K. Voudouris, Dr. Nikos Athanasopoulos, P. Tsiakas, D. Manor, and A. Mor

Multihop relay technologies offer potential for extending cellular communications coverage both indoors and outdoors. Proper application of a multihop relay station can enhance the capacity of even fourth-generation systems, such as 3GPP Long Term Evolution (LTE) and WiMAX, compared to their standard configurations.1 To demonstrate, the authors have developed a prototype relay station (RS) for WiMAX-based wireless networks, designed for operation from 3.3 to 3.8 GHz according to the IEEE 802.16j standard.2

The WiMAX RS operates as a nontransparent relay (NTR) within the system and implements media-access-control (MAC) functions as well as distributed scheduling. The RS supports time-division transmit and receive (TTR) operation in a time-division-duplex (TDD) mode while also enabling simultaneous transmit and receive (STR) operation. The RS is designed to support the WiMAX physical layer (PHY) profiles detailed in the table.3

Figure 1 shows the top-level architecture for the proposed WiMAX RS. The RS design incorporates a single-unit-relaystation (SURS) topology, implementing both access and backhaul capabilities within a common enclosure. The prototype RS consists of the following main modules:

1. The relay processor and interface (RP&I) module serves as the system's switchboard. In addition to providing system interfaces, it also executes routing and networking processing as management options and for optional network-based relay implementation.
2. The relay modems implement both MAC and PHY functions.
3. The relay front-end module (RFEM) performs RF processing, amplification, filtering, and antenna switching.
4. The power-supply unit handles the power-distribution chores.

An optional Global Positioning System (GPS) receiver can be incorporated into the WiMAX RS, as fallback capability for synchronizing the RS to a WiMAX network. Also, an optional backhaul module can be included for use in the case of a network-based relay implementation.

Figure 2 shows a suggested frequency-reuse plan in a reusethree- relay deployment scenario. The multiple-relay base station (MRBS) can be found in the center of the diagram. This scenario shows six RSs distributed among three WiMAX cells. Considering for example the north-west sector, also shown on the right-hand side of Fig. 2, a 10-MHz band, F1, is used by the MRBS to provide access for non-relay subscribers, and to provide wireless backhaul to the RSs. Band F2 is used by the RS to provide access to its subscribers. Additional RS units in the cell may use either band F2 or band F3 for access, according to spectrum availability and interference among the other relay cells. Figure 3 shows the SURS deployment method in TTR operation. In this case, the MRBS communicates with the RS and also directly with subscribers (CPEs). This is achieved through a time-division frame format, as shown in Fig. 4.

In the WiMAX RS, the RP&I module is the system's switchboard. It performs the following functions:

1. orchestrates system power-on sequencing;
2. enables management services such as web-based telnet/command-line-interface (CLI) and optional simple-network- management-protocol (SNMP) functionality, as well as alarms;
3. provides Ethernet connectivity and synchronization; and
4. provides network processing resources for network-based relay implementation.

The top-level architecture of the RP&I module is shown in Fig. 5. This module's primary role is to transfer and manipulate Internet Protocol (IP) traffic; its processing power is derived from the total access bandwidth that it can support. As a result, the appropriate central processing unit (CPU) is critical for the whole operational capabilities of the prototype WiMAX RS. In addition to CPU power, sufficient Flash and SDRAM memory is also required. The PR&I module is designed for three Ethernet interfaces. Two are used to exchange data and management transactions with the access and backhaul parts of the relay modem, while the third Ethernet interface is external and can be used for optional RS extensions. The WiMAX RS is also designed with a serial port (UART) for communication with the RFEM. To support optional extension of network-based relay functionality, the RP&I module provides two Universal Serial Bus (USB) host ports that are used for connection to the optional backhaul module. The RP&I module also provides debug ports such as a Joint Test Action Group (JTAG) port and a serial console for supporting related communications integrated-circuit (IC) and software development.

