However, we should realize that, in the OMUD with ideal data exchange, all the intracell users have the same asymptotic SINR.
To be more specific, under the MMSE-SIC with ideal data exchange, at every stage of detection, each of the three BSs detects one user.
Since (10) for the OMUD with ideal data exchange shares the same form of (21) for the MMSE-SIC without BS cooperation, the ASE of the OMUD with ideal data exchange can be directly obtained from the method adopted for deriving the ASE of the MMSE-SIC without BS cooperation, which will be detailed in Section 4.4.
Consequently, the ASE decreasing rate of the MMSE-SIC with ideal data exchange is even lower than that of the MMSE-SIC without BS cooperation, as analyzed in Section 4.4.
Furthermore, the average spectral-efficiency per user of the multicell systems using the MMSE-SIC with ideal data exchange is C = 1 K ∑ k = 1 K C k (24).
When comparing the two detection schemes with ideal data exchange, the spectral-efficiency of the OMUD-PIC-DE scheme is slightly higher than that of the MMSE-SIC-DE scheme, when the system load is low.
From Sections 3.2 and 3.4, we can see that the spectral-efficiency of the OMUD with ideal data exchange, as shown in (10), shares the same form as that of the MMSE-SIC without BS cooperation shown in (21).
When operated under the scheme of MMSE-SIC with ideal data exchange, multiple stages of detection in the principles of the MMSE-SIC are exploited, so that the data detected at a stage can be shared by the BSs, in order to cancel their interference on the following stages of detection.
From the above analysis, we are implied that, when a multicell system is lightly loaded, yielding β → 0, the schemes of MMSE-MUD and MMSE-SIC without BS cooperation, and the OMUD and MMSE-SIC with ideal data exchange achieve the same spectral-efficiency, which is equal to the single-cell bound.
