The Mobile WiMAX system profiles for Wave 2 of the WiMAX Forum’s® Radio Conformance Tests require a number of new features when compared to Wave 1.
N7615B Signal Studio for 802.16 WiMAX™ supports many new capabilities for Wave 2 testing. The main features are listed below:
DL/UL AMC 2x3 zones
The IEEE 802.16 standard allows various types of adaptive modulation and coding (AMC) zones. Wave 2 conformance testing tests AMC zones with a tile structure of 2 bins x 3 symbols.
2-antenna matrix A: space-time coding (STC) (DL-PUSC)
STC transmits each symbol of data twice with slightly different coding, giving the receiver two opportunities to receive the data. As shown in the equations in the upper right corner of Figure 1, if the receiver has knowledge of the channel (h0 and h1), it can recover the transmitted symbols s0 and s1. The channel is estimated based on the pilot measurements.
Matrix A is a diversity technique that increases the range and hence the effective data throughput that the receiver sees due to the improved reception, but the transmitted data rate is not increased. The data rate is referred as rate 1 in the standard. The preamble and the first DL-PUSC zone in a frame are not transmitted with STC since the first zone contains the basic FCH and MAP information that all subscriber stations need to receive, and these are transmitted from antenna 0 only. Subsequent DL-PUSC zones can be designated as an STC zone, and this zone will be transmitted with the appropriate encoding from both antennas.
Figure 1. Matrix A (STC) diagram
2-antenna matrix B: vertical encoding (2x2 MIMO) (DL-PUSC)
The other type of MIMO for Wave 2 is matrix B with vertical encoding. Vertical encoding means that the transmitter takes one set of data (referred to as a “layer”) and splits up the data among multiple transmit antennas. With two transmit antennas, the data rate can theoretically be doubled.
As with matrix A, the signal can be recovered with knowledge about the channel, which is estimated by measuring the pilots. The first DL-PUSC zone cannot be matrix B, and the preamble and first DL-PUSC zone are transmitted from antenna 0 only, while the matrix B zone is transmitted from both antennas.
For a 2x2 MIMO system, there are now 4 fading channels between each pair of antennas, shown in Figure 2 as hxx. Each channel is modeled with multiple fading paths.
The WiMAX Forum has developed a specific MIMO channel model for Mobile WiMAX that consists of the ITU Pedestrian B or Vehicular A channel models with the addition of a spatial correlation matrix that is applied to each tap.
Figure 2. Matrix B (2x2 MIMO) diagram
Collaborative spatial multiplexing for two subscriber stations (SS), each with one Tx antenna (UL-PUSC)
For uplink transmissions, collaborative spatial multiplexing allows two subscriber stations to transmit on the same subchannels in order to increase throughput on the uplink. The single transmit antennas on each SS effectively act as though they were two transmit antennas on a single device.
Each SS transmits on a different pattern with unique pilot tones for each SS as shown on the right of Figure 3.
Different channel models can be applied between each SS and base station (BS). For example, one SS may be moving at pedestrian speeds while the other SS may be in a vehicle.
Figure 3. Collaborative spatial multiplexing
IO-BF (beamforming) profile
DL-PUSC and AMC 2x3 with dedicated pilots
Normally a BS modulates all of the pilots belonging to the segment(s) that it is using, whether or not those subchannels have been allocated for data bursts, and the SS uses all of the pilots in the DL transmission for channel estimation. However, when beamforming is used, the pilots in a group of subchannels will be beamformed for particular subscriber stations, so it is important for the SS to focus only on the appropriate pilots. In dedicated pilots mode, a BS only modulates the pilots in the allocated subchannels, and the SS only uses the pilots that are specific to its allocation for channel estimation.
UL-PUSC without subchannel rotation
Subchannel rotation is part of the normal mode of operation for the WiMAX uplink for most uplink zones (except AMC and ranging/fast feedback/ACK regions). For each slot duration (e.g. every 3 symbols in UL-PUSC), the subchannels are renumbered so that different physical subchannels are assigned to a particular logical subchannel. This is described in section 126.96.36.199.6 of the 802.16 standard. This provides improved immunity to fading or interference, since the data gets transmitted at different frequencies at different times. In beamforming, the BS is trying to apply weights to the transmission from different antennas to direct the signal to particular SS, based on information about the channel conditions to that SS. This is very difficult to do if the transmission frequencies are changing with subchannel rotation, because the channel would be constantly changing also. Thus for beamforming, it is desirable to disable subchannel rotation.
Uplink sounding zone (type A)
In uplink sounding, a special sounding zone is allocated in the uplink which occupies all subchannels and may be one or more symbols long. When commanded by BS, the subscriber stations can use this allocation to transmit a sounding signal that is a known sequence of data to the BS. The BS measures the channel frequency response from this sounding signal and uses the information to determine the proper beamforming to use. There are two types of sounding, type A and type B. Type A uses sounding frequency bands that are composed of 18 consecutive subcarriers. Type B has frequency bands that are allocated based on a specified downlink subcarrier permutation such as PUSC. Only type A is required for Wave 2.
N7615B Signal Studio for 802.16 WiMAX is a flexible signal creation software tool which simplifies the design and test of components and receivers for WiBro and Mobile WiMAX. See the N7615B product page for more details.
N7615B Signal Studio for WiMAX supports DL/UL AMC 2x3 zones, matrix A, matrix B, collaborative spatial multiplexing for uplink PUSC zone, and the features listed above under the IO-BF profile. See N7615B Technical Overview for more details.