The WiMAX RS incorporates two relay modems, for access and backhaul links, respectively. Each relay modem executes MAC and PHY functionality for the WiMAX RS in TTR operation, with the potential for also supporting STR operation. The core of each relay modem is a model DAN2400 system-on-chip (SoC) device from DesignArt Networks.4 The DAN2400 SoC is essentially a base-station design with backhaul capability on a single chip. It integrates complete PHY and MAC access and backhaul capabilities, a DSP, embedded wireless network processor, control-plane CPUs, and a six-channel RF interface with integrated digital-toanalog- converter (DAC and analog-todigital- converter (ADC) cores.

The MAC module implements fullycompliant IEEE 802.16e wave-2 MAC functionality performing: packet classifications according to different profile requirements; packet enqueuing to external memory based on queue identification (ID) and packet dequeuing according to queue ID; automatic-repeat- request (ARQ) and hybrid-automatic- repeat-request (HARQ) packet reassembly and fragment management as well as management queue handling; received MAC-protocol-data-unit (MPDU) parsing according to the MPDU headers; and cyclic-redundancy-code (CRC) decoding, header-check-sequence (HCS) decoding, and decryption by means of dedicated hardware blocks.

The PHY module supports fullycompliant IEEE 802.16e wave-2 PHY functionality. It incorporates a powerful multiple-input-multiple-output (MIMO)/beamforming-network module capable of operating with as many as six channels, as well as an integrated digital signal processor (DSP), which performs all MIMO orthogonal-frequency-division- multiple-access (OFDMA) receive signal processing. The main functionalities of the PHY module include:

* scalable OFDMA (SOFDMA) PHY with 512- and 1024-point Fast Fourier Transforms (FFTs);
* channel bandwidths of 3.5, 5.0, 7.0, and 10.0 MHz;
* adaptive modulation schemes with quadrature phase shift keying (QPSK), 16-state quadrature amplitude modulation (16QAM), and 64-state QAM (64QAM) on both downlink (DL) and uplink (UL) channels;
* partially used subcarrier (PUSC) with or without dedicated pilots, PUSC for all subchannels, fully used subcarrier (FUSC), adaptive modulation and coding (AMC) 3 x 2 or 2 x 3 subchannels;
* multiple-zone support in DL, including MIMO zones;
* extensive smart-antenna capabilities, including transmit and receive (T/R) MIMO 2 x 2 configurations over six (T/R) antennas;
* DL MIMO in 2 x 2 or 2 x 4 configurations or in a 2 x 6 MIMO minimum mean square error (MMSE) configuration, as well as UL Collaborative MIMO in 2 x 2 or 2 x 4 configurations or in 2 x 6 MIMOMMSE configuration;
* support for MIMO matrix A and B schemes in DL;
* open- and closed-loop MIMO schemes;
* power control in open- and closed-loop forms;
* powerful backhaul link PHY ; and
* TDD operation.

A relay modem for the WiMAX RS is depicted in Fig. 6.

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The RFEM performs ADC and DAC functions, RF processing, amplification, filtering, and antenna switching (Fig. 7). It consists of two main submodules: the RF processor (RFP), which provides digital baseband signals to the intermediatefrequency (IF)/RF upconverter and signal conditioning prior to transmission, and the relay front end (RFE), which performs filtering, amplification, and antenna switching.

The RFP module consists of numerous function blocks. The ADC/DAC block includes an ADC and DAC, each with 14-b resolution. The baseband interface contains all necessary interfaces between the baseband RS modem and the RFEM. The baseband interface is based on a standard serial peripheral interface (SPI) for control and data transfer using digital I/Q or JC61 interface. The DC and power function block regulates and monitors the required supply voltage. The configuration control and monitoring block employs the RFP to configure monitors and control the frequency synthesizer, amplifier, power, and RF switching, and control communications with the RP&I unit. The clock generation and distribution unit serves clock distribution to clients inside and outside the RFP as well as to the optional GPS receiver. The DSP engine, which is implemented with a fieldprogrammable gate array (FPGA), performs RF/DSP processing for enhanced operational modes such as repeater and digital predistortion. The assembled RFP module is depicted in Fig. 8.

