# Efficient Preamble Design Technique for Millimeter-Wave Cellular Systems with Beamforming
###### Abstract  
The processing time for beam training in millimeter-wave (mmWave) cellular systems can be significantly reduced by a code division multiplexing (CDM)-based technique, where multiple beams are transmitted simultaneously with their corresponding Tx beam IDs (BIDs) in the preamble. However, mmWave cellular systems with CDM-based preambles require a large number of cell IDs (CIDs) and BIDs, and a high computational complexity for CID and BID (CBID) searches. In this paper, a new preamble design technique that can increase the number of CBIDs significantly is proposed, using a preamble sequence constructed by a combination of two Zadoff-Chu (ZC) sequences. An efficient technique for the CBID detection is also described for the proposed preamble. It is shown by simulations using a simple model of an mmWave cellular system that the proposed technique can obtain a significant reduction in the complexity of the CBID detection without a noticeable performance degradation, compared to the previous technique. m m  
the receive beam at a high resolution to compensate for the imperfections in the quasi-omni antenna pattern. However, in an mmWave cellular system, it is not possible to use the quasi-omni antenna pattern on the receive side because it requires a large transmission range unlike the wireless LANs [8]. Directional beamforming antennas are also necessary at both the BS and the MS to maintain a link budget in the beam-training period. The beam training technique in the IEEE 802.11 ad standard cannot be used for cellular communication systems because it is mainly targeted for indoor communication, without a handover capability. The beam training protocol in cellular systems requires a considerable training time and network resources because it needs to select a serving BS with the best beam pair after checking the link qualities of all the possible beam pairs for all the neighbor BSs. The best beam pair in the mmWave cellular system can be found by using a time-division beam switching (TDBS) during the beam-training period. In the TDBS scheme, single Tx beams are individually transmitted from the BS until all the Tx beams are transmitted. A Tx beam ID (BID) is distinguished by the location of the time slot where the corresponding Tx beam is transmitted. The Rx beam sweep is performed at the MS for each Tx beam for measuring the SNR of every Tx-Rx beam pair. The measurement of the SNRs for all the possible Tx-Rx beam pairs must be performed for all the neighbor BSs to select a serving BS with the best beam pair. The processing time required for beam training in the TDBS increases proportionally to the product of the number of Tx beams, the number of Rx beams, and the number of neighbor BSs. This considerable processing time will create a significant overhead for a moving MS because beam training should be performed periodically for a possible handover or for beam tracking. In order to reduce the processing time for beam training in mmWave cellular systems, a code division multiplexing (CDM)-based technique was proposed in [9], where multiple beams with their corresponding Tx BIDs are transmitted simultaneously during a beam-training or a beam-tracking period. In [9], the cell ID (CID) as well as the corresponding BID is transmitted in the preamble so that the BID can be detected in a multicell environment. The BID is mapped to a single Zadoff-Chu (ZC) sequence in association with its CID because the ZC sequence is suitable for preamble designs in OFDM systems owing to its good correlation property and low peak-to-average power ratio (PAPR) [10; 11; 12; 13]. However, the computational complexity required for the CID and BID (CBID) detection in [9] increases significantly because the correlation operation required for the CBID detection increases proportionally to the product of the number of CIDs and BIDs. In addition, the number of CIDs in mmWave cellular systems is expected to increase significantly because the cells should be deployed in a denser manner than in the existing systems.