A Quadruplexer Outdoor Branching Unit for Carrier Aggregation Radio Communications Systems

Junwei Dong*1, Wang Liu2, and Salah Saafan3

1,2 Tongyu Communication Inc. No.1 Dongzhendong Rd, Zhongshan, Guangdong, China 528437

djw@tycc.cn, liuwang@tycc.cn

3 Higher Technological Institute, 10th of Ramadan, Egypt dr-salah@hotmail.com

Abstract A physical channel branching unit for broadband communications is introduced. Such design facilitates carrier aggregation, a concept considered as the enabler for LTE technology (LTE is the last step toward the 4th generation of radio technologies designed to increase the capacity and speed of mobile telephone networks), to be promoted in the microwave frequency domain. What forms the proposed single outdoor unit is a four-channel circulator/filter chain, two for receiving and two for transmitting respectively. Such unit of certain frequency selectivity is ease of branching with another one of different frequency responses. Doing so yields radio-link bounding for higher bit-rate capacity while reusing single antenna system, meanwhile lowering demand for extra room in base station. In this paper, specification is defined and design method and implementation are addressed. Promising results for the initial prototype are demonstrated by simulation and measurement data. Lessons learnt and certain aspects for further phase of industrialized development are discussed as well.

1. Introduction The data rate of wireless communications systems has been experiencing steady growth ever since the invention of first mobile device. It is no surprise to foresee a future of extensive e-commerce, video conferencing, personal and business activities backed by cloud technologies revolved around mobile and base station networks. The rapid growth of 4G-LTE network is one way of solving the base station puzzle. However, there is still concern on using microwave wave links, one of the most prevailing methods, as the backhaul communications for new networks due to its relatively narrow frequency channel and lack of spectrum in hotspot sites. Luckily, a very similar technique of carrier aggression, the channel bounding, that enables LTE could be readily adopted in the microwave domain. It increases system level bit rate without dependency on using high-order modulations that may eventually become bottleneck due to linearity and phase noise. For instance, 1-Gbps Ericsson microwave link throughput in a 28MHz channel based on such technology has been seen in the exhibition in the Mobile World Congress in 2011 [1]. The research proposed in this paper represents one of means of achieving frequency bounding, which is suitable for usage in such broadband outdoor microwave radio networks based on carrier aggression. A few system level characteristics that affect the design specification are deducted. A normal microwave station comprises one microwave antenna and one easy detachable outdoor radio unit (ODU), while both units are selective according to gain and manufacture interfaces. In order to form channel bounding， the proposed design aims at attaching multiple ODUs and antenna of given manufactures. Consider the minimum number of channels for carrier aggression is two, thus it is perceived as a single outdoor product between antenna and radio, for duplex system, that would be one channel open for antenna and two channels for receiving aggression and two channels for transmitting aggression. As a standalone product, one can increases the number of aggression channels by attaching multiple the proposed unit. Thus, each unit should also have an open port that support further branching of identical interfacing unit. That is also the major reason why such types of devices are normally named as outdoor branching unit (OBU). Electrical stability over generally applicable -35-65℃ environmental conditions should be maintained. In this paper, we will first address the methodologies and form a target specification for a demo unit, and then discuss the considerations for electrical and mechanical design. Implementation result of single elements and their cascading testing are followed. Aspects for future improvement and next step plan are touched in the concluding section.

2. Methodologies

978-1-4673-5225-3/14/$31.00 ©2014 IEEE

Channel bounding, intrinsically, reflects a need for frequency band separation and combination. This belongs to a well established applied electromagnetism of multiplexer design. Typical methods of achieving multiplexing could either be directional filter approach , the manifold techniques, branching filter concept, or circulator/filter chain [2]. Among those methods, the circulator/filter is one most economic and suitable choice for outdoor microwave communications systems. It has advantages of flexible channel allocation and facilitate modular design, more importantly, ease of extension for additional devices without change of existing system setup. Comparing to other methods, its drawback is relatively high loss due to circulator, limited power handling and bandwidth. For detailed technical comparison between the above methods, one can refer to [2]. In this paper we adopt the circulator/filter chain approach thanks to relatively low power requirement and acceptable bandwidth in nowadays commercial microwave communications systems. Figure 1 shows the schematic diagram of two cascaded proposed OBUs. P0 is the port open for antenna adaptation, and P1-P2 represent the transmitting only channels unified by diplexer A and P3-P4 stand for the receiving only channels frequencies selected by diplexer B. The two diplexers with quadruple exterior channels are interconnected by two circulators as shown in Figure 1. Two OBUs could be cascaded accordingly via open port P5 and the input port Pa, noteworthily, the open port Pf being terminated by load. To indicate the transmission principle of each OBU, P1/P2 signals are combined by diplexer A and further circulated by C1 toward P0 and get radiated into free space; while Pb/Pc signals are first combined by diplexer A’ and further circulated toward Pa by C3, after passing C2 in the same manner, entering C1’s diplexer port and gets redirected into P0. The receiving mode operates in a similar manner with different signal selectivity by each circulator’s terminal.

Figure 1. Schematic diagram of quadruplexer outdoor branching unit (OBU) Based on existing microwave antenna profile and radio interface at 8GHz, a specification is arrived at by Table 1. The physical parabolic antenna at 8GHz by default has a waveguide flange interface, and to uniformly adapting with radio units, the radio port was assigned Type N interface. To shorten the development cycle, existing circulators with SMA connector was used. Thus one most critical part in this design was to achieve reliable and re-tunable diplexers and have all relevant components encapsulated in an outdoor ingress protection rating (IPV67) compliance container. Detailed design considerations are discussed in the next section.

