ZigBee RF Testing (2)

By David A. Hall, National Instruments

Page 1 of 2

RF Designline (12/04/2007 3:40 AM EST)

ZigBee is a wireless standard for personal area network (PAN) sensor monitoring and control. National Instruments’ Alliance Partner SeaSolve has developed a test suite including transmit (Tx), receive (Rx) and compliancetesting for ZigBee. In this two-part article, we will describe test methodologies and techniques for each type of testing.Part 1 introduced ZigBee testing requirements and focused on transmitters. This part looks at receivers, frame types, and compliance testing. ZigBee Receiver Testing The requirements for testing a ZigBee receiver can generally be broken into two requirements: media access control (MAC) layer emulation and impairments testing at the physical layer (PHY). The first type, MAC layer emulation, can be used to ensure that the ZigBee receiver is able to respond appropriately to the generated commands. With the second type, impairments testing, a receiver is tested by intentionally reducing the modulation quality of the test stimulus. The examples below use SeaSolve’s WiPAN LVSG signal generation solution combined with a PXI vector signal generator, as illustrated in the Figure 8. 8. WiPAN mapping on ZigBee protocol stack.In Figure 8, we have illustrated that the IEEE 802.15.4 standard defines the MAC and PHY layers of a ZigBee transmissions. Typical test procedures involve both MAC layer emulation through packet generation and PHY layer testing by adding impairments. ZigBee Frame Types The MAC (Media Access Control) layer of a ZigBee transmission defines the basic packet and frame structures. The IEEE 802.15.4 specification defines four basic frame structures that can be used for receiver test. These frame types include: A beacon frame is used by a coordinator to transmit beacons. The beacon packet enables a node to identify the presence of other nearby A data frame, is used for all transfers of data payloads An acknowledgment frame is used for confirmation of a successful frame reception MAC command frame is used to handle MAC peer-entity control transfers The MAC command frame is the most flexible. Thus, receiver testing also involves selection of specific sub-frames, listed by type, below: Association request is a request for association with a PANcoordinator. Association response is a reply from coordinator with association status (possibilities include: Association Successful, PAN at capacity, Access denied) Disassociation notification is used by device or coordinator to inform other nodes about disassociation. Data requestis used to request data from a coordinator. PAN ID conflict notification is transmitted when a PAN identifier conflict is detected Orphan notification is used by an associated device that has lost synchronization with its coordinator Beacon request is used for synchronization and to transmit superframe information Coordinator realignment is used by the coordinator to reply to an orphan notification command. It is also used when PAN attributes change with the logical channel information. It can be transmitted to the whole PAN or to a single orphan device. GTS request is used by an associated device to request the allocation of a new guaranteed time slot (GTS) or to request the deallocation of an existing GTS from the PAN coordinator. It also defines the GTS fields such as length, direction, and type. MAC Frame fields configuration In addition, MAC frame fields can be configured as well. Common fields include: frame type, encryption, acknowledgement, frame pending, inter/intra PAN, addressing fields, destination and source addressing modes, sequence number, destination PAN identifier, destination MAC address, source PAN identifier, and source MAC address.Generator Impairments Because tradeoffs must frequently be made between performance, power, and cost, it is common for ZigBee transceivers to operate with a relatively low modulation quality. Thus, testing a ZigBee receiver offers unique challenges to the test engineer. When performing tests, the worst-case environment must be simulated in the lab to ensure that transceiver meets performance specifications and complies with the IEEE 802.15.4 standard. The WiPAN LVSG software enables users to test for interoperability by applying various impairments to model imperfect transmissions and challenges of the physical channel. The specific impairments that can be added include: memoryless nonlinearity, additive white Gaussian noise (AWGN), frequency offset, DC offset, in-phase/quadrature (I/Q) gain imbalance, quadrature skew, and phase noise. Memoryless Nonlinearity Components such as a power amplifier are inherently nonlinear and introduce distortion into a transmission signal. Generally, non-linearity is particularly problematic to modulated signals because of their constant fluctuations in amplitude. Fortunately, ZigBee devices use an OQPSK modulation scheme that is less susceptible to distortion than most modulation schemes. However, because of power requirements, ZigBee transceivers are often designed such that the power amplifier is driven almost into saturation. To illustrate this concept, we show a basic simulated model of a power amplifier in the Figure 9. 9. Saturation of a non-ideal power amplifierAs a power amplifier approaches the point of saturation, significant distortion is often introduced to the Tx signal. Thus, receiver validation requires us to simulate this characteristic of a ZigBee transceiver. AWGN AWGN is the most common mechanism for simulating the signal-to-noise ratio (SNR) of a Tx signal. The affect reducing SNR is that instantaneous phase and amplitude uncertainty is applied. This is most commonly observed on a constellation plot, where we can observe that AWGN causes symbol spreading. This is illustrated in the Figure 10. 10. ZigBee transmission with 25 dB Eb/N0.Because SNR deteriorates with transmit distance, ZigBee transmissions over a longer distance will result in reduced EVM at the receiver. As illustrated in Figure 3, a higher EVM will increase the probability of bit errors and reduce system performance as a whole. Frequency Offset Frequency offset occurs when the Tx and Rx local oscillators of two different devices operate at slightly different frequencies. The effect of frequency offset on an RF signal is that it produces a slight carrier offset in the baseband waveform. Typically, small carrier offsets in the baseband waveform can be removed through signal processing algorithms. Thus, this characteristic is often tested during the design validation phase by applying a slight carrier offset to the test stimulus. If not removed appropriately, frequency offset will prevent the receiver from achieving carrier lock with the transmit signal