Benefits of Beamforming | Advanced Antenna Technologies in WiMAX

System Design Aspects on BS and MS

All the complexity and intelligence of a beamforming system is inside the base station. In addition to the spatial signal processing unit, the BS includes as many transceivers as the number of antenna elements. Usually, wireless systems implementing beamforming are operating between two to eight antenna elements and transceivers.

On the MS side, the impact for the support of beamforming is minor. Basically, beamforming can be applied to any MS. However, in order to provide better performance, the standards (and in particular IEEE 802.16e) define additional messages/procedures between the BS and the MS. Nevertheless, there is no hardware impact on the terminal side; only some additional software is needed for an optimised beamforming operation.

The beamforming algorithms aim to combine coherently the signal transmitted/received from different antenna elements. In order to achieve this correlation, the antenna elements need to be closely separated. For an optimum performance of beamforming, a spacing of half the wavelength λ is preferred. Consequently, assuming a four-element antenna array at 2.5 GHz would result in an antenna of about 22 to 25 cm width. The array itself is thus of a relatively small size, which is very beneficial for visual impacts on the environment.

Besides, in order to maintain the signal coherence, mechanisms for calibrating the different transmit/receive paths are required, the algorithm for doing this being vendor-specific.

Benefits of Beamforming

The benefits of beamforming are manyfold: range increase and power saving at the MS side, interference mitigation and capacity increase.

First, beamforming improves the link budget for the data transmission for both the downlink and the uplink. Indeed, by concentrating the energy in one direction, the resulting antenna gain in one direction is significantly increased (see Figure 1). This additional gain is beneficial for improving the coverage of the BS (less sites needed for a deployment) and/or for reducing the power needed by the MS to transmit signals (power saving).

Figure 1: Range extension with beamforming

Theoretical gains, compared with a conventional antenna, for an N-element antenna arrays are of 10×log(N) for the uplink and 20×log(N) for the downlink. For example, with a four-element antenna arrays the gains are respectively of 6 dB (12 dB) for the uplink (respectively the downlink). The gain in the downlink is higher since, on top of the beamforming gain, the power from each transmitter coherently increases. The value of those gains has been validated in many experiments on the field and proves to be in line with the theory. Additional gains are measured in the uplink due to the additional spatial diversity gain.

Second, because the energy is focused in the direction of the user, there is a general interference reduction in a cellular system employing beamforming. Indeed, when beamforming is deployed on the BS of a given geographical area, the beams are oriented as a function of the repartition of the users served in a cell; at one moment, on a given radio resource, a single user is served. As a consequence, the interference created by the communication of this user is only in a restricted angle compared to sectorised antenna deployment (see Figure 2).

Figure 2: Interference reduction with beamforming

The angle spread of the main lobe is approximately the total angle of the sector divided by the number of antenna elements N. For example, with a four-element antenna array and a 90° antenna, the resulting main lobe width (at −3 dB) is around 220°. Therefore, since the users are randomly spread, the beam directions change according to the user locations, which create additional interference diversity gain. The interference reduction is further improved with the use of explicit interference cancellation algorithms.

Two direct consequences of the interference reduction are: a better signal quality and availability across the cell area and a better capacity in the cell for systems using link adaptation. Indeed, since the CINR values are better, the possibility of using a better modulation and coding scheme is higher.

Beamforming or AAS Technogies | Advanced Antenna Technologies in WiMAX

Beamforming or AAS Technogies

Beamforming technologies may be encountered behind several wordings: smart antenna, beamforming and Adaptive Antenna System (AAS). In the following beamforming will be used.

Beamforming Basics

The main objective of beamforming technology is to take benefit from the space/time nature of the propagation channel. Indeed, due to multiple reflections, diffraction and scattering on the transmitter to receiver path in a cellular environment, the energy reaching the BS comes from multiple directions, each direction being affected by a different attenuation and phase.

In a macrocellular environment (i.e. the antenna of the BS is above the rooftop) the signals reaching the BS are inside a cone. The angular spread of the signal depends on the environment. In a urban environment, the angular spread is of the order of 20 degrees. In a more open environment, like in a rural environment, the angular spread is a few degrees.

In the uplink, the beamforming technology principle is to coherently combine the signals received for N antenna elements of an antenna array. A generic beamforming diagram is shown in Figure 1. A block diagram of a beamforming receiver (respectively transmitter) with an N-element antenna array is shown in Figure 1 (respectively Figure 2). In the case of a block diagram of a beamforming receiver with an N-element antenna array, a signal processing unit analyses the same signal received from the N antenna elements and computes weights (w_i) that are applied on each path for combining.

Figure 1: Example of a block diagram of a beamforming receiver with an N-element antenna array

Figure 2: Example of a block diagram of a beamforming transmitter with an N-element antenna array

On the downlink, the processing is very similar to the uplink. Based on the information measured on the signal received in the uplink, it is possible to estimate the Direction of Arrival (DoA) from the uplink signal and to apply different weights, z_i (amplitude and phase), to the different transmit paths of the same signal, so that the resulting antenna pattern focuses towards the direction of the user.

