LTE Flavors in Unlicensed Spectrum


Guest post by Faris Alfarhan*

The unprecedented increase in demand for high-speed broadband requires a bundle of solutions to satisfy the demanded capacity. Unlicensed spectrum is increasingly considered by cellular operators, internet service providers, and businesses as part of solution set. Unlicensed spectrum cannot match the quality of licensed spectrum, as the interference profile is much more stochastic. However, unlicensed spectrum offers a complimentary solution to licensed carriers for operators, and an opportunity to cable companies and internet service providers – who typically don’t own any licensed spectrum – to deploy wireless networks and hotspots. Read more of this post

Millimeter Wave MIMO Systems for 5G Access Networks

Guest post by Faris Alfarhan*

Cellular NetworkConventionally, millimeter wave (mmW) frequency bands have been either largely overlooked or treated solely as real estate for wireless backhaul and personal indoor networks. That is mainly due to higher atmospheric attenuation loss, penetration losses, and increased absorption and scattering in rainy conditions. However, recent measurements indicate good outdoor short range coverage – of 200 meters on average – when using directive antenna beams, even when radio line of sight conditions are not met [1-3]. The propagation characteristics of mmW bands vary considerably depending on whether LOS or NLOS conditions are present. Since mmW signals experience low diffraction due to their small wavelength, LOS signals propagate in conditions similar to free space (a path loss exponent of 2 on average). NLOS signals, on the contrary, experience more significant losses and hence a pathloss exponent of 5.7 on average [3]. However, the NLOS pathloss exponent is significantly reduced when directing the Tx and Rx antenna beams towards each other. In order to overcome the increased pathloss at mmW frequencies, directional beamforming or beamsteering is used to generate narrow beams towards users. Since the required antenna size is inversely proportional to the operating frequency, mmW antenna arrays could encompass as much as 64-256 antenna elements at the base station and 4-12 elements on a mobile device. For example, the required antenna element length is about 0.5 cm at 28 GHz,whereas it is about 20 cm at 700 MHz. Figure 1 shows measurement results for the maximum coverage distance of a mmW systems operating at 28 GHz as a function of the pathloss exponent and the combined Tx-Rx antenna gains, where acceptable coverage is deemed to have an SNR of 10 dB and higher. Read more of this post

Further Enhanced ICIC (FeICIC)

FeICIC LTE-AdvancedGuest post by Faris Alfarhan*

In an earlier post, R10-LTE enhanced inter-cell interference coordination (eICIC) techniques for heterogeneous networks were discussed, along with the concept of small cell range expansion. The purpose of cell range expansion is to offload more traffic from macro cells to small cells and hence achieve larger cell splitting gains. By adding a cell selection bias, the service area of small cells increases and more users are offloaded to small cells. The need for heterogeneous networks interference management schemes stems from the fact that users in the small cell range expansion area are vulnerable to stronger interference signals than useful signals from the associated serving small cell. In the previous post, it was explained how time domain partitioning based eICIC schemes – known as Almost Blank Subframes (ABS) – could be used to control the interference on the data channels in the range expansion region. Further, carrier aggregation based techniques – known as Cross Carrier Scheduling – could be used to control interference on the control channels (such as the PDCCH, PCFICH, and PHICH channels). However, R10 eICIC schemes did not address interference control on cell-specific reference signals (CRS), which cannot be blanked in order to ensure backward compatibility with R8 and R9 UEs. In this post, R11 improvements to eICIC schemes are discussed, along with the shortcomings of R10 eICIC schemes. First, the concept of Reduced Power Almost Blank Subframes (RP-ABS) is explained along with its advantages over ABS. I then discuss the R11 techniques of Further enhanced ICIC (FeICIC) to control the interference on CRS resources. Read more of this post

An Evaluation of LTE Frequency Selective Scheduling

Guest post by Faris Alfarhan*

Frequency selective scheduling

Channel dependent scheduling is commonly used in cellular systems. In LTE, orthogonal frequency division multiple access (OFDMA) in the downlink and single carrier frequency division multiple access (SC-FDMA) in the uplink allow scheduling to be performed orthogonally in both the frequency and time domains. Instead of exploiting the frequency diversity of the channel, frequency-selective scheduling leverages the channel’s time and frequency selectivity to allocate valuable radio resources in an optimal manner. The OFDMA and SC-FDMA shared channel transmissions incorporated in LTE offer great flexibility for integrating adaptive scheduling strategies. The minimum resource allocation corresponds to a resource block of 180 kHz and a time duration of 0.5 ms. Downlink resource allocation relies on the channel quality index (CQI) reported by the user. For frequency selective scheduling to be applicable, the CQI must be reported for all of the carrier’s resource blocks. Read more of this post