization and has less requirements on backward compatibility than earlier technologies
• The work on the evolved network aspects is known as System Architecture Evolution (SAE)
• Release 8 bitrate targets are 100Mbps in downlink and 50Mbps in uplink for 20 Mhz  channel bandwidth.
• LTE supports both FDD and TDD
• Performance requirements should be completely fulfilled for a cell radius up to 5km and only slight degradations are allowed for a cell radius up to 30 km.
• For broadcast, the requirement is an efficiency of 1 bit/s/hz corresponding to around 16 TV-channels using in the order of 300 kbit/s each in a 5 Mhz channel bandwidth.
• LTE should be able to operate in various frequency allocations, from 450 Mhz up to at least 3,5 GHz
• For release 8, support is included for scalable bandwidths of 1.25, 2.5, 5, 10, 20 Mhz
• Mobility targets:

-optimized for low speeds  < 15 km/h

-high performance at speeds up to 120 km/h
-maintain connection at speeds up to 350 km/h

• Latency is divided into control plane and user plane targets. For control plane, it is stated that the transition from an idle state to an active state should be less than 100ms and in case there is a dormant state such as cell_pch in hspa the transition to the active state should take less than 50 ms. In the user plane it is stated that the one way transmission from the terminal to the RAN edge node should take less than 5ms.
• For moving between different Radio Access Technologies (IRAT) it is stated that the interruption time should be less than 500ms for non-realtime services and less than 300ms for realtime services. 
• Spectrum efficiency:

-DL: 3-4 times HSDPA rel6

-UL: 2-3 times HSUPA rel6

• Supported antenna configurations:

-DL:  4*2, 2*2, 1*2, 1*1

-UL:  1*1, 1*1

Overall Time Domain Structure

• A radio frame of length 10ms consists of ten equally sized sub-frames of length 1ms.
• Each frame is identified by its System Frame Number (SFN).
• A sub-frame of length 1ms consists of two slots of length 0,5 ms.
• A slot then consists of six or seven symbols including cyclic prefix.
• A resource block consists of 12 sub-carriers during one 0,5ms slot
• A resource block pair is two contiguous resource blocks in one subframe.
• A resources element consists of one sub-carrier during one symbol interval.
• Each resource block consists of 84 resource elements (12 subcarriers * 7 OFDM symbols)

Cyclic prefix insertion

• In a time dispersive channel, sub-carrier orthogonality will be lost leading to inter-symbol-interference (ISI)
• Cyclic prefix: the last part of each symbol is copied and inserted at the beginning of the symbol which efficiently reduces the ISI as long as the time dispersion does not exceed the length of the cyclic prefix.
• However, cyclic prefix occupy some of the resources that could otherwise be used for data transfer which means that further extending the length of the cyclic prefix becomes inefficient at a certain point.

Channel estimation and reference symbols

• For the transmitter to be able to select the appropriate transport format (coding, modulation, TBS) it needs an accurate estimate of the channel quality or better, the amount of attenuation, noise, interference and phase shift which is the way the channel impacts the transmitted signal.
• In order to estimate each sub-carriers channel quality or channel impulse/frequency response, known reference symbols or pilot symbols are inserted into the OFDM time-frequency grid at regular intervals.

Resource block mapping

• Each resource block consists of 84 resource elements (12 subcarriers * 7 OFDM symbols)
• Some resource elements are used by downlink reference symbols and L1/L2 control signaling
• The physical resource to which the DL-SCH is mapped to is referred to as the Physical Downlink Shared Channel (PDSCH)
• To achieve frequency diversity, the transmission can be mapped to multiple frequency-non-contiguous resource blocks

Spatial multiplexing, MIMO

• In LTE spatial multiplexing, data can be transmitted in several layers.
• Each layer has to be configured to a separate pair of transmit/receive antennas.
• The number of layers is also known as the transmission rank.
• Each layer carries a separate data stream which is precoded in order to make it orthogonal to streams of other layers.
• LTE spatial multiplexing operates in two modes: closed loop and open loop where closed loop requires more extensive feedback from the terminal.
• An increase in the data transmission rate of the same order as the rank number is theoretically possible.
• Spatial multiplexing requires very good channel conditions and is mainly targeted for a small number of simultaneous users in the close vicinity of the base station.
• Or…. Spatial multiplexing has very strong capabilities for utilizing favorable channel conditions that may exist in the close vicinity of the base station.