Since the weights in the downlink depend on the uplink signals, this assumes certain channel reciprocity between the uplink and downlink signals since the BS do not know the downlink spatial channel response. Actually, the reciprocity can more realistically be assumed in the case of the TDD system since the uplink and downlink signals use the same frequency at different time intervals. On the FDD system, the reciprocity is more difficult to assess.

In fact, beamforming technology encompasses several techniques. First implementations of beamforming were based on simple antenna switching mechanisms: in that approach, the elements of the antenna array where simply switched on or off according to the received signals. This has the advantage of simplicity but the possibility for beamforming is limited. Today, beamforming uses an adaptive array: the amplitude and phase of each antenna element can be set independently. This has the advantage of having the possibility to achieve infinity of beams.

With adaptive beamforming, several optimising strategies may be used. The signal processing unit must maximise the received CINR. This can be achieved by having a resulting antenna pattern such that the antenna array creates a null in the direction of arrival of a strong interferer. However, the number of interferers that can be cancelled are limited by the number of elements constituting the array: with N antenna elements, it possible to have at most null N−1 interferers. In addition, this technique requires a good knowledge of the radio environment (which may imply additional overheads). This explains why in many implemented systems this method is mainly used in the uplink, where the BS can have maximum knowledge of the radio environment.

Finally, an advanced implementation of beamforming can enable SDMA (Spatial Division Multiple Access). Provided that two or more users are sufficiently separated in space, it is possible to send them at the same time, on the same physical resources, different information on different beams. Nevertheless, the use of SDMA is quite difficult in a mobile environment where MSs that may be well separated at a given moment may be in the same direction at the next moment. 

Radio Engineering Consideration for WiMAX Systems

WiMAX solutions include fixed WiMAX and mobile WiMAX. According to the type of terminal offered (outdoor CPE (Consumer Premise Equipment), self-install indoor CPE, mobile terminal–PCMCIA, laptop, smart phones), each solution has a different deployment method.

LOS/NLOS Propagation
Coverage from a BS is linked to several radio parameters. The first one is the propagation environment. Depending on the relative locations of the BS and the terminal, several models can be used to evaluate the losses due to propagation.

LOS and Near LOS Propagation
Line-of-Sight (LoS) and Near Line-of-Sight (NLoS) propagations may happen when the BS and the MS are deployed outdoor, above the average height of the environment. This may be the case of the deployment of a fixed WiMAX solution in a rural environment with the BS located on a high altitude point (or at the top of a mast) and the SS (then a CPE, for a fixed WiMAX) deployed on the rooftop of the customer's house.

LOS propagation requires that the first Fresnel zone is free from any obstacle. In that case, the propagation losses are proportional to the square of the distance between the BS and the SS. However, in a practical deployment, this happens very seldom. Usually, the Fresnel zone is obstructed and/or there are few obstacles on the BS to the CPE transmission path. In that case, near LOS models are used. However, in the case where there are few obstacles (a building, a water tower, a hill), the obstacles are modelled by knife-edge. An example of such a model can be found in Reference.

NLOS Propagation
NLOS (Non-Line-of-Sight) propagation occurs when the terminal is located indoor and/or at ground level. In this situation, there is in most cases no direct path between the BS and the terminal, there is a high number of obstacles on the BS to MS path (buildings, trees, cars, etc.) and the receiver may receive several copies of signal that experienced several reflections/diffractions on different obstacles. This type of propagation is typical of cellular deployments.

In the case of WiMAX, NLOS propagation corresponds to a deployment of a fixed or mobile WiMAX using self-install indoor terminals (wireless DSL deployment), or a deployment of a mobile WiMAX with mobile terminals (PCMCIA, laptop with integrated chipset, multimode mobile phones, etc.).

In NLOS, the losses versus the distance are much higher that in LOS. Usually, the distance decay exponent is between 3 and 4. An example of a propagation model that can be used to evaluate propagation losses in NLoS is the Erceg model.

Radio Parameters and System Gains
In order to evaluate the range, additional parameters are required. First, the value of some propagation parameters depends on additional radio parameters such as:

• Frequency band. WiMAX could be deployed at different frequencies (2.3 GHz, 2.5 GHz and 3.5 GHz); the higher the frequency, the higher the propagation losses.

• BS antenna height. Propagation losses decrease if the antenna height is increased.

• Terminal antenna height. The lower the terminal, the higher the losses (for mobile deployment, the height of the terminal is usually taken to be between 1.5 and 2m).

Then, depending on the deployment scenario, a margin modelling the fluctuations of the radio environment needs to be considered.

In the case of the LOS/near LOS scenario, the BS and the MS are fixed. However, the propagation loss fluctuates (according, for example, to geoclimatic parameters). Hence, to evaluate the range, a margin needs to be evaluated according to the signal availability required (e.g. 99%) and the geoclimatic parameters. A method to evaluate the margin is available in Reference.

