LTE UNLICENSED

LTE Advanced : Real 4GLTE-A is designed keeping in view the ever increasing data needs.  LTE has higher bitrates in a cost efficient way and, at the same time, completely fulfil the requirements set by ITU for IMT Advanced, also referred to as 4G. Increased peak data rate, DL 3 Gbps, UL 1.5 Gbps.

Higher spectral efficiency, from a maximum of 16bps/Hz in R8 to 30 bps/Hz in R10.

Increased number of simultaneously active subscribers

Improved performance at cell edges, e.g. for DL 2x2 MIMO at least 2.40 bps/Hz/cell.

Carrier Aggregation The most straightforward way to increase capacity is to add more bandwidth.

Each aggregated carrier is referred to as a component carrier. The component carrier can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a maximum of five component carriers can be aggregated. Hence the maximum bandwidth is 100 MHz.

The number of aggregated carriers can be different in DL and UL, however the number of UL component carriers is never larger than the number of DL component carriers. The individual component carriers can also be of different bandwidths

MIMO  is used to increase the overall bitrate through transmission of two (or more) different data streams on two (or more) different antennas - using the same resources in both frequency and time, separated only through use of different reference signals - to be received by two or more antennas.

MIMO, Multiple Input Multiple Output (Spatial multiplexing)

Coordinated Multi Point operation (CoMP)

The main reason to introduce CoMP is to improve network performance at cell edges.

In CoMP a number of TX (transmit) points provide coordinated transmission in the DL, and a number of RX (receive) points provide coordinated reception in the UL.It is of two types :

Types of Transmission

Coexistence with Wi-Fi

Wi-Fi is a main user of unlicensed spectrum. Moving LTE to unlicensed spectrum poses a condition of coexistence with Wi-Fi.Type of coexistence mechanism depends on the regulation on the unlicensed spectrum which differs from country to country.

Mechanisms for non-LBT marketsChannel

SelectionLTE-U small cells will scan the unlicensed band and identify the cleanest channels for the SDL carrier transmission. If interference is found in the operating channel and there is another cleaner channel available, the SDL transmission will be switched to the new channel.

Channel Sense Adaptive Transmission (CSAT)

The goal of these algorithms then is to provide coexistence across different technologies in a TDM fashion. In CSAT, the small cell senses the medium for longer (than LBT and CSMA) duration (around 10s of msec to 200msec) and according to the observed medium activities, the algorithm gates off LTE transmission proportionally.

The duty cycle of transmission vs gating off is dictated by the sensed medium activity of other technologies.

CSAT ensures fair and efficient channel sharing, with the impact of a LTE-U node to its neighbouring Wi-Fi APs no worse than a neighbouring Wi-Fi AP.CSAT in spirit is similar to CSMA except that it has longer latency, an impact that is mitigated by avoiding channels where Wi-Fi APs use for discovery signals and Qos traffic.

Opportunistic SDL Since the anchor carrier in license band is always available, the SDL carrier in unlicensed band can be used on an opportunistic base.

When the DL traffic of the small cell exceeds a certain threshold and there are active user within the unlicensed band coverage area, the SDL carrier can be turned on for offloading. Opportunistic SDL mitigates the interference from continuous RS transmission from LTE-U in unlicensed channel, reducing the interference in and around a given channel.

Mechanisms for LBT marketsWi-Fi has DIFS mechanism which is absent in LTE-U. This causes the LTE to posess the channel most of the time not giving chance for Wi-Fi. Thus there must be modifications to the PHY and MAC layers of LTE in order to accommodate in LBT markets.

Challenges• Unlicensed frequencies serves as catalyst for innovation.

• Holders of spectrum licenses should not have special claim to unlicensed spectrum.

• Robust coexistence mechanisms with other RATs in the unlicensed bands.

ConclusionSmall cells and efficient and healthy utilization of unlicensed spectrum is the only answer for the 1000x data challenge and LTE-U lays the pathway for meeting it.

Data intelligent devices will be coming in future which we will use all the available technologies to give the user the best of whatever required.

References Dr T. Bheemarjun Reddy, HOD, Dept Of CSE, IIT Hyderabad.

‘ Best use of unlicensed Spectrum’ by Durga Malladi, Qualcomm Inc.

Qualcomm Research LTE in unlicensed spectrum

Nokia White Paper on LTE in unlicensed spectrum

DSA Postion Paper on Unlicensed LTE

3GPP Rel 10/11/12/13