LTE (Long-Term Evolution, commonly marketed as 4G) is a quite new standard for wireless communication of high-speed data for mobile phones and data terminals. The standard is developed by the 3GPP (3rd Generation Partnership Project) and is specified in its Release 8 document series, with minor enhancements described in Release 9.
The LTE specification provides downlink peak rates of 300 Mbit/s, uplink peak rates of 75 Mbit/s and QoS provisions permitting a transfer latency of less than 5 ms in the radio access network. LTE has the ability to manage fast-moving mobiles and supports multi-cast and broadcast streams, supports scalable carrier bandwidths, from 1.4 MHz to 20 MHz and supports both frequency division duplexing (FDD) and time-division duplexing (TDD).
The IP-based network architecture, called the Evolved Packet Core (EPC) and designed to replace the GPRS Core Network, supports seamless handovers for both voice and data to cell towers with older network technology such as GSM, UMTS and CDMA2000. The simpler architecture results in lower operating costs: for example, each E-UTRA cell will support up to four times the data and voice capacity supported by HSPA. Naturally costs will decrease just after the first investment from Mobile Operators.
LTE Downlink Throughput Calculation
The two most important factors for the radio performance in LTE are fading and attenuation due to distance.
LTE throughput depends on the following parameters:
- Channel Bandwidth: According to 3GPP specifications LTE channel bandwidth can be 1.4, 3, 5, 10, 15, 20 MHz. The wider bandwidth the higher throughput. All available spectrum is divided into Resource Blocks (RB). A table to map bandwidth on number of available RB is presented below.
- Channel quality: Radio conditions impact user bit rates. The better radio conditions the higher throughput is available and vice versa.
- eNB (eNodeB, base station) selects Modulation and Coding Scheme (MCS) based on current radio conditions.
- The higher MCS the more bits can be transmitted per time unit. UE (User Equipment, mobile terminal) measures radio channel quality and sends CQI (Channel Quality Indicator) to eNB.
- eNB selects MCS based on the following table (it’s presented for information only and will not be used for further throughput calculations).
- Network load: Available radio resources are divided among active subscribers. So the more subscribers are active and receive/transmit data the less resources are allocated to a given subscriber. It also depends on subscriber and connection (bearer in terms of LTE) priorities.
Maximum Data Rate
Shannon’s Law is a statement in information theory that expresses the maximum possible data speed that can be obtained in a data channel. It has been formulated by Claude Shannon, a mathematician who helped build the foundations for the modern computer.
LTE Advanced key features
With work starting on LTE Advanced, a number of key requirements and key features are coming to light. Although not fixed yet in the specifications, there are many high level aims for the new LTE Advanced specification. These will need to be verified and much work remains to be undertaken in the specifications before these are all fixed.
Currently some of the main headline aims for LTE Advanced can be seen below:
- Peak data rates: downlink – 1 Gbps; uplink – 500 Mbps
- Spectrum efficiency: 3 times greater than LTE
- Peak spectrum efficiency: downlink – 30 bps/Hz; uplink – 15 bps/Hz
- Spectrum use: the ability to support scalable bandwidth use and spectrum aggregation where non-contiguous spectrum needs to be used
- Latency: from Idle to Connected in less than 50 ms and then shorter than 5 ms one way for individual packet transmission
- Cell edge user throughput to be twice that of LTE
- Average user throughput to be 3 times that of LTE
- Mobility: Same as that in LTE
- Compatibility: LTE Advanced shall be capable of interworking with LTE and 3GPP legacy systems
These are many of the development aims for LTE Advanced. Their actual figures and the actual implementation of them will need to be worked out during the specification stage of the system.
As LTE uses different channel bandwidths both for FDD and TDD (1.4, 3, 5, 10, 15 or 20 MHz).
Let’s take the example for LTE using FDD, where channel bandwidth can be 5 MHz, 10 MHz and 20 MHz. In LTE release 8 there is no carrier aggregation, so let’s just consider simple cases.
Here we are calculating maximum data rate at the physical layer. The data rate always depends on the channel bandwidth. For example, in case of 5 MHz channel bandwidth, 300 subcarriers are used. For a perfect idle condition 64 QAM can be used. That means each symbol is now allowed to carry 6 bits.
So the total bits carried by 300 subcarriers for the duration of a symbol is 300 X 6 = 1800 bits.
Again 1 symbol is of 71.4 microseconds for LTE. So the data rate is 1800 / 71.4 = 25.2 Mbps
So the formula for calculating maximum data rate at physical layer is:
(Number of subcarriers x 6) / 71.4 microseconds
For 10 MHz using the same formula the maximum data rate in downlink is 50.4 Mbps and for 20 MHz it is 100.8 Mbps.