Beamforming in LTE FDD for Multiple Antenna Systems

Abstract

High data rate is still the most crucial factor in cellular systems, and with 5g approaching new techniques are studied to achieve higher bit rates. Using MIMO technology, the spatial domain was exploited through beamforming and multiplexing multiple users’ data together assured more spectrum efficiency which resulted in higher bit rates. Reciprocity based beamforming is feasible in TDD systems due to channel reciprocity assumption between uplink and downlink, the same is not true for FDD systems, as a consequence of using different frequencies between uplink and downlink. Nonetheless, FDD systems are widely deployed because of the existing spectrum assignments and earlier technologies. In addition, FDD systems have advantages such as greater coverage, lower latency over TDD systems.

The thesis objective is to study the feasibility of applying beamforming in FDD systems. We investigate for appropriate techniques based on uplink channel measurements and study its performance for various conditions like Doppler shift, number of users, and also compare those results with beamforming in TDD. Results are generated by implementing LTE physical layer and a practical channel model in MATLAB. Mainly three techniques are implemented, all of these used reciprocity in term of Angle of Arrival between uplink and downlink for an FDD channel.

The obtained results showed that channel reciprocity based TDD systems perform much better than channel non reciprocity based FDD system in terms of the number of users which can be beamformed with an acceptable Bit Error Rate (BER). Also, the used beamforming techniques showed around 50% improvement in overall capacity for one cell, when compared to FDD system with no beamforming applied.

Popular Abstract

Transmitting multiple information simultaneously requires them to be separated. when separated in time and frequency is robust, spatial separation, possible due to multiple antennas, is less robust but can lead to a capacity increase.

When transmitting data for different receivers at the same time through wired connection, it is easy for each receiver to receive their corresponding data, without significant interference, as each receiver has its dedicated wire. But, that's not the case in wireless systems, and due to the waves propagation property (imagine throwing stones in a lake), they will be mixed with each other, thus, lose their embedded information. Therefore, in order for receivers to receive their corresponding data, their signals must be separated in frequency, time or space. While time and frequency domains are used extensively in modern wireless systems, spatial domain is still not fully exploited, which only can be achieved using multiple antennas at the transmitter.

Imagine a wireless system that relies on switching light bulbs on and off for transmitting information, to support multiple receivers, the transmitter can't use the same bulb color, as receivers can't distinguish their information, one solution is to use different colors for each receiver, however, the spatial domain can be exploited by using directional laser light with the same color, so resources will be saved, this what can be called beamforming. However, in order for the transmitter to direct the beam towards receivers, it must have knowledge about the propagation environment, and this what can be called in wireless systems the channel knowledge.

In a wireless full duplex system, again information in both directions should be separated in either time or frequency domain. Wireless systems which separate uplink and downlink information in time is known as TDD systems and in frequency is FDD systems. While acquiring channel knowledge at the transmitter in TDD systems is straightforward, because channel behavior remains constant in both direction, So transmitter can measure the channel in one direction and use the same information to perform beamforming. Same is not true for FDD system, as channel behavior changes. In mobile communications, FDD systems are more common in practice.

Therefore, this thesis is about checking if is it possible to do beamforming in FDD systems, where the transmitter doesn't have full channel knowledge and comparing it to beamforming in TDD systems where transmitter have full channel knowledge. Various techniques were studied and the results showed that beamforming in FDD systems is possible. However, beamforming in TDD systems is far better than the studied techniques. In addition, Increase by 50% in capacity was obtained when using one of the techniques compared to conventional systems.

• author: Hatahet, Atef and Jain, Ankit
• supervisor: Fredrik Tufvesson
• organization: Department of Electrical and Information Technology
• course: EITM02 20181
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Beamforming, FDD, MIMO, LTE, AOA
• report number: LU/LTH-EIT 2018-664
• language: English
• id: 8952784
• date added to LUP: 2018-08-29 10:03:48
• date last changed: 2018-08-29 10:03:48