In the case of the NLOS scenario, in order to reflect the distribution of the obstacles in the coverage area, a margin, called the shadowing margin, needs to be taken into account for the evaluation of the range. This shadowing effect is modelled by a lognormal distribution with a standard deviation, which depends on the environment (typically from 10dB in an urban environment to 5 dB in rural environments). The resulting margin also depends on the signal probability availability on the cell area (usually between 90 and 95%). In addition, other margins may be included in the propagation losses: indoor margin (in the case of indoor coverage requirements), interference margin (to reflect the interference level generated by other cells transmitting at the same frequency), body losses, etc.

Finally, to evaluate the range, the system gain may be evaluated. The system gain is the maximum signal level difference that can be accepted on the BS-to-terminal path so that the receiver may decode a signal with sufficient quality. The system gains depend on many specific parameters:

• Transmission power of the BS/terminal.

• Antenna gain of the BS/terminal. The gain at the terminal side depends on the type of terminal. Outdoor CPEs may have gains in excess of 14dBi while mobile terminals may have only a 0dBi antenna gain.

• Receiver sensitivity. The receiver sensitivity depends on the signal bandwidth, the modulation and the coding scheme. The IEEE 802.16 standards are providing minimum reference values but vendors may come with better performance solutions.

WiMAX Radio Features that Enhance the Range
For fixed WiMAX solutions deploying outdoor CPEs, the coverage may be of several km. However, to get range figures in line with an exiting cellular system for WiMAX cellular-like deployments, additional features are needed to compensate for the extra losses due to lower terminal antenna gains, indoor penetration and NLOS behaviour.

The radio enhancement feature applicable to fixed and mobile WiMAXs is subchannelisation. This is the possibility to concentrate the transmit power on a few subchannels and thus to play with trade-off between the coverage and the maximum data rate a terminal can get at the cell edge. Other enhancement features that are only applicable to the mobile WiMAX are:

• Convolutional Turbo Coding (CTC).

• Repetition Coding. For each retransmission, up to 3 dB gain can be obtained. This again allows a trade-off between the maximum range and data rate at the cell edge.

• Hybrid ARQ (HARQ). This is the capability of the receiver to combine several transmissions of the same MAC PDUs.

• Advanced antenna technologies: beamforming and MIMO.

Frequency Planning Guidelines
Again, according to the deployment scenario, the frequency planning guidelines are different.

• Fixed WiMAX

In the case of a fixed WiMAX with an outdoor CPE deployed in LOS/near LOS, similar frequency planning to that of Fixed Wireless Access (FWA) systems (e.g. PMP microwave transmission) can be used. At the radio site, several alternatives for sector configurations may be employed (1, 3, 4 and 6 sectors per site). In FWA-like deployment, the CPE has an external antenna with high gain and hence a reduced antenna beam width. Consequently, the antenna of the CPE provides significant interference protection; hence in order to achieve the CINR target on the service area, frequency planning with 3 and 4 frequencies may be used. For the fixed WiMAX with an indoor self-install CPE, which also have directive antennas, a similar frequency plan may be used.

For densification, more sectors (up to six) or more frequencies per sector can be deployed on the existing sides.

•  Mobile WiMAX

In the case of a mobile WiMAX deployment with mobile terminals, there is no interference reduction due to the MS antenna (omnidirectional antennas). However, because of the use of OFDMA and permutations (which gives some randomness in the use of subcarriers), and because of the better radio performance (more coding capabilities, beamforming and/or MIMO), it is possible to deploy an efficient system with a frequency planning reuse scheme of 3 and even 1.

In the case of MBS (see below), the frequency channel assigned for transmitting multicast/broadcast connections must be deployed in a Single Frequency Network (SFN) manner.

For densification, the same techniques used for existing cellular systems are employed: more frequencies per site, cell splitting, microcellular layer, indoor coverage from the indoor, etc.

Base Station Synchronisation
IEEE 802.16-2004 indicates that there are three options of BS synchronisation:

• Asynchronous configuration. Every BS uses its own permutation. The frame lengths and starting times are not synchronised among the base stations. This configuration can be used as an independent low-cost hot-spot deployment.

• Synchronous configuration. All the BSs use the same reference clock (e.g. by using GPS, or Global Positioning System). The frame durations and starting times are also synchronised among the BSs but each BS may use different permutations. Due to the time synchronisation in this scenario and the long symbol duration of the OFDMA symbol, fast handovers as well as soft handovers are possible. This configuration can be used as an independent BS deployment with a controlled interference level.

• Coordinated synchronous configuration. All the BSs work in the synchronous mode and use the same permutations. An upper layer is responsible for the handling of subchannel allocations within the sectors of the base station, making sure that better handling of the bandwidth is achieved and enabling the system to handle and balance loads between the sectors and within the system.

The standard indicates that, for TDD and FDD realisations, it is recommended (but not required) that all BSs should be time-synchronised to a common timing signal.