Table 1. Specification for demo system

3. Design Considerations

Freq CH Space

BW IL Loss Variation

Group delay Variation

Rejection ANT RL ODU RL Isolation between TX/RX

(GHz) (MHz) (MHz) (Tx+Rx) (dB) Max (dB) Max (ns) Min (dB) Max (dB) (dB) (dB)

-35~65℃ -35~65℃ -35~65℃ -35~65℃ -35~65℃ -35~65℃ -35~65℃

7125 MHz – 8500 MHz

56 28 1.5±0.5 2.5 35 15 24 15 40

Demo Freq: P1/P2: 8300/8356MHz; P3/P4: 8426/8482MHz

Diplexer A’

Diplexer B’

Pb Pc Pd Pe

OBU2

Pf Pa

Diplexer A

Diplexer B

P1 P2 P3 P4

OBU1

P5P0

Load

C1 C2 C3 C4 ANT

To achieve the above electrical specifications, a 5-order Chebyshev H-iris waveguide filter was adopted for each filter branch, as shown in Figure 2. One of the diplexers’ simulation results was indicated in Figure 3. To reduce the overall size, filter chambers were purposely bent and co-designed together with the SMA and Type N adapters. Another very important property one has to take into account during design stage, is the temperature drifting effect. Take aluminum for example, it has a linear thermal expansion coefficient of 0.023mm/K, which reflects a mm-scale mechanical displacement across -35~65 ℃ temperature range, rather significant extent. Thus, temperature drift compensation design has to be considered after finishing the basic filter simulation process. A cavity fully implemented by invar could achieve stable response in nearly all conditions [3], however it significantly increases the product cost. To achieve so in accordance with -35~65℃ specification at reasonable cost, we adopted a drifting compensator underneath each resonant cavity, as shown in Figure 2. Figure 4 shows a single resonant cavity’s temperature drifting simulation result using the Ansoft’s eigenmode solver, the materials of the rod being chosen as invar and the cavity as aluminum alloy. After optimization, it achieved a frequency drift less than 6MHz across -35~65℃, a feasible number for realistic outdoor usage. Figure 2 Diplexer Model Figure 3 Diplexer simulation result Figure 4 Temperature Drift Analysis Upon knowing the general performance of the involved elements in each OBU, cascaded circuit simulation is necessarily performed for verification purposes or further optimization. Figure 5 shows the circuit diagram of the proposed OBU and a final simulation result was indicated in Figure 6. Ideal circulator was assumed, it is noted that using physically measured circulator’s S-matrix in the simulation will achieve more realistic numerical predictions. Figure 5 The circuit diagram of OBU Figure 6 Subsystem simulation result Beside of electrical parameters, one has to focus on the environmental stability as for any commercial product. The related tests include but not limit to water sealing, vibration, salt moist stabilities etc. A final worth mentioning issue is the exterior mounted waveguide load, most commercially available load may not be water proof, so corresponding customization or redesign is necessary for reality.

4. Implementation and Result

Figure 7 is a photograph of one unsealed proposed OBU prototype. As shown, two diplexers were stabilized on the ceiling and floor of the capsule chamber respectively. The antenna port is on the right hand, and the branching port locates on left. Figure 8 shows how two branched OBUs appear. Each quadruplexer OBU has four Type N connectors

Drift Compensator

on two opposite side of the capsule, and the exterior structure could be adjusted for different real application scenarios. For instance, the unit could be integrated on the antenna with proper designed weight supporting structure or used as a stand along device on base station floor if the antenna gets connected using flexible waveguide. The exterior surface of the OBU was treated by anodic oxidation, an environmental process offering satisfying slat moist resistance under hazardous conditions. Figure 9 shows the typical measurement results of the four-channel OBU prototype. All primary channels P0-P4 demonstrates return loss better than -15dB with sufficient design margins and insertion loss of transmitting channels P0 -P1/P2 is around 1.4dB (Figure 9 A-B) and receiving channels P0-P3/P4 is around 1.6dB (Figure 9 C-D). Other measured results for relevant specifications in Table 1, not attached to this paper due to space constraint, were achieved as well. Figure 7 One of the fabricated OBU Figure 8 Two cascaded OBUs

Figure 9 Measured result of proposed quadruplexer OBU

5. Conclusion

A quadruple channel outdoor branching unit (OBU) was proposed for broadband radio systems that are suitable for minimum two channel carrier aggression. Multiple carrier aggressions could be achieved by multiple branched identical draduplexer OBUs. Design method and implementation were presented with applicable exhibited frequency responses. Proposed quadruplexer provides meaningful insertion loss, harmonic rejection and superior phase and amplitude tracking in a relatively suitable package. Good electrical performance was received. Future aspect of the design involves in using more integrated and customized circulators so that dual diplexers could be further integrated. If a further implementation via LTCC style processes gets available, full branching units could be integrated inside complex radio system in a much more compact manner.

7. References

1. J. Hansryd and J. Edstam, “Microwave Capacity Evolution,” Ericsson Review. 1, 2011 2. J. Uher, J. Bornemann, and U. Rosenberg, Waveguide Components for Antenna Feed Systems: Theory and CAD, Artech House, Boston - London, 1993. 3. U. Rosenberg, J. Ebinger, M. Knipp, “Channel Branching Equipment for Outdoor Radio Transceivers Serving High Capacity (n×STM-1) Short Haul Radio Links”, German Microwave Conference, 2005

C. P0-P3 D. P0-P4

A. P0-P1

B. P0-P2