The RFE supports 2 x 2 MIMO operation and is comprised of five main function blocks. The RF relay antenna switch directs an antenna to either transmit or receive mode at the designated time zone. The RF switch isolates the LNA from the WiMAX power amplifier during the RS receive period. The RF switch contains a circulator that increases isolation and protects the power amplifier from damage due to unterminated operation. The WiMAX transmit power amplifier boosts signals to the desired power levels within the transmission mask specifications. The power amplifier also includes temperature-compensation and powerdetection circuitry as part of its thermal management and protection circuitry. The LNA serves the WiMAX receiver by boosting low-level signals from the antenna. With several stages, it provides small-signal gain that can be adjusted to adapt to different input power levels. The converter block shapes RF signals into the required spectral emission masks at the required power levels for transmission; shaping is performed by means of channel filtering at low IFs.

Because of space restrictions, all required function blocks for the RFE had to fit into a limited printedcircuit- board (PCB) area. The design was also guided by requirements for heat and power dissipation. Figure 9 shows the top and bottom layouts of the RFE, with several PCB areas worth mentioning. The frequency source (FREQ SRC) area, for example, uses the 112-MHz reference signal from the RFP to lock signals from a phase-lock loop (PLL) and voltagecontrolled, temperature-compensated crystal oscillator (VCTCXO) to generate low-phase-noise signals to feed the LOs. The DC area (PWR) includes dedicated power sources for the transmitter and receiver chains. The receivechain area (RX1 and RX2) is limited to the bottom side of the PCB to reduce interference from the transmitter. It is roughly partitioned into two mirror images implementing the MIMO A and B receive chains. Since the RFE supports STR operation, interference caused by the transmitter chain affects the receiver chain. This problem is mitigated by filtering transmit signals from the receive chain while maintaining high linearity over the wide dynamic range of the receiver. This was achieved by means of superheterodyne layers throughout the receiver chain and by using surfaceacoustic- wave (SAW) filters in the receive chain. The excellent rejection of these filters helps provided enhanced dynamic-range performance.

The RFE's transmit area (TX1 and TX2) is limited to the top side of the PCB, and is roughly partitioned into mirror images with MIMO A and B transmit chains. A superheterodyne approach was used to meet the WiMAX transmission mask requirements. The transmit sections employ a topology to enable highpower transmission for long-range access. Transmit PAs are placed on the top of the PCB and interface with the chassis heat sink using a metal platform for enhanced heat dissipation. The switch matrix area (matrix switches 1 and 2) directs backhaul or access antennas to their corresponding receiver (LNA) or transmitter (PA) ports. The RFE switching element includes the RF relay antenna switch and the RF switches. Switches between backhaul and access modes are synchronized with backhaul and access time zones determined by the relay modems. Figure 10 shows the final integration of all RS modules and how they are mounted within the compact SURS enclosure. The mounting arrangement provides adequate isolation between modules to ensure proper thermal and electromagnetic management of the different functions

See Associated Table

ACKNOWLEDGMENT
The present work has been performed in the scope of the REWIND ("Relay-based WIreless Network and StandarD") European Research Project and has been supported by the Commission of the European Communities Information Society and Media Directorate General (FP7, Collaborative Project, ICT-The Network of the Future, Grant Agreement No. 216751).

REFERENCES
1. V. Genc, S. Murphy, Y. Yu, and J. Murphy, "IEEE 802.16j Relay-based Wireless Access Networks: An Overview," IEEE Wireless Communications Magazine, Special Issue on Recent Advances and Evolution of WLAN and WMAN Standards, 2008.
2. IEEE Standard for Local and Metropolitan Area Networks, "Part 16: Air Interface for Broadband Wireless Access Systems. Amendment 1: Multiple Relay Specification", IEEE, June 2009, www.ieee.org.
3. IEEE Standard for Local and Metropolitan Area Networks, "Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems. Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1", IEEE Std., February 2006, www.ieee.org.
4. "DAN2400 SoC data sheet," DesignArt Networks Ltd., Tel-Aviv, Israel.